Chapter V
THE MINOR MINERALINGREDIENTS OF SOILS; MINERAL FERTILIZERS; MINERALS INJURIOUS TO AGRICULTURE.
(A.) MINERALS USED AS FERTILIZERS.
Of minerals important in soil-formation, not usually present in large amounts in rocks, but extensively used in fertilization, the following require mention:
Apatite; phosphate of lime containing more or less of the chlorids and fluorids of the same metal; the mineral from which the phosphoric acid of the soil is mostly derived. In the crystallized condition when perfectly pure it is colorless; but it is mostly of a greenish tint (hence “asparagus stone”). The pure crystalline mineral rarely occurs in large masses (as in Canada); but small to minute crystals are found widely disseminated in many rocks (granites, “basalts” of the Pacific Northwest), thus passing into the soils formed from these rocks. These crystals are readily recognized, being regular six-sided prisms with a flat or obtusely pyramidal termination (distinction from quartz), and do not effervesce with acids (distinction from calcite). By far the largest deposits of this mineral occur in connection with carbonate of lime, in the rock materials known as phosphorites. Lime phosphate being, like the carbonate, soluble in carbonated water, the two naturally frequently pass into solution, and are subsequently deposited together. Most limestones contain a small proportion of lime phosphate, being, as already stated, formed from the shells and the framework of animal organisms usually containing also phosphates. But the content of phosphates in limestones is not readily apparent to the eye, and the richest deposits, save such as contain animal bones, have long passed unsuspected as to their being anything else but limestone. Systematic search has now revealed the presence of phosphate rock in numerous localities, chiefly where limestone formations occur. In the United States, in South Carolina, Florida, Alabama, Tennessee, Kentucky, Nevada; in South America, on Curaçoa island, Venezuela; in the Antilles on Sombrero, St. Martins and Navassa islands. In Africa, in Algiers and Tunisia; in Europe, in Spain (Estremadura, one of the first deposits known), France, Belgium and the adjacent parts of Germany; in Bohemia and Galicia in Austria; and very extendedly in European Russia. Many islands of Oceanica supply phosphorites derived from the decomposition of bird guano by the coral limestone.
Unfortunately the percentage of phosphate in a large proportion of these materials is not sufficiently high to make their conversion into water-soluble superphosphate economically possible at the present time; since all the calcic carbonate present must also be converted into comparatively worthless sulphate (gypsum) by the use of sulfuric acid; and as yet no practicable method for avoiding this difficulty has been found.
“Thomas Slag.”—Probably the nearest approach to such a method is indicated by the fact that a compound containing four instead of three molecules of lime to one of P₂O₅, such as is contained in the “Thomas slag” of the basic process of steel manufacture, is nearly or in some cases (“sour” soils) quite as effective for the nutrition of plants as the water-soluble superphosphate. This discovery has rendered available for agricultural use the phosphoric acid contained in the enormous deposits of limonite iron ore known as bog ore, which contains a large proportion of ferric phosphate and from that cause has until lately been excluded from the manufacture of wrought iron and steel. It is reasonable to hope that by some analogous process the low-grade phosphorites, such as those of Nevada and the plains of Russia, will also in the course of time become available for agricultural use. Extremely fine grinding and washing (producing “floats”) has been resorted to for the purpose of rendering the raw phosphorites effective in fertilization. But while this is successful on some soils, on others the “floats” remain almost inert; so that this method has found only limited acceptance.
Animal bones, which consist of from 24 to 30% of animal substance and 70 to 76% of “bone earth,” (or when fossil are free from the former), are largely used for the manufacture of superphosphate. The bone-earth consists in the main of tri-calcic phosphate with from one to two per cent of calcium fluorid (much as in natural apatite), a small amount of magnesic phosphate, and about 4 to 6% of calcic carbonate. Bone meal can therefore supply to plants both phosphoric acid and nitrogen, and the presence of the latter has been largely the cause of a material overestimate of its efficacy as a fertilizer in the past. Wagner’s and Maerker’s experiments have shown that at least in sandy soils poor in humus, it cannot be considered an adequate source of phosphoric acid for annual crops, and that in these soils its immediate effects are almost wholly due to its nitrogen-content. The slow availability of the phosphoric acid renders it unprofitable as a source of the latter, outside of the heavier lands with abundance of humus; in “sour” lands (notably on meadows) bone meal produces its best results. In soils naturally calcareous, or in such as have received heavy dressings of lime either as carbonate or in the caustic condition, the manurial effects of bone meal are seriously diminished. Nagaoka (Bull. Coll. Agr. Tokyo, Vol. 6, No. 3) shows that the crop of rice fertilized with bone meal was reduced to less than half when limed, and that the phosphoric acid taken up by the crop was reduced to one-sixth. In any case it is most important that bone meal should be as finely ground as possible, as in the case of the phosphorites; and this can best be done when it has first been freed from fats by boiling with water, and then steamed under pressure. It can then also be most readily converted into superphosphate.
The phosphate minerals and the fertilizers manufactured therefrom are of primary importance to agriculture. The phosphoric-acid content of soils is mostly very small, and only a fraction of it is usually in an immediately available form. Hence for permanent productiveness, and especially for intensive farming or gardening, a cheap supply of phosphate fertilizers is of first importance in all soils and climates.
Other phosphate minerals occur frequently, but as a rule only in small amounts, in connection with the ores of most metals. The only ones of these of interest to agriculture are Vivianite and Dufrenite, the phosphates respectively of the protoxid and peroxid of iron. The former occurs in mineral deposits as small blue crystals, or more frequently as blue earthy masses or streaks, in the substrata of rich alluvial ground (Louisiana, California). Dufrenite sometimes results directly from the oxidation of the protoxid mineral, which then turns greenish and finally brown. Unfortunately these minerals, rich as they are in phosphoric acid, cannot readily be utilized as sources of phosphate fertilizers, because of the difficulty of getting rid of the iron. Their occurrence usually suggests the presence of abundance of phosphoric acid in the soil. But that which is actually combined with the iron oxids is practically unavailable to plants; especially so in the case of the peroxid compound, the formation of which is a common source of loss of phosphoric acid when soils rich in iron are submerged for any length of time; a point which is discussed below (chapt. 13).
Among the iron phosphate minerals, may also be mentioned “bog ore,” which results from the reductive maceration of swamped ferruginous soils, and accumulates in the subsoils and in the bottom of swamps or moors, forming “moorbedpan”; a dark brown, rather soft mass, which is sometimes used as an iron ore, especially since the invention of the “basic process” of iron smelting, one of the products of which is the phosphate or Thomas slag. (See above).
Nitrate of Soda or Chile saltpeter.—This mineral being (like all nitrates) easily soluble in water, can only occur in regions nearly or quite destitute of rainfall. Such is the case in the Plateau of Tarapacà in Northern Chile, where it occurs in large quantities; it is likewise found, but to much smaller extent, in Nevada, southern California, Egypt and India. By far its most extended occurrence is that in Chile, where, together with common salt, it fills cavities and crevices in a gravelly clay that forms the surface of a plateau from three to six thousand feet above the sea. It is never pure, but always mingled with a large proportion (up to 50% and over) of common salt; also some Glauber’s salt (sulfate of soda) and some sodic perchlorate and iodid; hence it forms an important commercial source of iodine.
The mixed mineral mass, called “Caliche,” when taken out of the ground is dissolved in water; and the solution boiled down, during which process the common salt is first deposited and is raked out of the pans; the nitrate is afterward farther purified by crystallization. As brought into commerce for agricultural purposes it constitutes a moist gray saline mass, somewhat resembling common salt, of which substance it usually contains a few per cent; occasionally also a small amount of sodic perchlorate (which acts injuriously on vegetation). Aside from its use as a fertilizer, Chile saltpeter serves for the manufacture of nitric acid; and either directly, or after previous transformation into potassic nitrate, for that of gunpowder.
The Chilean locality is the only one from which the commercial article is derived; the deposits elsewhere are too limited in extent to compete commercially with the South American product. Caliche ranging as high as 80% of nitrate of soda has been sent to the writer from the Colorado Desert in Southern California, but the exact locality of occurrence has not been divulged. Extended areas of clay hills impregnated with nitrates exist in the Death Valley region of California, but in the absence or extreme scarcity of water in that region, it is doubtful whether these impregnations can be made practically available. Another locality is that near White Plains, Nevada, where Caliche averaging about 50% purity is found in cavities and crevices of a reddish volcanic rock. The rainfall in this region is so slight that the greater part of the dust or sand blown about by the wind consists of Glauber’s salt. Here also, as in Chile, the niter deposits appear to be restricted to within a short distance from the surface, and the total amount thus far observed appears to be insufficient to encourage large-scale exploitation.
Origin of Nitrate Deposits.—The probable origin of these niter deposits has given rise to a great deal of discussion, and a wide difference of opinion exists as to the source from which the nitrogen may reasonably be supposed to have been derived. According to the present state of our knowledge, it must be presumed that its sources have been organic, and that the niter has been produced by the activity of the same bacteria which now produce nitrates in our soils, rendering the nitrogen of humus available to plants. But it is by no means clear what that organic material could have been; for at the present time the plateau of Tarapacà is almost wholly destitute of vegetation, if not of animal life. The latest and apparently most reasonable suggestion is that of Kuntze, who calls attention to the fact that the vicuñas and llamas which are at home in this portion of the Andes, and are known to have roamed over that region in countless herds, have the curious habit of always depositing their manure in one and the same place whenever at liberty. Each herd of these animals has its definite dunging place at some convenient point. That such herds have existed in the region from time immemorial is obvious from historical as well as collateral evidence; and as their manure accumulated, its nitrification would progress rapidly under the prevailing arid conditions. The common salt would naturally be derived from the urine and excrements, and the alkaline salts which exist throughout this region as the products of soil decomposition, would be quite sufficient to account for the alkaline bases in the caliche. On the other hand, the presence of iodine points to seaweeds as the organic source.
Intensity of Nitrification in Arid Climates.—Of the efficacy of nitrification under arid conditions abundant evidence may be found within the State of California. In the alkali lands of southern California the nitrates of soda, lime and magnesia are almost universally present; they form at times as much as one-fifth and even more of the entire mass of alkali salts, and in one case the total amount in the soil has been found to reach two tons per acre, with an average of twelve hundred pounds over ten acres. In the plains of the San Joaquin Valley, spots strongly impregnated with niter are found, especially under the shadows of isolated oak trees, where the cattle have been in the habit of congregating for a long time; a case quite analogous to that supposed by Kuntze to exist in the Chilean locality. Of course it is only in arid climates that the accumulation of nitrates can usually occur; for in the region of summer rains the nitrates formed during the warm season will inevitably be washed into the subdrainage, unless restrained by absorption by the roots of vegetation. The heavy losses occasionally occurring from this cause in the course of a rainy winter on summer-fallowed land have been amply demonstrated by many investigations.
POTASH MINERALS.—By far the most abundant occurrence of potash in the earth’s crust is that in silicates and notably in orthoclase or potash feldspar, which contributes so largely to soil-formation. But in the absence of any economically successful artificial method for producing potash compounds from feldspars on a commercial scale, almost the entire supply of potash salts was, until a comparatively late period, derived from plant ashes, viz., the “potashes” of commerce. At the same time, almost the entire demand for alkalies for industrial uses bore upon the same product, until the invention, toward the end of the last century, of LeBlanc’s process for the manufacture of soda from common salt; for until that time, soda in the various forms in which it was imported from the Orient or prepared from seaweed ashes, was a comparatively costly product. LeBlanc’s invention was most timely in that it very quickly diminished materially the production of potashes which, in view of the increased demand for alkalies for industrial uses, seriously threatened the depletion of agricultural lands, and of woodlands as well, of one of its most essential ingredients. Yet as there are many industrial uses in which soda cannot replace potash, the manufacture of potashes continued to a greater or less extent, as no other available source except the ashes of land plants, was then known. The production of potassic chlorid from the mother-waters of sea salt in the spontaneous evaporation of sea water for the manufacture of common salt, was on too small a scale to influence materially the manufacture of potashes.
Discovery of Stassfurt Salts.—The depletion of potash had become so serious a matter in the agricultural lands of Europe, that for a time much research was bestowed, and prizes offered for an economical method of producing potash salts from feldspar, on a commercial scale. But the problem had not been satisfactory solved when, in the year 1860, attention was called to the fact that the saline deposits overlying certain large rock-salt beds that had been developed by borings near Stassfurt in Prussia, contained so large a proportion of potash salts, as to render their purification and conversion into fairly pure sulphate and chlorid technically feasible. The impulse having been given, the potash industry developed rapidly in that region as well as in the adjacent portions of Saxony, where the same formation underlies; the production of “Stassfurt Salts” rapidly assumed a greater development than that of the rock-salt which had originally prompted the enterprise, and numerous additional boreholes demonstrated an unexpectedly wide extension of the same beds. At the present time, in consequence of such development, the manufacture of potashes from plant ash has almost ceased, outside of Canada and Hungary; and the production of potash salts in the Stassfurt region now supplies the demand of the entire world, both for industrial and agricultural purposes.
The cheapening of potash as a fertilizer has rendered possible the profitable cultivation of large areas of land which were naturally too poor in that substance for ordinary cultures; and has likewise rendered possible the restoration to general culture of lands that had ceased to produce adequately, on account of the depletion caused by long-continued cropping. It has likewise served to intensify agricultural production wherever desired; and between this supply and that of phosphoric acid from the phosphorites (see above), and the discovery of the nitrogen-absorbing power of leguminous plants, which can be used for green-manuring, farmers have been enabled to dispense, in many regions, with the production and use of stable-manure, which until then had been considered an indispensable adjunct to agriculture everywhere. Even within the last fifty years it was proclaimed by high authority in Germany that stable-manure constituted, as it were, the farmer’s raw material, from which he manufactured the various products of the field through the intervention of the plant-producing power of the soil.
Origin of the Potash Deposits.—The manner in which this accumulation of potash salts has been formed deserves explanation. It is abundantly evident that nearly all deposits of rock-salt thus far known have been formed by the evaporation of sea-water at times when bays or arms of the sea were cut off from open communication with the ocean. The composition of sea-water has already been given and discussed (chap. 2, p. 26); and by the slow evaporation of sea-water on a small scale we can quite successfully imitate the phenomena observed in natural rock-salt deposits. When sea-water is heated a slight deposit of lime carbonate (usually containing a little ferric oxid and silica) is soon formed; and a corresponding thin deposit of ferruginous limestone is commonly found at the base of rock-salt-bearing deposits. Next above this we almost invariably find a deposit of gypsum, sometimes of great thickness; in the artificial evaporation of sea-water the same thing occurs so soon as the brine has reached a certain degree of concentration. It constitutes the major portion of the “panstone” of salt-boilers. Next above follows a deposit of rock-salt, at base somewhat mixed with gypsum; its thickness varies greatly according to circumstances. Above it lie the potash salts.
In the manufacture of sea-salt by evaporation in shore lagoons or “saltpans,” the solution remaining after the salt has been deposited (known as “mother-waters,” or “bittern”), of course remains on the surface of the salt unless allowed to drain off, as is done in the process of manufacture. When not drained off, the water gradually evaporates, and there remains a saline crust of a composition exactly resembling that of the upper layers at Stassfurt, containing a large proportion of potash salts.
If it be asked why the Stassfurt salts are not found overlying every rock-salt deposit in the world, the answer is that in a great many cases the concentrated mother-waters have had an opportunity to flow off from the surface of the rock-salt by the action of tides, the inflow of fresh water from the land or from other causes. Their presence therefore depends upon the fulfilment of accidental conditions not nearly always realized in the natural evaporation of sea-water, but which happened to occur on a very large scale in that portion of the North-European continent.
Nature of the Salts.—The potash is present in the Stassfurt salts in the form of complex sulfates and chlorids containing, besides, sodium, calcium and magnesium in various proportions and modes of combination. The most abundant of the potassic chlorid minerals is carnallite, a hydrous chlorid of potassium and magnesium. The chlorids characterize chiefly the upper portions of the deposit, the sulfates the lower.
Kainit.—Of the products derived from the Stassfurt salt industry for agricultural use, the two requiring special consideration are “kainit,” a natural mixture of the several chlorid minerals in varying proportions; and “high-grade sulfate.” Being a natural product, “kainit” is the cheapest source of potash available to the farmer; but on account of its variability in composition it must be sold and purchased on guaranteed assay. On account of its large content of chlorin it is not desirable in the production of certain crops, especially in the arid region, where alkali soils, and even those not visibly alkaline, often contain already large amounts of chlorin. Moreover, kainit usually contains a considerable proportion of common salt. For the arid region therefore the sulfate is generally preferable, although it is somewhat higher in price for the same amount of potash. The potash content of commercial kainit (calculated as K₂O) ranges from 16 to 35%, while the sulphate frequently ranges from 80 up to 95% of the pure sulfate; thus costing materially less in freight charges than the lower-grade kainit. Its potash content ranges from 43 to over 50% of K₂O.
Potash Salts in Alkali Soils.—The sulfates and chlorids of potassium, however, occur not only in connection with rock-salt deposits, but are also found in the alkali soils of the arid region. They are, in fact, never absent where such salts occur at all, and their percentage in the total of salts ranges all the way from about 4 to as much as 20% of potash sulphate. In numerous cases it has been found that the content of this salt to the depth of four feet amounts to from 1200 to 1500 pounds per acre. In such lands, of course, additional fertilization with potash salts is totally uncalled for, the more as such soils invariably contain, besides the water-soluble potash, an unusually large percentage of the same in the form of easily decomposable silicates, or zeolites.
Farmyard or Stable Manure.—In connection with the subject of mineral fertilizers, it will be proper to discuss briefly the uses and special merits of stable manure, composts, etc. Up to within the last century, these were practically the only fertilizers known and used, and the exclusive use of this manure might have continued indefinitely but for the discovery that as time progressed, stable manure and with it grain crops, for the production of which it was necessary, became less and less in amount, so as to threaten bread famines. The cause of this diminution was, of course, the incompleteness of the return of the soil-ingredients taken off by the crops, when these were exported to feed the cities or foreign countries. Thus the attention of chemists, and notably that of Liebig, was attracted to the solution of the problem of keeping up production even with an insufficient supply of stable manure; and the discovery of the use of mineral fertilizers was the result of their activity.
The chemical composition of stable manure does not, alone, suffice to explain its remarkable efficacy and the difficulty of replacing it by any other material. The composition of manure of course differs not only with different animals but also with the different feeds consumed by them; but the average composition of farmyard manure is approximately given thus by Wolff and others:
ANALYSES OF VARIOUS FARMYARD MANURES.
========================================================== | 1. | 2. | 3. | 4. | 5. | | | | | Water | 71.00 | 75.00 | 79.00 | 79.95 | 72.33 Dry Matter | 29.00 | 25.00 | 21.00 | 20.05 | 27.67 Ash ingredients | 4.40 | 5.80 | 6.50 | | 5.87 Potash | 0.52 | 0.63 | 0.50 | 0.84[23]| 0.69 Lime | 0.57 | 0.70 | 0.88 | | 0.85 Magnesia | 0.14 | 0.18 | 0.18 | | 0.14 Phosphoric acid | 0.21 | 0.26 | 0.30 | 0.40 | 0.30 Ammonia | | | | | 0.02 Total Nitrogen | 0.45 | 0.50 | 0.58 | 0.78 | 0.46 ----------------+-------+-------+-------+----------+-------
1. Average composition of fresh farm manure (Wolff).
2. Average composition of moderately rotted farm manure (Wolff).
3. Average composition of very thoroughly rotted farm manure (Wolff).
4. Mixed cow and horse manure from a bed two feet thick, accumulated during the winter in a large covered yard, and packed solid by the tramping of cattle (The analysis by F. E. Furry).
5. “Box Manure,” consisting of mixed manure of bullocks, horses, and pigs (Way, Royal Agric. Soc. Journ., 1850, II., 769).
[23] And soda.
It is thus seen that the percentage of the important plant-foods in stable manure are minute when compared with those commonly found in “commercial” fertilizers. Nor are they so much more available for plant absorption than the latter; a very large proportion is not utilized at all the first year, and unless the amount applied is very large it hardly carries the supply needed for the usual crops.
It is now well understood that its efficacy is largely due to the important physical effects it produces in the soil. It helps directly to render heavy clay soils more loose and readily tillable. If well “rotted” or cured it also serves to render sandy, leachy soils more retentive of moisture; and the humus formed in its progressive decay imparts to all soils the highly important qualities discussed later on (chapt. 8). More than this, the later researches have shown that stable manure acts perhaps most immediately upon the bacterial activity in the soil, greatly increasing it not only directly by the vast numbers of these organisms it brings with it, but also in supplying appropriate food for those normally existing in the soil (see chapt. 9). In so doing it serves indirectly to render the soil ingredients more available, and to impart to the soil the loose condition required in a good seed-bed—a “tilth” which cannot be brought about by the operations of tillage alone.
The only possible substitute for the use of stable manure is found in green-manuring with leguminous crops conjointly with the use of commercial or mineral fertilizers. Unless this is done the use of the latter, alone, ultimately leads to a depletion of humus substances, which renders the acquisition of proper tilth by the seed-bed impossible, and causes a compacting of the surface soil which no tillage can remedy.
Proper method of using stable manure in humid and arid climates.—In the humid region it is a common practice to spread the stable manure on the surface of the fields and leave it there without any special operation to put it into the soil; trusting to the rains, earthworms and subsequent tillage for its being brought into adequate contact with the roots; it is rarely plowed in. In the arid region this mode of using it is impracticable; it would remain on the surface indefinitely without advancing in its decay because of the dryness, and unless plowed in very deep the ordinary, strawy manure would ruin the seed-bed by rendering it too pervious to the dry air, thus preventing germination. Much of this valuable material has therefore been, and to some extent is still being burnt, thus causing a severe depletion of the land, both of humus and of mineral plant-food. The best way to deal with stable manure in the arid regions is to thoroughly rot or cure it before putting it on the land, and then plowing it in. To do this of course it must be put in piles and wetted regularly; a procedure which at the high prices of labor is thought to be too expensive, but which in the end would be found eminently profitable, unless green-manuring is regularly done. The very small proportion of humus generally present in arid soils renders this precaution indispensable, if production and proper tilth is to be maintained. The saving of stable manure and of all composting material, even if less needful as a means of supplying plant-food in the rich soils of the arid regions, is fully as essential in order to maintain the humus supply.
(B.) MINERALS UNESSENTIAL OR INJURIOUS TO SOILS.
The minerals heretofore mentioned contribute to soil formation either one or several ingredients, important to plant growth either by their mechanical or chemical action. It remains to consider some not intrinsically desirable, but frequently present in certain soils, which should be known to the farmer in order that he may be enabled to counteract or remove their injurious effects. Leaving aside such as are of only casual or rare occurrence, the following may be mentioned as among those which not unfrequently affect soils desirable for culture to such extent as to make them unavailable for general farming purposes:
Iron Pyrite; sulphid of iron containing two molecules of sulphur to one of iron; a mineral exceedingly common in deposits of metallic ores, and whose deceptive gold-like color has caused it to be mistaken for gold so often as to cause it to be designated as “fool’s gold” among miners. While it frequently does contain some gold and is often associated with valuable ores, it is practically valueless when occurring outside of mineral veins, in rock-masses; and more especially in sedimentary rocks, such as sandstones, limestones, shales and clays.
When present in soils it sometimes becomes a source of trouble to the farmer, because in contact with air it is soon transformed into ferrous sulfate or copperas, which, like the carbonate referred to above, is injurious to plants. Sometimes indeed iron pyrite is actually formed in badly-drained soils alongside of the carbonate of iron, when much sulfate (such as gypsum) is present; and then its injurious effects subside more slowly than do those of the carbonate (see above, p. 46).
Recognition of Iron pyrite.—The mineral is easily recognized by its golden or brass-yellow tint; the latter color being the one most commonly shown in the “sulphur balls” occurring in marls or soft limestones. A very easy test is to pulverize it and then heat it on a shovel over a fire, when it will soon itself take fire, burning with a blue sulphur flame, and upon more complete roasting, leaving behind a red powder, viz., “Venetian red” or red ochre; that is, ferric oxid. In clays it commonly occurs in large, well-defined cubes, which do not readily form copperas but rather become covered with a crust of limonite or brown iron ore.
When a subsoil is found to contain pyrite, or when “sulfur balls” have been accidentally introduced with dressings of marl, the remedy is thorough and persistent aeration of the material. In the case of marls nothing more need be done; but in that of ill-drained subsoils it is best to add lime in moderate dressings, to accelerate the transformation into ferric hydrate or iron rust, and gypsum; whereby the copperas becomes not only innocuous but adds two beneficial ingredients to the soil. The same policy will render available manure or other materials which have been disinfected by means of solution of copperas.
Halite (rock-salt), or common salt, has already been mentioned as to its occurrence in connection with the Stassfurt potash salts (see above, page 71); but as rock-salt it rarely exerts any injurious influence upon lands. It is, however, a common ingredient of seashore lands, and is also present to a certain extent in the alkali lands of the arid countries. While it is true that occasionally small quantities of common salt are used as an ingredient in fertilization, its usefulness in that direction is exceedingly subordinate; and it is far more generally to be considered as an injurious ingredient of all cultivatable soils whenever present to a larger extent than a few hundredths of one per cent It is usually considered that one-fourth of one per cent of common salt renders lands unfit for most culture plants. Only a few, such as asparagus, the beet, the saltbushes and some others, succeed when it is present in this or in larger amounts. In the case of sea water it is usually accompanied by a still more injurious ingredient, magnesic chlorid or bittern; which is detrimental to plant growth in much smaller quantities than the common salt itself.
Recognition of Common Salt.—The presence of common salt may, as a rule, be detected by the taste, well-known to every one; when this taste is very intense or somewhat bitterish, it indicates the presence of bittern. The presence of salt, however, is easily verified without the use of chemical reagents, by slowly evaporating some of the clear water leached from the soil in a clean silver spoon. If the last few drops are allowed to evaporate spontaneously, it will be easy to distinguish, even with the unaided eye, the square, cubical crystals, sometimes combined into cross-shape, which are characteristic of common salt. It is always an unwelcome addition to the land, and as its action cannot be neutralized in any way, it can be gotten rid of only by leaching-out. This process is usually accomplished in seashore lands by the action of rain, or by the overflow of fresh-water streams, after the tide has been excluded by means of drains provided with check-valves to prevent the inflow of tidewater; or else by underdrainage, and flooding when possible.
Mirabilite, (Glauber’s salt) or sulfate of soda, exists not unfrequently in the soils of the arid region and sometimes encrusts extended areas of lowlands during the dry season. When present in the soil it will commonly be seen blooming out on the surface after a rain, in light, feathery, needle-shaped crystals, sometimes to such an extent that it can be collected by the handful. Subsequently, when wafted by the wind, it is reduced to a fine white dust, which constitutes a goodly proportion and sometimes the entire mass of the “alkali dust” that is so annoying on the plains of Nevada, and in the desert regions generally, during the hot summer. Near White Plains, Nevada, it forms a thick layer of “white sand,” in which the foot sinks deeply, and which is carried about by the wind with great ease.
Glauber’s salt is never a desirable soil-ingredient. It is largely produced as a by-product in several industries, but cannot be utilized for agricultural purposes to any extent. It is, however, much less injurious to plant growth than common salt; according to experience in California it may be considered about three times less so. It constitutes the major portion of what is commonly known as “white alkali,” which is well known to be much less injurious to crops than the “black” kind, which contains carbonate of soda.
Trona and Urao are natural forms of carbonate of soda or salsoda. Like Glauber’s salt, it commonly occurs as a surface efflorescence or crust in dry or desert regions; either from the evaporation of standing water, as in the case of the soda lakes of Nevada, Hungary and Egypt, or as an efflorescence on the surface of the soil, as in the western United States, Mexico (“urao”), North Africa (“trona”), and at many points in the Old Continent. In the United States it is commonly known as “black alkali,” because of the black spots formed on the surface by evaporation; practically the same name (“kara”) is given it in Arabia and Asia Minor, whence impure soda has long been imported into Europe; while in north India it forms part of the “reh” salts that incrust large areas (usar lands) in the Indo-Gangetic plain.
The natural mineral always contains an excess of carbonic acid over the “normal” salt, nearly in the proportion of four parts of carbonic dioxid to three of soda; it is sometimes designated as sesqui-carbonate. In hot sunshine it may lose most of this excess for a time; while within the soil itself it may, in presence of abundant carbonic acid, become temporarily converted wholly into hydrocarbonate or “bicarbonate,” which is less corrosive than the monocarbonate or common salsoda.
Injury caused in soils.—Like common and Glauber’s salt, carbonate of soda is always an unwelcome soil ingredient; more so, in fact, than either of the other two, since less than a tenth of one per cent is sufficient to render certain soils wholly untillable, by the deflocculation or puddling of the clay; at the same time rendering it impervious to water. It is by far the most injurious ingredient that ordinarily occurs in otherwise good, arable soils; for in addition to the physical effect just mentioned, it dissolves the humus-substance of the soil, forming an inky-black solution which, especially when evaporating on the surface and forming black spots, has given rise to the popular name of “black alkali.” As will be more fully explained hereafter, wherever such is the case, the first step necessary toward reclamation is the transformation of the carbonate of soda, at least in part, into the relatively innocuous sulfate, by means of gypsum in the presence of water; while carbonate of lime remains in the soil.
In its direct action on the plants themselves, soda is also most injurious; as when accumulated to any extent near the surface by evaporation it will corrode the root-crown or stem, and sometimes completely girdle the same, destroying the bark. Farther details on this subject are given in chapter 22.
Epsomite, or Epsom salt, or sulfate of magnesia, is another one of the water-soluble minerals frequently found efflorescent on the surface of the ground; more commonly in saline seashore lands than in the alkali region proper, although it is rather common in the northeastern portion of the arid region of the United States. Whether on the soil surface or in the crevices of rocks, its needle-shaped, feathery crystals greatly resemble those of Glauber’s salt, but are readily distinguished by the more intensely bitter taste. Epsom salt is frequently the last remnant of sea-salts left in the soil after reclamation. Though probably somewhat more injurious to plant growth than Glauber’s salt, the mineral Kieserite, one of the Stassfurt salts and consisting essentially of Epsom salt, is sometimes used as an application to calcareous lands instead of gypsum, and with good results. Yet gypsum is usually the safer, and equally effective.
Borax (bi-borate of soda) occurs much more rarely than the salts just described; most frequently in certain portions of California, forming part of the “alkali” in the soil. It is injurious to plant growth, but is as readily dealt with as is the carbonate of soda, by dressings of gypsum, whereby inert borate of lime is produced.
It is hardly necessary to say that saline waters containing any of the above salts in notable amounts must be used for irrigation very cautiously. The measures to be observed in this respect will be discussed later.
PART SECOND.
PHYSICS OF SOILS.
VI. PHYSICAL COMPOSITION OF THE SOILS
As has already been stated (chapt. 1, p. 10), the general physical constituents of soils are rock powder or sand and silt, more or less decomposed according to the nature of the original rocks; clay, the product of the decomposition of feldspars and some other silicates; humus, the complex product of the decomposition of vegetable and animal matters on and in the soil mass; as well as vegetable matter not yet humified. Each of these several constituents must now be considered more in detail. Since clay is the substance whose functions and quantitative proportions influence most strikingly the agricultural qualities of land, it should be first discussed.
Clay as a Soil Ingredient.
The plasticity and adhesiveness of clay, together with the extreme fineness of its ultimate particles (said to reach the 1-25000 of an inch), explains its great importance as a physical soil ingredient. It serves to hold together and impart stability to the flocculent aggregates of soil particles that compose a well-tilled soil; for without clay the sand would collapse into close-packed single grains so soon as dried, and loose tilth would be impossible. Sand drifts illustrate this condition.
On the other hand, the fineness of the particles serves to render clay very retentive of moisture as well as of gases and of solids dissolved in water, imparting these important properties to soils containing it; while coarse sandy soils are oftentimes so deficient in them as to render them unadapted to any useful culture, despite the presence of an adequate supply of plant-food.
When to these essential physical properties of clay, there is added the fact that usually the clay-substance as it exists in soils contains the most finely pulverized and most highly decomposed portions of the other soil-minerals, and therefore the main part of the available mineral plant-food, it is easy to understand why soils containing a good supply of clay should be called and considered “strong” land by the farmers of all countries. “Poor” clay soils are exceptional; but sometimes the clay content reaches such a figure that the difficulties of tillage render them too uncertain of production for profitable occupation.
Amount of Colloidal Clay in Soils.—Any and all of the kinds of clay mentioned (p. 57) as occurring naturally may, of course, enter into and form part of soils. But as the amount of true, plastic clay substance contained in them is very indefinite, it becomes necessary, in order to classify soils in respect to their tillableness, to ascertain more definitely the amount of pure, or nearly pure, colloidal clay substance contained in the several classes of soils ordinarily recognized and mentioned in farming practice. That this determination can at best be only approximate, is obvious from the fact mentioned above (chapt. 4, p. 59), that pure kaolinite itself is not plastic, and only becomes so by the indefinite comminution and hydration it experiences in the processes of soil-formation. As the progress of this process is also indefinite, the same soil containing particles ranging from the finest to the chalky scales of pure kaolinite, the drawing of a line must be more or less arbitrary and empirical.
From numerous experiments and comparisons made, the writer has been led to place the limits of “plastic clay” at and below such grain sizes as will remain suspended (afloat) in a water column eight inches high, during 24 hours. To go beyond this point in the examination of soils for practical purposes, would render such examinations so laborious and hence so rare, that this kind of work would be practically excluded from ordinary practice. According to this view the following percentages of such “clay” correspond approximately to the designations placed opposite:
Very sandy soils .5 to 3% clay Ordinary sandy lands 3.0 to 10% “ Sandy loams 10.0 to 15% “ Clay loams 15.0 to 25% “ Clay soils 25.0 to 35% “ Heavy clay soils 35.0 to 45% and over.
It must be distinctly understood, however, that these figures make no claim to accuracy or invariability. For, the tilling qualities of a soil containing one and the same amount of such “clay” may be very materially modified according to the kind and amount of each of the several grain-sizes of rock powder or sand they contain.
Influence of fine powders on plasticity and adhesiveness.—An admixture of a large amount of fine powders diminishes materially the adhesiveness of a clay soil, even though it may render it even more “heavy” in tillage; while the admixture of coarse sand, even in very considerable proportions, does not greatly influence the adhesiveness of the clay. The latter alone cannot therefore serve as a proper guide or basis for the classification of soils in respect to tillage; we must also take into consideration the nature and amount of the several granular sediments mixed with it.
Moreover, the nature and especially the adhesiveness of the clay substance as obtained by analysis may vary considerably in the presence of a very large amount of the finest grain-sizes; among which ferric hydrate or iron rust is especially apt to accumulate predominantly in the clay, considerably increasing its apparent weight and greatly diminishing its adhesiveness.[24] In strongly ferruginous soils, therefore, it becomes necessary to take into special consideration the amount of the ferric hydrate or rust which accumulates in the clay substance. The presence of large amounts of humus or vegetable mold also influences materially the adhesiveness and physical properties of the clay obtained by the method described, although most of it remains with the finer powdery sediments or grain-sizes. There are, besides, other colloidal or at least amorphous substances present in all soils, such as silicic, aluminic and zeolitic hydrates, which are all non-plastic, and yet sufficiently fine to form part of the “clay” obtained as above specified.
[24] This fact emphasizes the impossibility of explaining the plasticity and adhesiveness of clay simply as a function of fineness of grain.
Despite these imperfections, (which however can in a measure be taken into consideration in judging of a soil’s tilling qualities by its clay content), the figures given in the above table approximate much more nearly to a tangible basis for such estimate, than the utterly indefinite mixtures which under the older methods of analysis have been, and still are to some extent, used as a basis for soil classification by writers on agriculture.
Rock Powder; Sand, Silt and Dust.
The powdery (sandy and silty) constituents of soils usually constitute the greater part of their mass; and the proportions present of the several grades of fineness exert a most decisive influence upon their cultural qualities, and very commonly upon their agricultural value also. It is needless to add that the kind of mineral of which they consist or from which they were formed, is also of great importance in determining the quality of soils from the standpoint of the chemist, with respect to their content of mineral plant-food.
WEATHERING IN HUMID AND ARID REGIONS.
Sands of the Humid Regions.—As has already been stated, “sand” is usually understood to be, in the main, quartz more or less finely pulverized, generally intermingled with a few grains of other minerals. With this understanding, since quartz is practically inert with respect to plant nutrition, it follows that soils consisting mainly of this substance contain but little plant-food; hence the common expression “poor, sandy land,” the outcome of the experience had in Europe and in the Eastern United States, and which until recently has been held to be of general application. The “sands of the desert” have, both in ordinary life and in poetry, always stood as the symbol of sterility.
Thus the sandy lands (“sand hammocks”) of Florida, the (long-leaf) pine lands of the Gulf States, the “pine barrens” of New Jersey and of Michigan, are noted both for their sandy soils and their sterility after brief cultivation; necessitating fertilization within a few years from the time of occupation. In Europe, the “Heide” (heather) soils of northeastern Germany are of the same cultural character.
Sands of the Arid Regions.—The experience of arid countries however, has long ago shown that some very sandy lands—e. g., such as form the oases of the north African deserts—may be extremely productive when irrigated, and also of considerable durability. Actual experience and close investigation given this subject in the arid regions of the United States has fully demonstrated that lands appearing to the casual observer to be hopelessly sterile sandy deserts, very commonly prove to be even more productive than the more clayey lands of the same regions. Examination of the sand shows, in these cases, that instead of mere grains of quartz, the minerals of the parent rock, partially decomposed, themselves constitute a large proportion of the sandy mass. But in the regions of deficient rainfall, as has already been stated, (p. 47) the formation of clay (kaolinization) is exceedingly slow; hence the decomposition of the rock powder results in the production of predominantly pulverulent instead of clayey soils. But the mineral plant-food is not on that account less available, provided other physical conditions necessary for the success of plant growth are fulfilled. Among these moisture stands foremost; hence the relative proportions of the several grain-sizes are of vital importance, since upon this depends to a great extent the proper supply and distribution of moisture, without which no amount of plant-food will avail. Moreover, the finest and most highly decomposed powder is the portion from which the roots draw their chief food-supplies.
The point last mentioned is well shown in the results obtained by Dr. R. H. Loughridge, from the analysis of each of the several grain-sizes into which he had resolved a very generalized soil of the State of Mississippi, representing a very large land area in that State as well as in Tennessee and Louisiana. The details of this investigation are given farther on; but summarily it may be stated that he found practically the whole of the acid-soluble mineral plant-food accumulated within the portion of the soil the fineness of whose grains was below .025 millimeters (one-thousandth of an inch); ingredients so fine as to be wholly impalpable between the fingers. Moreover, two-thirds of the total amount was found in the portion described above as “clay.” It is thus readily understood why clay soils are in the regions of summer rains commonly designated as “strong” lands.
The corresponding later investigations of Rudzinski (Ann. Agr. Inst. Moscow, Vol. 9, No. 2, pp. 172-234; Exp. Sta. Record, Dec. 1904, p. 245) and of Mazurenko (Jour. Exp. Landw. 1904, pp. 73-75; Exp’t Stn. Record, Dec. 1904, p. 344) fully corroborate Loughridge’s conclusions, for typical soils of European Russia.
In the arid or irrigation regions, however, the case is different, for the reason that much of the decomposed rock-substance remains adherent to the surface of the larger grains, and plastic clay is formed to a much less extent. Much available plant-food may therefore, in arid lands, be present even in rather coarsely sandy soils almost devoid of clay; such as in humid climates would be likely to be found wholly barren. (See chapt. 19).
PHYSICAL ANALYSIS OF SOILS.
Use of Sieves.—Down to a certain point the separation of the soil into its several grain-sizes may be accomplished by means of sieves. We may thus separate coarse gravel from fine gravel and from sand; and the latter may itself be separated into several sizes by the same means. This presupposes, of course, that the soil has been previously prepared for the purpose by crushing the lumps consisting of aggregates of finer particles, that in the operation of tillage would again be resolved into their fine constituents, or be penetrated by roots. But this preparation of the soil for sifting must not be carried beyond the point mentioned, for a grain consisting of particles somewhat firmly cemented together will under ordinary conditions play in the soil precisely the same part as a solid sand-grain, and must not therefore be broken up, if the soil is to be examined in its natural condition. The pressure of the fingers or of a rubber pestle is as far as trituration should go. The disintegration of these compound particles by means of acids, as prescribed and practiced by the French soil chemists, may wholly change the physical nature of the soil by the breaking-up of mechanical aggregations which in the usual course of tillage would remain intact. This is especially true of strongly calcareous soils, and particularly those containing calcareous sand.
The sieves used for this purpose should not be ordinary wire sieves, but should have bottoms of sheet brass perforated by round holes of the various diameters desired, of fractions of inches, or preferably of millimeters. For the finer grain sizes, silk bolting cloth is used by the U. S. Bureau of Soils.
In the sifting process it will be found that so soon as the finer grain-sizes of the sand are approached, the sieve fails to act satisfactorily; the more so, the more clay was originally contained in the material. The fine particles flock together, forming little pellets, which refuse to be separated by the sieve. This difficulty can, of course, be partly overcome by previously separating the clay from the sand by means of water, as detailed above; but even then it will be found that so soon as the grain-sizes fall much below ¹/₅₀ of an inch (½ millimeter) the same difficulty is experienced, so long as the sand is dry. By playing a small stream of water upon the sieve, however, all the particles beyond the ¹/₅₀₀ of an inch may be successfully separated from the coarser portion; and for many practical purposes the separation need be carried no farther.
Use of Water for Separating Finest Grain-Sizes.—The scientific investigator, however, must of necessity proceed to separate the finer grain-sizes from each other, since, as will presently be shown, they influence the tilling qualities of the soil to a much greater degree than do the coarser particles. Such farther separation can be accomplished only by the aid of water.
Subsidence Method.—When a small amount of soil is stirred up in water, and is afterward allowed to stand for some time, the different grain-sizes will settle consecutively in accordance with their sizes (or weights); the smallest ones settling latest, and the clay only remaining suspended, as stated above. So long, however, as any considerable amount remains suspended in the water, the latter is not only denser but especially more viscid than if the clay were absent. In order therefore to obtain correct results by any method involving the use of water, it is necessary to remove the clay before proceeding to the separation of the granular sediments. This, as has been already stated, is approximately accomplished by allowing the soil, when diffused in water after proper disintegration, to settle for 24 hours from a column of water 200 mm. high, whereby all grain-sizes, of and above .01 mm. diameter are removed from the turbid liquid. This sedimentation is then repeated until after 24 hours the water becomes clear. The clay is then determined in the “clay water” by evaporation or precipitation; the granular sediments may then be successfully separated by sedimentation.
The U. S. Bureau of Soils uses for the separation of clay, instead of subsidence for 24 hours, the more expeditious process of centrifuging the turbid soil water in appropriate glass cylinders, by the aid of an electric motor; and thus in a relatively short time obtains “clay” in which the upper limit of size is one-half of that mentioned above, viz., .005 mm. But for the costliness of the appliances required, including the entire time of an operator, this method of separating the clay would undoubtedly be preferable to the elimination by subsidence; the more as a more minute grain-size for the clay group is thus secured.
The separation of the clay having been accomplished, the various sizes of silt and sand may be separated by again suspending them in water; and interrupting the settling process at stated times, the grain-sizes corresponding to definite velocities in settling may be segregated and weighed. When this process of settling and decanting is carefully and repeatedly carried out, very good results are obtained.
Hydraulic Elutriation.—The sedimentation (or “beaker”) method, long practiced in the arts is, however, quite tedious, requiring the constant close attention of a skilled observer. The desired results may, in the writer’s judgment, be more conveniently obtained by the hydraulic method, whenever no very large volume of work of this kind is required to be done at once.
When instead of allowing the soil to settle in quiet water, the latter is used as an ascending current of regularly graded velocities, it is clear that the soil particles will be carried off by this current in exact conformity with their several sizes (or strictly speaking, volume-weights); and when maintained in such a current for a sufficient length of time, the entire quantity of the sediment corresponding to the prevailing velocity will be carried away. It is of course easy to ascertain to what grain-sizes certain velocities of the upward current (regulated by a stopcock with arm moving on a graduated scale) correspond, and to regulate accordingly the intervals between the different velocities to greater or less detail, as may be desired. A number of instruments have been devised for this purpose.
Schöne’s Elutriator is the one commonly used in Europe; in it the upward current ascends in a conical glass tube, (see figure 6) entering through a narrow, curved inlet tube, in which the soil sample is kept agitated by the current itself. The objection to this plan is twofold: first, the narrow, curved inlet-tube is readily clogged by the soil mass at the lower velocities, which are thereby changed, so that, unless a very small amount of soil only is employed, the whole mass is not kept properly stirred; second, the circulating currents brought about by the conical shape of the tube cause the sediment-particles to coalesce into complex, larger ones (floccules), which will then settle down and fail to pass over at the current-velocity corresponding to their individual component parts.
Churn Elutriator with Cylindrical Tube.—The errors just alluded to are obviated by an arrangement devised by the writer, in which a rapidly revolving stirrer, placed at the base of a cylindrical tube in which the washing process is conducted and which eliminates counter-currents, continually disintegrates these compound particles, and thus enables the entire quantity of the sediment corresponding to the prevailing current-velocity to pass off with a comparatively slight expenditure of time on the part of the operator (see figure 7). A wire screen interposed between the churn and cylindrical glass tube prevents communication of the whirling motion to the column. As the apparatus works automatically, the analyst has only to observe from time to time whether or not the turbidity near the top of the tube has disappeared; and as the sediment accumulates at the bottom of the tall receiver bottle,[25] no harm is done if the attendant should neglect to change the velocity in time, except that water will run to waste.
[25] The figure given of this elutriator in Bulletin No. 24, on physical soil analysis, published by the U. S. Bureau of Soils, shows as the receiver a bottle entirely too low to insure the complete retention of the sediments by settling. The receiving bottle should not be less than twelve inches high and five inches wide.
The conical relay glass below the churn serves to retain the coarser grades of sediments which are not concerned in the velocities employed in the elutriator tube, and thus prevents injurious attrition. But these sediments can at any time be stirred up by the incoming current and brought into the washing tube if desired. In the same manner the passing-off of the finer sediments can be materially accelerated by running off rapidly about two-thirds of the turbid column of water every twenty minutes.
It should be fully understood that prior to attempting such separation, the “colloidal clay” must first be removed by the subsidence or centrifugal method, since otherwise much larger grain-sizes may be carried off at a given velocity.
Yoder’s Centrifugal Elutriator.—A very ingenious instrument which combines the elutriation and sedimentation processes into one, has been devised by P. A. Yoder, of the Utah Expt. Station. The elutriator bottle is placed in a centrifuge driven by an electric motor; it is closed by a glass stopper carrying a delivery tube to a short distance above the bottom of the elutriator bottle, as well as an outflow tube ending at the base of the stopper; the latter also carries a funnel coinciding with the center of rotation. Into this funnel flows gradually the muddy water containing the soil in suspension; and the rate of its flow, together with the velocity of rotation, determines the size of the sediment-granules that will be deposited in the slack-water below the mouth of the delivery tube. The muddy soil-water is kept agitated in a funnel-shaped reservoir by air-bubbles from a constant-pressure chamber.
While the principle of this instrument is good, it is quite complicated and the results obtainable from it in practice have not as yet been made public. The inventor claims that an analysis may by its means be completed in less than three hours.
In all hydraulic elutriators a provision for constant pressure in the reservoir supplying the current of water is needed; although in Schöne’s and some other instruments a gradually decreasing pressure in a plain reservoir is employed. A large glass bottle or carboy fitted with the proper tubes so as to constitute a Mariotte’s bottle (in which the air enters near the bottom of the vessel), is a very convenient arrangement.
Number of Sediments.—The number of grain-sizes or sediments into which the soil mass is to be segregated is of course entirely within the option of the operator. Experience has shown that it is unnecessary to discriminate very closely between the several sizes of the coarser portion of the sand, such as those lying between one-fourth and one-half of a millimeter. But below this point, and especially between one-tenth of a millimeter and the clay, a proper discrimination becomes very important. The series first devised by the writer in 1872 is based upon a consecutive doubling of the velocities of the current from a quarter of a millimeter per second to thirty-two millimeters per second; the sediment of sixty-four millimeter-velocity corresponding to a diameter of one-half of a millimeter, will remain in the elutriator. Above this, as before remarked, the sieve (especially when aided by a jet of water) effects a satisfactory segregation.
The table below shows the elements of these series both as regards current-velocities and maximum quartz-grain diameters carried off by each. In a great many cases, however, it is altogether unnecessary to go into such detail, and a subdivision into six or seven divisions is quite sufficient. Such a subdivision, based upon the doubling of grain-sizes instead of current-velocities, has been adopted by Prof. Milton Whitney, of the U. S. Department of Agriculture, and others.
TABLE OF DIAMETERS AND HYDRAULIC VALUES OF SEDIMENTS.
=============================================================== |Velocity per second,|Maximum diameter Designation of materials.| or hydraulic | of quartz | value. | grains. -------------------------+--------------------+---------------- | Mm. | Mm. | | Grit | (?) | 1-3 { | (?) | .5-1 { | 32-64 | .50 Sand { | 16-32 | .30 { | 8-16 | .16 { | 4- 8 | .12 { | 2- 4 | .072 { | 1.0- 2 | .047 Silt { | .5- 1 | .036 { | .25-0.5 | .025 { | 0.25 | .016 { | < 0.25 | .010 | | Clay | < 0.0023 | -------------------------+--------------------+----------------
Results of such analyses.—A tabular presentation of the results of analyses made in accordance with the above plan will give a good idea of the differences between the various grades of soils recognized in farm practice, to any one accustomed to the study of figures. But a much more satisfactory showing is made by placing the several grain-sizes segregated, into small vials or tubes of identical diameter and placing them in parallel series alongside of each other.[26] The curves formed by the surfaces of the several sediment-columns in each series show to the eye very strikingly the relations of the several grades of soils to each other, and suggest at once that while gentle slopes or gently undulating curves belong to soils of intermediate, loamy character, steep grades and zigzags show soils of extreme types. This is exemplified in the subjoined Figures:
[26] Convenient stands for this purpose, used by the writer since 1872, may be cut from L-shaped moldings of wood, such as can be readily ordered from any planing mill. The vials can be cemented, wired or tied.
Physical composition corresponding to popular designations of Soil quality.—The subjoined table illustrates the physical composition of a number of soils from the State of Mississippi, selected for their representative character, in order to deduce therefrom approximate definitions of physical character corresponding to popular designations. This table, published in 1873 in accordance with results obtained during the two preceding years, does not require any material modification on account of subsequent investigations. It lacks, however, a characteristic representative of the predominant soils of the arid region, viz., the silty soils so prevalent in dry climates, only approximately represented by No. 165 of the table; hence two such, from California, exemplifying respectively the valley deposits of the Sacramento and Colorado rivers, have been added to the list.
It must not, however, be understood that these typical soils necessarily represent correctly the physical constitution of all soils falling under the same popular designation; for we are far from being able as yet to predict accurately in every case the tilling qualities of a soil material from its physical composition. To do this it would be necessary not only to know with some degree of precision the several physical coefficients of each of the several grain-sizes, and perhaps of many more intermediate ones; but we would also have to construct a formula according to which each could be given its proper weight when present in varying proportions, and of varying shapes, surface condition, and material. For this our present knowledge is wholly inadequate, if indeed the problem is not beyond the limits of mathematical computation. We must for the present at least be satisfied with the empirical approximations afforded us by the constantly increasing number of such analyses, correlated with farming experience.
Since the finest grain-sizes above those classed as “clay” do not tend to “lighten” soils, but even to render them more intractable (“putty soils”), while coarser ones gradually change the dense clay-texture into the “loamy,” it is clear that in between there must be a neutral point, some grain sizes which by themselves do not influence soil texture either way. Discussion of numerous physical analyses, and some direct experiments, have led the writer to conclude that this theoretically neutral grain-size lies at or near the diameter of .025 mm., or .5 mm. hydraulic value. In correlating the results of analysis with the tilling qualities of the soil as to “heaviness and lightness,” therefore, that grain-size may usually be left out of consideration.
PHYSICAL ANALYSES OF SOILS AND SUBSOILS.
(A) = 238 White Pipe Clay. Tishomingo Co. (Z) = Hygroscopic Moisture (+7 = to +21°C) ===========+==============+================+======= | Diameter | Velocity | |(Millimeters) | (Hydr. V.) | Designation| | | of +--------------+----------------+------- Materials. | | | | | Millimeters | (A) | | per sec. | -----------+--------------+----------------+------- Grit | 1-3 | | | | | { | .5-1 | | { | .50 | 64 |} Sand { | .30 | 32 |} 0.06 { | .16 | 16 |} { | .12 | 8 | 0.08 | | | { | .072 | 4 | 0.02 { | .047 | 2 | 0.04 Silt { | .036 | 1 | 0.08 { | .025 | 0.5 | 0.08 { | .016 | 0.25 | 2.00 { | .010 | <0.25 | 21.15 | | | Clay | ? | <0.0023 | 74.65 +------- | 98.16 | (Z) | 9.09 Ferric Oxid | 0.13 -------------------------------------------+-------
(B) = 248 Tallahoma Subsoil. Jasper Co. (C) = 165 Flatwoods Soil. Chickasaw Co. (D) = 206 Pine Hill Soil. Smith Co. (E) = 209 Pine Hill Subsoil. Smith Co. (F) = 397 Oxford Subsoil. Lafayette Co. (G) = 219 Table Lands Subsoil. Benton Co. (H) = 173 Prairie Subsoil. Monroe Co. (I) = 230 H’y Flatwoods Soil. Pontotoc Co. (J) = 246 Red Hills Subsoil. Attala Co. (K) = 196 Hog-wallow Subsoil. Jasper Co. (Z) = Hygroscopic Moisture (+7 = to +21°C) ==========+=============================================================== | MISSISSIPPI UPLANDS. +------------------+------------------+------------------------- Designation| Sandy. | Loam. | Clay. of +-----+------+-----+-----+-----+------+-----+------+-----+------ Materials.| | | | | | | | | | | (B) | (C) | (D) | (E) | (F) | (G) | (H) | (I) | (J) | (K) | | | | | | | | | | ----------+-----+------+-----+-----+-----+------+-----+------+-----+------ Grit | 6.94| 2.90 | 0.36| 0.36| |} |} | 0.33 |} | 0.83 | | | | | |} 0.23|}2.10| |}1.97| {|17.65| 6.96 | 2.98| 0.83| |} |} | 0.35 |} | 1.19 {|18.81| 2.81 | 6.62| 6.21|} | 1.47|} | | 0.72| 1.96 Sand {|10.16| 4.41 | 7.75| 3.38|}0.79| 2.33|}0.62| | 2.32| 1.64 {| 2.66| 3.13 | 3.01| 3.85|} | 1.17|} | | 2.09| 0.88 {| 1.66| 2.02 | 1.59| 1.49| 0.18| 0.78| 0.20| 0.23 | 0.70| 0.26 | | | | | | | | | | {| 1.02| 2.23 | 1.19| 0.64| 0.78| 0.76| 1.26| 0.18 | 1.29| 0.19 {| 0.88| 5.06 | 3.56| 2.63| 3.56| 9.79| 2.92| 1.61 | 1.80| 2.49 Silt {| 1.96| 9.67 | 6.50| 5.40|13.12| 7.26| 7.36| 2.66 | 3.60| 3.67 {| 7.89|14.18 |13.97| 7.77|16.64| 13.14| 8.81| 9.13 | 2.73| 5.39 {| 8.40|22.03 |14.20|16.65|27.28| 15.07| 7.85|26.64 | 3.30| 10.31 {|15.53|15.62 |29.36|37.75|18.87| 26.50|35.22|32.35 |25.33| 24.18 | | | | | | | | | | Clay | 8.63| 7.86 | 4.58|10.70|17.23| 19.19|33.16|25.48 |40.25| 47.03 +-----+------+-----+-----+-----+------+-----+------+-----+------ |99.28|98.68 |95.67|97.77|98.35| 97.65|99.50|97.87 |96.11|100.00 | | | | | | | | | | (Z) | 1.80| 3.36 | 2.48| 7.69| 8.79| 7.24|11.35| 9.33 |18.60| 14.48 Ferric Oxid| 1.10|(1.45)| 1.25| 4.45| 2.53| 5.11| 5.42|(5.90)|10.50| 4.00 ----------+-----+------+-----+-----+-----+------+-----+------+-----+------
(L) = 390 Buckshot Soil. Issaquena Co. (M) = 237 Loess. Claiborne Co. (N) = 365 Tallahatchie All. Soil. Panola Co. (O) = 377 Frontland Subsoil. Sunflower Co. (P) = 395 Dogw. Ridge Soil. Coahoma Co (Q) = Southwest Pass. Plaquemine Par. (R) = Southwest Mud-lump. Plaquemine Par. La. (Z) = Hygroscopic Moisture (+7 = to +21°C) ==========+======================================================= | MISSISSIPPI RIVER BOTTOM. +-------+-------+--------------------------------------- Designation| Swamp.| River.| River Deposit. Delta of +-------+-------+-------+-------+-------+-------+------- Materials.| | | | | | | | (L) | (M) | (N) | (O) | (P) | (Q) | (R) | | | | | | | ----------+-------+-------+-------+-------+-------+-------+------- Grit | 0.09 |} |} | | | | | |} 0.24 |} 0.09 | | | | {| 0.05 |} |} | | | | {| | 0.37 | 0.04 |} 0.32 | 0.15 |} 0.18 |} 0.10 Sand {| 0.36 | 0.61 | 0.05 |} | |} |} {| | 0.93 | 0.21 | 2.79 | | 0.47 | 5.02 {| 0.31 | 1.65 | 1.30 | 2.41 | 3.75 | 7.03 | 3.68 | | | | | | | {| 0.27 | 1.95 | 2.68 | 16.90 | 21.46 | 12.38 | 5.34 {| 1.56 | 14.25 | 9.38 | 19.97 | 21.83 | 13.27 | 10.09 Silt {| 2.23 | 16.20 | 9.88 | 13.90 | 14.01 | 15.87 | 5.58 {| 3.68 | 20.08 | 20.37 | 4.27 | 9.93 | 8.25 | 9.54 {| 8.97 | 5.59 | 19.79 | 1.89 | 9.58 | 7.26 | 8.01 {| 38.19 | 33.38 | 25.30 | 30.08 | 8.65 | 19.67 | 34.46 | | | | | | | Clay | 44.30 | 2.51 | 9.64 | 5.51 | 10.35 | 12.20 | 18.18 +-------+-------+-------+-------+-------+-------+------ |100.01 | 97.74 | 98.73 | 98.04 | 99.72 | 96.58 |100.00 | | | | | | | (Z) | 14.31 | 4.18 | 6.12 | 5.68 | 3.95 | | Ferric Oxid|(5.82) | 3.27 | 2.58 | 2.31 | 2.69 | | ----------+-------+-------+-------+-------+-------+-------+-------
(S) = 10 Sacramento. Sacramento Co. (T) = 506 Gila. San Diego Co. (Z) = Hygroscopic Moisture (+7 = to +21°C) ===========+================== | CALIFORNIA. +------------------ Designation | River Deposit. of +---------+-------- Materials. | | | (S) | (T) | | -----------+---------+-------- Grit | | | | { | | { | | .13 Sand { | | .15 { | | .11 { | .32 | .75 | | { | 3.16 | 2.51 { | 10.27 | 8.32 Silt { | 13.67 | 12.64 { | 13.11 | 11.28 { | } 43.61 | } 31.79 { | } | } | | Clay | 12.06 | 23.97 +---------+-------- | 96.20 | 91.71 | | (Z) | 9.18 | 9.26 Ferric Oxid | | -----------+---------+--------
Number of soil grains per gram.—It is of some interest to consider the number of grains of different sizes that may be contained in, e. g., a gram of soil. If for this purpose we assume all the soil grains to be spherical, we shall obtain the minimum figures, for most other shapes will pack more closely. King (Physics of Agriculture, p. 117) calculates such figures for different grain-sizes, assuming the density to be that of quartz (2.65), with the result that while with a diameter of one millimeter (1-25 inch) the number of grains would be 720, and with one-tenth of a mm. 720,000; if made of the finest particles only, viz., one thousandth of a mm., the number would be 720,000 billions. Probably few of the clayey soils we ordinarily deal with are of this order; it is doubtless approached in certain fine plastic clays.
Surface afforded by various grain-sizes.—The amount of surface afforded by a similar amount of soil must naturally be considered in this connection, since upon it depends not only the amount of moisture which the soil may hold in the form of superficial films, but also the extent of surface upon which the weathering agencies as well as the root hairs of plants may act. Quoting again from King’s work, we find on the same premises given above for the number of grains, that their surface would in the case of grains of one mm. diameter be eleven square feet per pound (about half a pint) of material; while in the case of the finest grade we should have 110,538 square feet, or more than two and a half acres.
From actual experiments made with the flow of air through various soils, King calculates that while in ordinary loam soils the total surface is about an acre per cubic foot, in fine clay soils it rises to as much as four acres. If we imagine this large surface to be covered with even a very thin film of water, it is readily seen how large an amount may be present in a cubic foot of moist soil.
E. A. Mitscherlich (Bodenkunds für Land-und-Forstwirthe; Berlin, 1905) attributes to the surface offered by the soil particles supreme importance in determining the productiveness of soils. According to him the internal soil-surface determines directly the ease with which roots can penetrate the soil; and he proposes the determination of this factor by means of the heat produced in wetting the soil (“Benetzungswärme”), measured in a calorimeter, as a substitute for all methods of physical soil analysis, which are vitiated by the varying shapes and densities of the particles; while his method gives directly the actual surface. To the consumption of energy required by difficult penetration he attributes most of the differences in production, and hence refers to the internal soil-surface as governing nearly all the other physical factors. The introduction of many arbitrary assumptions, and the failure to show that the admitted inaccuracy of the ordinary mechanical soil analyses are of any practical importance, greatly detract from the cogency of the rigorous mathematical discussion carried through his work by Mitscherlich.
Influence of the several grain-sizes on soil texture.—Undoubtedly the most potent of all the sediments appearing in the above table in influencing soil texture, is the “clay.” That the materials included under this empirical designation may vary considerably in different soils, has already been sufficiently insisted on; and it is doubtful that in the present imperfect state of our knowledge of the functions of the several physical grain-sizes, we would be much wiser were we to go to the extreme advocated by Williams (Forsch. Agr. Phys., vol. 18, p. 225, ff.), of determining with precision the actual amount of such extremely fine clay particles as cease altogether to obey the law of gravity when once suspended in water. It is at least doubtful that the essential property of adhesive plasticity belongs only to these, for this property doubtless increases gradually as the size diminishes, although unquestionably not a mere function of the latter, since it belongs only to the hydrated silicate of alumina.
Ferric Hydrate.—Probably the body which most commonly modifies materially the adhesive and contractile properties of the clay substance, is ferric hydrate; the more as on account of its high density it tends to exaggerate materially, in many cases, the apparent content of true clay, and the estimate of the soil’s plasticity based upon it. A good example in point is the case of soil No. 246 (Miss.) of the above table. This is a heavy clay soil, yet not excessively adhesive; scarcely as much so as No. 230 (Miss.), the heavy gray “flatwoods” soil, and not nearly as “sticky” when wet as No. 173 (Miss.), the prairie subsoil, although containing apparently 15% more clay than the former, and 7% more than the latter. But No. 246 is a highly ferruginous clay, in which the ferric hydrate is in a very finely divided condition, and materially influences the physical qualities of the clay substance. Were it all accumulated in the “clay,” it would diminish the percentage of true clay by 11.75%, reducing the clay-percentage to 28.5% which accords more nearly with the soil’s only moderate adhesiveness, and not excessively heavy tillage.
But it must be remembered that the iron oxid shown in the analysis is not nearly always in this finely diffused condition. Frequently it incrusts the sand grains; quite commonly it forms small concretions of limonite, which themselves act as sand grains; and again, it may be present in the form of “black sand” or magnetic oxid, as is commonly the case in California and on the Pacific slope generally. To take this point properly into account, therefore, it would be necessary to determine the amount of ferric hydrate actually present in the “clay” as separated by subsidence of the granular constituents.
Other substances.—This circumstance as well as the inevitable presence of other modifying substances, clearly shows the desirability of being enabled to examine the physical properties of this “clay” directly, by collecting its entire amount as obtained in analysis, instead of merely determining it by weighing fractional portions. When this is done the analysis is much more valuable as indicating the true tilling qualities of the land. The increase of bulk suffered by this substance after wetting, is a very fair index of its content of true clay, and is preferable to the chemical analysis proposed by some investigators. For it is quite impossible to distinguish the silica and alumina derived from the kaolinitic substance proper, from that which is due to the decomposition of zeolites.
It is possible, however, to determine the possible maximum of the kaolinite ingredient by taking into consideration the quantitative ratio according to which silica and alumina combine to form it, viz., approximately 46% of the former to 40 of the latter, the rest being water. By using this calculation we can often demonstrate clearly the presence in the “clay” of considerable amounts (up to 33%) of aluminic hydrate; since no zeolitic mass can contain as much alumina as does kaolinite. Whether the aluminic hydrate be in the form of gibbsite, bauxite, diaspore,[27] or in the gelatinous state, the nature of the soils containing it proves that it is totally destitute of plasticity and adhesiveness; and this consideration will often serve to explain the fact that soils showing in their chemical analysis high percentages of alumina, nevertheless show quite low degrees of plasticity, adhesiveness and water absorption. What part it may take in modifying the physical properties of the soil we can thus far only conjecture.
[27] Bauxite is not only the most abundant of the three hydrates of alumina known to occur naturally, but also stands nearly midway between the two others in its water content, viz., a little over 25%; that of diaspore being nearly 15%, gibbsite about 35%.
Influence of the granular sediments upon the tilling qualities of Soils.—Considering the granular sediments by themselves, in the absence of clay, it may be stated in a general way that while in a moist condition they flocculate sufficiently to produce a fair tilth, they will nevertheless on drying collapse into a close arrangement resulting from the single-grain structure. The form of the grains being angular instead of rounded, they are apt to form a very closely packed mass far from suitable to vegetable growth; as will be seen by an example taken from one of the culture stations of the University of California, from a piece of land which on the surface would be called a very sandy loam, but after we descend increases in its content of fine grains until at a depth varying from eighteen inches to three feet we find what appears to be a hardpan, which is equally impervious to roots and water and causes the water to stagnate to such an extent that after heavy rains the land becomes so boggy as to render plowing almost impossible without endangering the team. A close examination of this hardpan shows that, unlike others, it is devoid of any cement, and when taken out can be readily crushed between the fingers, and softens in water, but does not become plastic. Its imperviousness is therefore due solely to the close packing of the sand grains, for it contains practically no plastic clay, and under the microscope the grains are seen to be angular-wedge-shaped and composed of the remnants of granite. The physical analysis shows the following result:
MECHANICAL ANALYSIS OF HARDPAN.
================================== Designation.|Diameter.|Percentage. ------------+---------+----------- { | .50 mm. | 10.93 Sand { | .30 “ | 21.23 { | .12 “ | 7.27 | | { | .072 “ | 9.63 { | .047 “ | 12.00 Silt { | .036 “ | 7.19 { | .025 “ | 1.25 { | .016 “ | 14.20 | | “Clay” | ? | 8.64 ------------+---------+-----------
It is doubtful whether this condition of things can be remedied by the usual measure of breaking up the hardpan either by hand or by means of giant-powder blasting. Experience seems to show that the effect is only temporary, and that in the course of time, by the action of the percolating waters, the particles settle back into their original impervious condition. It is just possible, however, that if once penetrated by roots, the intervention of these would permanently destroy the close structure, so as to make this a fair subsoil for the growth of trees and other plants. The writer is not aware that this kind of purely physical hardpan without cement has ever been observed elsewhere.
This physical condition is doubtless responsible for two other phenomena, viz., the “putty soils,” and also certain difficulties experienced in irrigation.
“Putty Soils” is the name popularly given in the Cotton States, and probably elsewhere, to soils usually occurring in low ground and also known as “cray-fishy.” They consist of very uniform, powdery sediment, with little or no coarse sand and still less of clay to render them coherent. When wet these soils behave precisely as would glazier’s putty, adhering to the surface of even the best-polished plowshare, so that no furrow-slice can be turned and the plow is soon dragged out of the ground. At a very closely limited condition of moisture such lands may plow fairly well; but when this limit is passed in the least (as sometimes happens in the course of a single day), it turns up only hard clods, which in a few hours of sunshine become so hard that no instrument of tillage short of a sledgehammer will make any impression upon them. The physical analysis of these usually gray soils shows that they contain only a trifling amount of clay; perhaps 1 or 2%, playing the part of linseed oil in making putty out of whiting. Even the addition of lime does not help such soils much, because there is little or no clay to flocculate. They are, as a matter of fact, among the most refractory lands the farmer has to deal with. A soil showing similar behavior, though not quite as extreme as in the case of the Gulf or Cotton States’ soils in question, occurs at the culture substation at Paso Robles, California, and is probably closely correlated to the physical hardpan referred to above. The physical analysis of this soil yielded the following result:
MECHANICAL ANALYSIS OF SOIL.
================================== Designation.|Diameter.|Percentage. ------------+---------+----------- { | .50 mm. | 14.24 { | .30 “ | 15.17 Sand { | .16 “ | 8.88 { | .12 “ | 5.60 | | { | .072 “ | 6.75 { | .047 “ | 8.35 Silt { | .036 “ | 8.55 { | .025 “ | 6.03 { | .016 “ | 17.77 | | “Clay” | ? | 7.50 ------------+---------+-----------
It would seem the best and almost only remedy to be applied to such soils as these is the introduction of vegetable matter or green-manuring, by which their texture is loosened: for the hauling of mere clay upon the land would hardly accomplish the purpose intended, within the limits of farm economy.
Dust Soils, which during the dry season are even in their natural condition so loose as to rise in clouds and render travel very uncomfortable, are not uncommon in arid countries, e. g., in Washington and adjacent parts of Oregon, on the uplands bordering the Columbia, Yakima and Snake rivers. The physical analyses of three of such soils, given in the table below, will convey some idea of their peculiarities in this respect.
PHYSICAL ANALYSIS OF DUST SOILS.
========================================================= |Hydr. Value.|Diameter. |No 17.|No. 37.|No. 79. ----------+------------+----------+------+-------+------- Clay |<.0023. mm. |<.10--? | .93| 3.59 | 1.27 { |<.25 mm. | .010 | 30.93| 13.06 | 32.29 Silt { | .25 to .5| .016 | 3.20| 5.82 | 12.75 { | .5 to 2.0| .025-.047| 7.18| 27.37 | 37.51 { |2.0 to 8.0| .047-.120| 21.88| 43.78 | 10.92 Sand |8.0 to 64.0| .12- .50 | 32.39| 49.57 | 3.97 | | +------+-------+------- Total | | | 96.57| 98.18 | 98.72 ----------+------------+----------+------+-------+-------
Slow penetration of Water.—Soils of this class are wetted with extreme slowness by irrigation water; so that when first taken under cultivation it sometimes takes twenty-four hours to soak the land for twelve inches in each direction. Irrigation furrows must be placed very close together and in large numbers, in order to ensure the wetting of the soil so that the crop shall not suffer from lack of moisture at a distance of two or not more than three feet. Where the irrigation furrows are drawn farther apart a fine stand of grain may be seen within eighteen inches of the same, while farther away the crops may be dying from lack of moisture. This difficulty is by no means infrequent in the arid region, and is difficult to overcome except by frequent and thorough tillage, which gradually increases the rapidity of water-penetration; as has been shown in the soils of the alluvial prairies of the Yakima country in the State of Washington. It is necessary, however, to take care that they shall always contain an adequate amount of humus or vegetable matter, in order to prevent re-consolidation by the burning-out of the humus during the warm, rainless season.
There is an unmistakable resemblance between these dust soils of the Northwest and the “putty” soils mentioned above; both showing a very low percentage of clay with a relatively large amount of the finest sediments, with a sudden downward break of the curve before the coarser grain-sizes are reached. It would seem as though the absence of these intermediate grains favors the close packing of the fine sediments in the interstices of the coarse ones, thus bringing about the imperviousness, which is the chief obstacle to their cultivation.
Effects of coarse Sand.—Coarse sand intermingled with heavy clay soils has but little effect in improving the tilling qualities, unless carried to such excess as renders it financiallyimpracticable. In actual practice it is frequently possible to improve such soils by properly distributing upon them the washings of the adjacent hills, which will always carry sands of many grades; and when it is intended to improve garden land by hauling sand it is important to choose the latter so as to complement the deficient grain-sizes of the soil. The sand of wind drifts or dunes is generally well adapted to such improvement, being, as Udden[28] has shown, of a fairly definite composition of sufficiently wide range of grain-sizes for the purpose.
The effects of humus in modifying soil texture are discussed farther on.
[28] The Mechanical Composition of Wind Deposits, Bull. No. 1, Augustana Library Publications; 1898.
VII. THE DENSITY AND VOLUME-WEIGHT OF SOILS
Aside from the humus-substances the specific gravity of the common soil constituents, taken individually, do not vary widely; kaolinite being the lightest (2.60), feldspar next (2.62); then quartz (2.65), calcite (2.72). Mica and hornblende range (according to their iron contents) from 2.72 to over 3.0. The average specific gravity of soils of ordinary humus content only will thus range between 2.55 and 2.75; sandy soils approaching very closely to that of quartz alone.
Volume-Weight.—The specific gravity of the soil is, however, of little practical consequence compared with the “volume-weight,” i. e., the weight of the natural soil as compared with an equal bulk of water. A cubic foot of water weighs 62½ pounds; a similar volume of soil usually weighs more, but in the case of peaty lands may actually (when dry) weigh less. The extreme range is from 110 pounds for calcareous, and somewhat less for siliceous sand, to as little as 30 to 50 pounds in the case of peaty and swamp soils. It may be conveniently remembered that while average arable loams range from 80 to about 95 pounds per cubic foot, “heavy” clay soils range from 75 pounds down to 69, observed by the writer in the case of certain alluvial soils, poor in humus,[29] of the Sacramento river, California. Manured garden soils, and the mold surface soil of deciduous forests, generally contain so much humus as to depress their weight considerably, varying according to their state of tilth from 66 to 70 pounds per cubic foot.
[29] This remarkable soil seems to have been derived from the finest “slickens” of the hydraulic gold mines.
Weight per acre-foot.—As for practical purposes and calculations it is often desirable to know approximately the weight in pounds of an acre (43,560 square feet) one foot deep, it is convenient to remember that in the case of sandy land, this weight (per “acre-foot”) may be assumed at four millions of pounds; for loams, at 3½ millions; for clay lands, 3¼ millions; for humus or garden land and woods earth, about 3 millions of pounds; for reedy swamp and peaty lands, 2 to 2½ millions.
The loose tilth and humus-content of the surface soil will in general cause it to weigh less, bulk for bulk, than the underlying subsoil, even when the latter is more clayey; moreover, the continuous pressure from above will tend to consolidate the subsoil and substrata. Warington (Phys. Properties of Soils, pp. 46, 47) gives interesting data on this point from the Rothamstead fields, as follows:
Old pasture, first nine inches 71.3 pounds per cub. ft. Same, fourth “ “ 102.3 “ “ “ “ Arable land, first “ “ 89.4 “ “ “ “ Same, fourth “ “ 101.4 “ “ “ “
The influence of humus and unhumified organic matter, as well as of tillage, in diminishing the volume-weight of soils is here strikingly shown.
Air-space in Natural Soils.—The difference between the specific gravity as usually determined, and the volume-weight of soils, is of course caused by the large amount of air contained in them when dry, but which in wetting them is partially or wholly replaced by water.
Theoretically, assuming all soil grains to be globular, and packed as closely as possible (in oblique order), the space not filled by them would be the same for all sizes, whether that of marbles, or so minute as to be hardly felt between the fingers; and would be 25.95 per cent of the soil volume.[30] If the same globular particles were packed as loosely as possible, i. e., in square instead of oblique order (see figures 10 and 11), the vacant space would be 47.64 per cent If however we imagine each sphere to be itself composed of a number of smaller ones, the empty space will obviously be greatly increased, to an extent proportionate to the diminution of solid mass thus brought about. The pore-space might in that case, with the oblique arrangement of the globules as shown in Fig. 10, be as high as 74.05 per cent But since the soil particles may be of all shapes and sizes within the same soil, and usually fit much more closely than would globular grains, the empty space rarely approaches (only in certain alluvial soils and in loose mulches) to the figure last named. In sandy soils it may fall as low as 20%, and in coarse gravelly soils even as low as 10%. Most cultivated soils range between 35 and 50% of empty space.
[30] King, Physics of Agriculture, p. 116, ff.
Effects of Tillage.—That these figures can be only approximations is obvious from the consideration that one and the same soil will vary materially in its volume-weight according to its temporary condition of greater or less compactness. After land has been beaten by winter rains, its volume-weight will be found to have materially increased from the well-tilled condition brought about by thorough cultivation. This difference is strikingly seen when, in plowing, the height of the ground on the land side is compared with that of the turned furrow-slice in well conditioned loamy land. This loose condition is called tilth, and it results from the formation of relatively large, complex crumbs[31] or floccules, between which there are large air spaces that were wholly absent in the untilled land; the floccules themselves being also more loosely aggregated than was the case before tillage.
[31] The word crumbs, which is generally understood as meaning a relatively large, loose aggregate, seems preferable to the word kernels, suggested for the same by King (Physics of the Soil, p. 110). Kernels are understood to be bodies rather more solid than the surrounding mass, and do not convey the idea of loose aggregates. The word “Krümelstructur” (crumb-structure), adopted by Wollny for this phenomenon, has both fitness and priority in its favor.
Crumb or Flocculated structure.—Figure 11 illustrates the difference between the unplowed land, consolidated especially on the surface by winter rains, and in its upper portion consisting largely of single grains; while the plowed land, toward which the furrow-slices have been turned, is greatly increased in height and volume and consists almost wholly of variously-shaped and-sized aggregates or floccules, loosely piled upon one another and separated by large interspaces. The increase in volume from consolidated clay to crumb-structure is given by Wollny (Forsch., vol. 20, p. 13, 1897) at 41.9%, to powder as 33%. On moistening dry clay increased 36.9%, quartz powder 8.01%. When land is plowed in the proper moisture-condition the crumbs of floccules are held together by the surface tension of the capillary films (menisci) of water at the points of contact. In the case of sands, the crumbs will collapse into single grains whenever the water-films evaporate, unless some cementing substance was dissolved or suspended in the water. (See figure 12). Lime carbonate is one of the substances most commonly found permanently cementing the floccules; hence the ready tillage of most calcareous soils, and especially the loose texture of the “loess” of the western United States, and of Europe and Asia. In these deposits we find sandy and silt aggregates or concretions ranging from ten or more inches in length (loess puppets) to microscopic size, held together by lime carbonate, but collapsing into silt and sand when the material is treated with acid so as to dissolve the cement. The rough surfaces of these aggregates, gripping into each other, explain the stability of the steep loess cliffs in the United States, as well as in northeastern China, as observed by Von Richthofen and Pumpelly.
Clay is most frequently the substance which imparts at least temporary stability to the crumbs and crumb-structure; this is one of its most important functions in soils, as it serves to maintain tilth once imparted by cultivation, even after the land dries out. Beating rains, and cultivation while too wet, will in this case of course destroy the crumbs and the loose tilth.
Other substances which greatly aid the maintenance of tilth are the several humates (of lime, magnesia, iron), which when fresh are colloidal (jelly-like) like clay itself, but unlike the latter, when once dried do not resume their plastic form by wetting (Schloesing). The crumbs thus formed are therefore quite permanent and contribute to the looseness of soils rich in humus. One part of lime humate is said by Schloesing to be equal in cementing power to eleven parts of clay.
Silica, silicates and ferric hydrate are sometimes found cementing soil crumbs, wholly or in part.
The importance of the ready penetration of air, water and roots thus rendered possible is obvious; and the question arises how it happens that wild plants are able to do without tillage.
How Nature Tills.—When we examine the undisturbed soil of woods or prairie in the humid region, we will as a rule find the natural surface soil in a very good condition of tilth; the obvious cause being the presence in it of an abundant network of surface roots and rootlets of grasses and herbs, which in connection with the fallen foliage prevent the beating and compacting of the soil surface; which can be seen to happen before the observer’s eyes whenever a heavy rain falls on a bare land surface, however well tilled.
Crusting of Soils.—In some soils, especially of the Gulf States, the beating of rain followed by warm sunshine so effectually compacts the surface that in the case of taprooted plants like cotton, it becomes necessary to cultivate after each rain, so as to break the crust that would otherwise not only prevent the proper circulation of air, but would also serve to waste the moisture of the land. The same land in the wild condition suffered no such change, being protected by the native vegetation, and by fallen leaves. (See chapt. 8).
Soils of the arid region.—In the regions of deficient rainfall the conditions are modified in several respects. Grass sward rarely exists, nearly all grasses assuming the habit of growing in tufts or bunches some distance (a foot or two) apart; hence the name of “bunch grass” commonly used, which however means not any one definite kind of grass, but serves to distinguish the grasses of the uplands from those of the moist lowlands, where true sward may be found. Between these bunches of grass the soil is fully exposed, and being free from roots and leaf-covering is compacted, unless its nature is such that the usually gentle rains do not produce a serious crusting of the surface.
That such is actually the predominant nature of the soils formed under arid influences has already been stated; and thus the hard-baked soil-surface so often seen in the Eastern United States in unplowed bare land, or during the prevalence of a drought, is rarely seen in the arid region. The clay lands that do exist are usually sufficiently calcareous to possess the property of “slaking” into crumbs whenever wetted after drying. But where this is not the case, the stony hardness brought about by the long dry and warm season is long in being removed by the winter rains.
Changes of soil-volume on wetting and drying.—The behavior of colloidal clay in the above respects has already been described above (see chapt. 4, page 59). It is obvious that whenever soils contain a large proportion of such clay, their behavior on wetting and drying will approximate to those of the pure clay. This is exemplified in the heavy clay, or so-called “prairie soils” of the United States, which when thoroughly wetted in spring will, during a dry summer, form wide, gaping cracks. These in the long summers of the arid region may extend to the depth of several feet, with a width of as much as three and more inches at the surface of the ground. This, of course, contributes greatly to the drying-out of the soil to the same depth, and results as well in the mechanical tearing of the root-system of growing plants; sometimes causing the total destruction of vegetation. In some clay soils it happens that after a rain or irrigation, the shrinkage occurring upon the advent of warm sunshine will cause the surface crust to so contract around the stem, e. g., of grain, as to constrict and injure the bark, causing serious injury to the crop. In soils of this character very thorough tillage in preparing for a crop, and the maintenance of a loose surface during its growth, are of course extremely essential.
In the arid region it will frequently happen that such soils when not tilled to a sufficient depth, will during the later part of the summer so shrink and crack beneath the shallow-tilled surface layer that the latter will bodily fall into the cracks, exposing the roots to all the deleterious influences of mechanical lesion and drying-out. It is thus obvious that the cultivation of such soils should not be undertaken at all by those not naturally able and willing to bestow upon them, to the fullest extent, the deep and thorough tillage which is absolutely essential in the utilization of their usually high productive power.
Extent of Shrinkage.—The extent of this shrinkage in drying, and subsequent expansion in wetting, have been measured by the writer by the use of the sieve cylinder described below (chapt. 11, p. 209), as serving for the determination of the water capacity of soils. When a soil of the kind above referred to is placed in the sieve cylinder in the tilled (flocculated) condition, then allowed to absorb its maximum of water and then dried at 100 degrees C., the contraction in drying can be very strikingly seen, and its amount measured by filling up the empty space with mercury; then measuring the latter after expelling the surplus by means of a ground glass plate laid on top. The contraction of several heavy clay soils, thus measured, has been found by the writer to range from 28 to as much as 40 per cent of the original bulk.[32] The soil thus contracted, when again wetted, does not return altogether to its original bulk, but remains in a more or less compacted condition, like that of a soil which has been rained upon.
[32] Wollny (Forsch. Vol. 20, p. 13 ff., 1897) records similarly high shrinkages in his experiments.
The expansion and contraction of a heavy clay soil on wetting and drying are well illustrated in the figure below, in which the soils are shown in the shallow cylinder which serves for the determination of water-holding power (see chapt. 11, p. 209). The middle figure shows in profile the expansion of a dry, pulverized “black adobe,” struck level, when allowed to absorb its maximum of water; it rises above the rim of the sieve-box to nearly the half height of the latter. The outside figure to the right shows the same soil after drying; that to the left, a red clay soil similarly treated. It is easily seen that these variations in volume may bring about very marked results in the fields; the surface of which, apart from the cracks usually formed, may be several inches lower in the dry season than during wet weather.
Contraction on Wetting.—In the case of alkali soils containing much carbonate of soda, a very notable contraction occurs in wetting the loose, dry soil. The cause is here obviously the collapse of the crumbs, formed in dry tillage or crushing, into single grains, closely packed. The same result is observed in the naturally depressed “alkali spots” (see chapt. 22).
“Hog-wallows.”—In the field the wetting of cracked clay soils produces some very curious effects. The effect of the first light rains usually is to crumble off the edges or angles near the surface, the materials thus loosened falling into the lower portion of the cracks. This is repeated at each successive shower followed by sunshine, the crevices thus becoming partly filled with surface soil. When, subsequently, the heavier and more continuous rains wet the land fully, also causing the consolidated mass in the crevices to expand, the latter cannot close on account of the surplus material having fallen into them; the result being that the intermediate portions of the soil are compelled to bulge upward, sometimes for six or more inches, creating a very uneven, humpy surface, well-known in the southwestern United States as “hog-wallows.”[33]
[33] A totally different kind of “hog-wallows,” occurring in California and the arid region generally, have been described in a previous chapter under the head of Aeolian soils (See chapt. 1, p. 9).
Such a surface is always therefore an indication of an extremely heavy soil, difficult to cultivate; yet embracing some of the most highly and permanently productive lands known in the United States, and in India, where the “regur” lands of the Deccan are of this character; they have been cultivated without fertilization for thousands of years. The subjoined physical analyses of lands of such extreme character as to be almost uncultivatable will serve to exemplify their physical composition.
PHYSICAL ANALYSES OF HEAVIEST CLAY SOILS.
=================================================================== |No 242 Miss.|No. 643 Cal. +------------+------------ |Hog-wallows |Black Adobe. | soil. |Contra Costa | Jasper Co. | Co. |Mississippi.|California. -----------------------------------------+------------+------------ Weight of gravel over 1.2 mm. diameter } | .83 | “ “ between 1.2 and 1 mm } | | “ “ between 1 and 0.6 mm | 1.19 | Fine earth | 97.98 | 100.00 +------------+------------ | 100.00 | 100.00 FINE EARTH. | | | | Hydr. Value. | Diameter. | | -------------------------+---------------+ | Clay <.0023 mm | ? | 48.00 | 45.96 | | | { <0.25 mm | .010 | } 35.18 | 37.64 { 0.25 mm | .016 | } | Silt { 0.5 mm | .025 | 5.50 | 2.74 { 1.0 mm | .036 | 3.74 | 3.31 { 2.0 mm | .047 | 2.54 | 2.95 { 4.0 mm | .072 | .20 | 2.39 | | | { 8.0 mm | .120 | .27 | 1.68 { 16.0 mm | .160 | .90 | .79 Sand { 32.0 mm | .30 | 1.67 | 2.36 { 64.0 mm | .50 | 2.00 | | +------------+------------ | | 100.00 | 100.00 -------------------------+---------------+------------+------------
It will be noted that in both these extremely heavy soils the sum of the clay and finest sediments is a little over 83%.
It should be stated that both these soils after being thoroughly wetted become so adhesive that it is almost impossible to travel over the tracts occupied by them, and that they are practically almost untillable, being too adhesive when wet; yet if allowed to dry to a certain extent (varying within very narrow limits) they turn up by the plow in large clods, which after a few hours of sunshine become of stony hardness and will resist all efforts at pulverization or the production of tilth.[34]
[34] In driving a light carriage over the land represented by No. 643 above, after a light rain, the wheels gathered up so much soil within a hundred yards as to render it necessary to stop and chop it off the tires by means of a hatchet. This is a common experience in the black prairie lands of Texas.
Calcareous Clay Soils crumble on drying.—The heavy clay soils of some of the calcareous prairies of the Southwest, instead of contracting into a stony mass on drying, on the contrary resolve into a mass of crumbs, thus producing excellent tilth. This occurs even though the land may have been plowed when wet, and of course is a great advantage. The most striking exemplification of this peculiarity occurs in the heavy but profusely fertile “buckshot” clay lands of the Yazoo bottom, in Mississippi, where it is usual to plant corn and sweet potatoes in the semi-fluid mud left after an overflow, after turning a shallow furrow, then covering by turning another. To the onlooker it seems impossible that such plantings could be successful; but within a short time the muddy surface becomes a bed of crumbs (“buckshot”), forming a seed-bed not readily excelled by any made by artificial means. Hence, largely, the almost invariable success of crops in the Yazoo region.
Port Hudson Bluff.—The same clay produces a most unpleasant result at the foot of the Port Hudson bluff, where it crops out some feet above low water. When after a freshet the water level falls below this stratum, on drying the clay disintegrates into crumbs just as does the Yazoo buckshot soil; with the result that at the next rise, the loose mass subsides into the river as a flood of mud. Thus the foot of the bluff is being constantly undermined, and the falling of the bluff scarp has obliged the town above to recede many hundreds of feet from its original historic site.
The exact proportions of lime carbonate necessary to produce this phenomenon, and its necessary relations to clay substance and other physical soil ingredients, yet remain to be investigated.[35]
[35] Schübler (Grundsätzed. Agrikulturchemie, 1838) ascribes the crumbling of calcareous clay soils to the difference in the contraction of calcareous sand and the clay substance. But it is doubtless more directly connected with the flocculation of the latter by lime.
Loamy and Sandy Soils.—It is largely the absence of these extreme changes of volume that renders the cultivation of loamy or even sandy lands so much more easy, and the success of crops so much more safe, than is the case in clay soils. Whenever the content of colloidal clay diminishes below 15%, the shrinkage in drying from the wet condition becomes so slight as to cause no inconvenience; while in sandy soils properly speaking, no perceptible change in volume occurs.
Peaty soils, however, and all those containing a relatively large amount of humus, are also liable to visible shrinkage when passing from the wet to the dry condition. But on account of their looseness and porosity such shrinkage does not usually result in the formation of cracks or rupture of the roots, as is the case in heavy clay lands. The entire mass of the soil then shrinks downwards, but rarely forms cracks on the surface. Hence the introduction of humus into “heavy” soils is among the best means of improving their tilling qualities.
Formation of Surface Crusts.—Some soils, especially those of a clay-loam character, are very liable to the formation of hard surface crusts from the beating of rains, and from surface irrigation; owing, doubtless, to the ready deflocculation of their clay substance. It is not easy to define the precise physical composition conducive to this crust formation; but the subjoined physical analyses show examples of soils in which this tendency is very prominent and is frequently annoying, in that when they occur in the regions of frequent summer rains, it becomes necessary after each one to till the surface in hoed crops (e. g., in cotton-fields) in order to prevent the injurious effects of such consolidation of the surface. It may, of course, be prevented by mulching, or on the large scale by green-manuring, to such extent as to prevent contraction.
The subjoined physical analyses of two soils from the Brown-Loam region of Northern Mississippi (see chap. 24), shows the composition of lands excellent in every respect other than the tendency to crust after each rain:
PHYSICAL ANALYSES OF CRUST-FORMING SOILS.
=========================================================== |Diameter.|Hydr. Value.|No. 219.|No. 197. ------------------+---------+------------+--------+-------- Coarse materials | 1-3 mm. | | } | | | | } .23 | { |.5-1 “ | | } | { | .50 | 64 mm. | 1.47 | Sand { | .30 | 32 “ | 2.33 | .79 { | .16 | 16 “ | 1.17 | { | .12 | 8 “ | .78 | .18 | | | | { | .072 | 4 “ | .76 | .78 { | .047 | 2 “ | 9.79 | 3.56 Silt { | .036 | 1 “ | 7.20 | 13.12 { | .025 | .50 | 13.11 | 16.64 { | .016 | .25 | 15.07 | 27.28 { | .010 | <.25 | 26.36 | 18.87 | | | | Clay | ? | <.0023 | 19.10 | 17.23 ------------------+---------+------------+--------+--------
These soils agree in having a sufficient amount of clay (17 to 19%) to characterize them as clayey loams, associated with a very large proportion of the grain-sizes of less than .025 mm., or .5 mm. hydraulic value. A higher proportion of clay, even though associated with a similarly high or even larger proportion of these fine sediments, seems to prevent crusting, probably because the swelling of the clayey ingredient on wetting and its extravagant contraction in drying breaks up the continuity of the surface. The heaviest clay soils, such as those shown on a preceding page, neither crust nor crumble on drying after wetting, but contract into lumps of stony hardness, as a whole.
The burning-out of the humus from well-tilled surface soils during the extended heat and dryness of rainless summers, brings about such a contraction or packing of the surface soil of orchards in California as to greatly reduce their productiveness, and to render necessary diligent green-manuring as the only practical remedy. In many cases, liming of the surface also serves well to prevent this injurious effect, which to some extent of course follows surface irrigation as well as rains.
In most soils, repeated alternate wetting and drying in place produces a loose, flocculated texture, so long as no deflocculation is brought about by mechanical causes, such as beating rains or running water.
Effects of Frost on the Soil.—The expansion suffered by water in freezing necessarily tends to separate the soil particles previously held together by the surface tension of the capillary water, or otherwise flocculated or cemented. Freezing of the soil is therefore of material assistance in disintegrating cloddy, ill-conditioned soils, leaving them in loose, crumbly condition after the ice has melted and the surplus water drained off; so as to materially facilitate tillage and root penetration. When, however, soils thus circumstanced are tilled or trodden while too wet, they quickly become puddled, being practically reduced to single-grain structure. (See this chapt. p. 110). Hence the injury caused by allowing cattle to range in winter on cultivated land subject to freezing and thawing, which it sometimes takes years to correct.
A disagreeable effect often produced by the freezing and thawing of wet lands is the “heaving-cut” of grain, resulting from the upward expansion of the surface soil in freezing, that may readily rupture the roots; while on thawing, the soil surrounding the upheaved stool is apt to settle down, especially in case of a rain, leaving the stool and roots exposed either to drying or freezing, as the case may be. Hence the desire of grain farmers in northern climates, for a sufficient covering of snow to protect the fall-sown grain, rather than an “open winter,” during which the grain is exposed to alternate freezes and thaws, or extreme cold.
In certain soils, notably in those liable to crusting (p. 117), instead of heaving the soil, the water in freezing emerges bodily from small cracks, in foliated or wire-like forms (“ice-flowers”) resembling those of native silver, and formed substantially in the same way, by a kind of “wire-drawing” process, aided by crystallization.
Small ice-crystals formed on the surface of small crevices filled with water cause others to be formed at their lower ends, and the expansion occurring in freezing, forces the ice upward; the process repeating itself under favorable conditions, until the stalks or sheets of ribbed ice grow to a height of several inches. This phenomenon is especially frequent in the middle cotton States—Arkansas, Tennessee, northern Mississippi, etc., where frequent changes from rainstorms or thaws to cold northwest winds occur in winter.
VIII. SOIL AND SUBSOIL
CAUSES AND PROCESS OF DIFFERENTIATION. HUMUS.
Soil and Subsoil Ill-defined.—While the general mass of rock debris formed by the action of the agencies heretofore discussed as soil-material, may under proper conditions become soil capable of supporting useful plant growth, universal experience has long ago recognized and established the distinction between soil and subsoil: by which are ordinarily meant, respectively, the portion of the soil-material usually subjected to tillage, and what lies beneath. There can be no question about the practical importance of this distinction; but the definition of the two terms, as commonly given in some works of agriculture, is both incomplete and, in its application to many cases, partly misleading.
The differentiation of soil and subsoil is due partly to the action of organic matter and micro-organisms, partly to physico-chemical causes, now to be discussed in detail.
THE ORGANIC AND ORGANIZED CONSTITUENTS OF SOILS.
Humus in the Surface soil.—The most obvious mark of distinction between soil and subsoil is, usually, the darker tint of the former, due to the presence of humus or vegetable mold, which becomes most apparent by darkening of the tint when the soil is moistened. Thus soils having a gray tint when dry, may become almost black when wetted. When no such deepening of color occurs in wetting, the absence or great deficiency of humus may safely be inferred. The only other substance whose presence may invalidate the conclusions based upon the darkening of the soil tint, is ferric hydrate (iron rust), which itself possesses the property of darkening on wetting, and may effectually cover either the presence or the absence of humus.
Since the formation of the humus depends upon the decomposition of organic matter (mostly of the cellulose group) derived partly from the roots, partly from the leaves and stems of plants growing and dying on the soil, its accumulation near the surface is natural. But since the depth to which roots penetrate varies greatly not only with different plants, but very essentially in conformity with the greater or less penetrability of the soil and subsoil, the depth to which the dark humus tint may reach vertically varies correspondingly, from two or three inches to several feet. In the case of soils that have been formed by the gradual filling-up of swamps or marshes, the humus-tint may reach to several yards depth.
Surface Soil, and Subsoil.—It is thus apparent that the term “surface soil,” while commonly confined by the farmer to the portion turned by the plow or usually reached in cultivation by any implements, may or may not belong, functionally, to layers of greatly varying thickness. Similarly the term subsoil may or may not refer, in individual cases, to parts of the soil mass materially different from the surface soil. Yet this distinction is of no mean practical importance, because the efficacy of one of the most common measures of soil improvement, viz., subsoil plowing or “subsoiling,” depends materially upon the differences between soil and subsoil in each particular case. Most of the diversity of opinion regarding the merits of this operation is simply the result of a corresponding diversity in the natural facts and cultural practice of each case.
Causes of the Differentiation of Soil and Subsoil.—One of the prominent points of difference between surface soils and subsoils has already been mentioned in the usual predominance of root-mass in the upper layers; to which is added a part at least of the substance of fallen leaves and stems of its vegetation. How much of this vegetable mass ultimately becomes converted into humus, as well as the nature of the product formed, depends upon a great variety of circumstances; some of which have already been mentioned in connection with the general discussion of humification (chapt. 2, p. 20). Briefly stated, the main controlling conditions are: the amount of water or moisture present, the access of air (oxygen), a proper temperature, and the presence of the several organisms which in the course of time take part in the process of soil-formation.
Ulmin Substances; Sour Humus (Germ. Rohhumus).—In the presence of so much moisture or liquid water as will materially impede the access of air, and with the concurrence of reasonably low temperatures, the organisms that at first take the chief role in the transformation of the vegetable tissues into humus-like substances are bacteria. But the antiseptic nature of the compounds thus formed[36] soon puts an end to their activity, and thereafter the process seems to be a purely chemical one, and very slow. In peat bogs, the transition from the fresh, dead stems and roots to brown peat is easily followed downward, white cellulose fibers remaining apparently unchanged to some depth; so that such fiber has been used for tissues and paper. The solid decomposition-products are brown substances, partly soluble in water and imparting to it a brown or coffee color (frequently seen in the drains of marshes) and an acid reaction; the latter due to ulmic (as well as apocrenic) acid, readily soluble in caustic and carbonated alkalies, and forming insoluble salts with the earths and metals; while another portion, ulmin, is insoluble in the same, but gradually becomes soluble by oxidation.
[36] The antiseptic properties of sour humus are well exemplified in the perfect state of preservation in which the remains of animals, wood implements, etc., are found in bogs into which they have sunk in prehistoric times.
The gaseous products formed under these conditions are carbonic dioxid and “marsh gas” (methan, CH₄), the former predominating in the early stages; while later, the carburetted hydrogen predominates, rendering the gas readily inflammable.
Sour Soils.—The “sour” soils thus produced in nature in presence of excess of water bear only “sour” growth, such as sedges and rushes, of little agricultural value; they usually require reclamation processes before becoming adapted to ordinary crops. In old forests of northern climates a peaty and more or less acid layer is sometimes formed on the surface, above the black woods-earth, and retards somewhat the full production of such land when taken into cultivation.[37]
[37] See Müller, Natürliche Humusformen.
Marshes and swamps, both fresh and salt, as above stated usually show coffee-colored waters, which are also characteristic of the streams that drain them, until by intermixture with waters containing lime salts, the ulmic substances are neutralized and precipitated. Such neutralization, preferably by means of lime, is the first step towards the reclamation of lands bearing “sour” vegetation. The acid reaction characterizing the ulmic substances is also characteristic of many woodlands, notably in the United States of the soils of the “Long-leaf-pine” region of the Cotton States, both upland and lowland, as well as of many deciduous forests in northern climates. Hence liming, whether artificial or natural, effects a most notable improvement, together with a marked change of vegetation, in these lands.
It has been long known that after long-continued cultivation, soils originally of neutral or slightly basic reaction become acid: and the liming of such lands is an ancient practice in Europe. The matter, however, received but scant attention until Wheeler and Hartwell, of the Rhode Island Experiment Station, demonstrated the almost universal acid condition of the older lands of that State, and the excellent effects produced by neutralization with lime, or even with the alkali carbonates.[38] The current neutralization of the humus-acids is unquestionably one of the cardinal advantages of calcareous lands; for such as contain only small amounts of lime carbonate will of course become acid more quickly under cultivation.
[38] Reports of the Rhode Island Exp’t Station, 1895, and ff.
Humin Substances.—In the presence of only a moderate amount of moisture, therefore under the influence of a more or less rapid circulation of air, and in the presence of earthy carbonates (especially that of lime) to prevent the formation of acids, or to neutralize them as formed, the normal process of humification occurs; mainly under the influence of fungous instead of bacterial growths. The various molds take a prominent part in the conversion of the vegetable substance into black, neutral, insoluble humus compounds. Such fungous vegetation is always accompanied by the evolution of carbonic gas, and the resulting fungous tissues are markedly richer in nitrogen and carbon than the substance of the higher plants from which they were derived (see chapt. 9). Comparative analyses show that in the normal process of humification of vegetable substances, oxygen and hydrogen are eliminated in the form of water and carbonic dioxid, while at the same time there is an increase in the percentage of carbon, and generally also of nitrogen; the latter more particularly in the case of vegetable matter not very rich in that element. When once humification is complete, oxidation, especially under arid conditions, bears mainly upon the carbon and hydrogen, so that the nitrogen content may rise to very high figures; while another portion is ultimately wholly oxidized, with the formation of nitrates, under the influence of the nitrifying bacteria, this being the process chiefly efficient in the nutrition of vegetation with nitrogen.
As a matter of course, the several organic compounds contained in plants may continue to exist in soils for some time, varying according to conditions of temperature and moisture. Thus dextrin, glucose, and even lecithin and nuclein have been reported to be found. The activity of the numerous fungous and bacterial ferments under favoring conditions will, of course, limit the continued existence of such compounds somewhat narrowly, so that they can hardly be considered as active soil ingredients save in so far as they favor the development of the bacterial flora.
Porosity of Humus.—One of the essential features of natural humus is its great porosity, whereby it not only becomes highly absorbent of water and gases, but is also gradually oxidized, probably under the influence of bacteria. For this oxidation, as measured by the evolution of carbonic gas, progresses most rapidly under the same conditions as to moisture, temperature and access of air, that are known to be most favorable to fungous and bacterial growth. Hence the formation of carbonic dioxid in the soil is assumed to be the measure of the intensity of such activity.
Physical and Chemical Nature of the Humus Substances.—The humus substances are gelatinous when moist, but are neither markedly adhesive or plastic. Like the other colloidal substances of the soil, they serve to retain both gases and vapors, including moisture, liquid water, and its dissolved solids. In the natural, porous condition they are powerfully absorbent of gases, including especially aqueous vapor. Dry humus swells up visibly when wetted, the volume-weight increasing to the extent of two to eight times; so that humus stands foremost in this respect among the soil constituents. The density of natural humus is about 1.4, being the lightest of the soil constituents. Hence soils rich in humus are “light” not only in the farmer’s sense of being easily tilled when not too wet, but also of light weight for equal volumes when compared with clayey and sandy soils. Some data bearing upon these points are given in the table[39] below, for the substances moderately and uniformly packed:
VOLUME-WEIGHTS OF
Humus.[40] Clay. Quartz Sand.
.3349 1.0108 1.4485
When saturated with water, the same substances gave the following figures:
Air-dry. Saturated Increase. with water. %
Humus[41] .3565 1.1024 209.2 Clay 1.0395 1.6268 55.9 Quartz sand 1.4508 1.8270 25.9
[39] Wollny, Zersetzung der Organischen Stoffe, pp. 242, 243.
[40] Peat pulverized and extracted with alcohol and ether to remove resinous substances.
[41] Peat pulverized and extracted with alcohol and ether to remove resinous substances.
These data show strikingly the effects produced by the several physical soil constituents upon some of its physical properties.
Chemical Nature.—While humus artificially produced by the action of caustic alkalies upon sugar or cellulose is free from nitrogen, all naturally occurring humus contains the latter.
It is not, however, present in the form of ammonia, as it cannot be set free by treatment in the cold with lime or alkalies. When, however, natural humus is boiled with these substances, ammonia is slowly given off, but the process continues indefinitely and it seems to be impossible to expel all the nitrogen in this manner. This behavior being characteristic of amido-compounds, it is presumable, in view of the slightly acid nature of the humus substances, that natural humus is largely of an amidic constitution. Artificial humic acid, formed by the action of caustic alkalies upon sugar, gums or cellulose, combines with ammonia as with other bases, and at first the ammonia can be readily expelled from this as from other ammonia salts. But after the lapse of some time it seems that the amidic condition is assumed, so that caustic lye acts but very slowly and cannot expel the whole of the nitrogen present. This is very important in connection with the practice of fertilization, as any ammonia taken up by or generated in the soil is thus in the course of time rendered comparatively inert, and unavailable to vegetation until nitrified.
Progressive Changes.—The natural neutral humin and ulmin, as found, e. g., in the lower portions of peat beds, are in the course of time by oxidation converted into ulmic and humic acids, capable of combining with bases; by still farther oxidation they form apocrenic and crenic acids, readily soluble in water and in part forming soluble salts with lime, magnesia and other bases. These acids act strongly upon the more readily decomposable silicates of the soil, and in the course of time may dissolve out, and aid in the removal by leaching, of most of the plant-food ingredients as well as the ferric hydrate of a soil. Thus red or rust-colored soils may be rendered almost white by continued “swamping” with stagnant water, and be greatly impoverished; and it is doubtless largely through this agency that the underclays of coal beds and the lower portions of peat beds, as well as peat and coal ashes, are almost wholly destitute of mineral plant food.
The Phases of Humification.—The progressive changes involved in the process of humification of vegetable matter are illustrated in the table below,[42] together with the farther changes by which such matter may ultimately be transformed into the several varieties of coal, and finally into anthracite, which already represents nearly pure carbon, but in nature has sometimes been still farther transformed into graphite (black-lead) and diamond.
[42] Data recalculated, omitting ash.
PROGRESS OF HUMIFICATION, AND FORMATION OF COAL. (MOISTURE AND ASH OMITTED FROM CALCULATIONS.) ========+==========+======================+============+ | | Oak Wood. | | | +------+--------+------+ Humin | |Cellulose.|Fresh.|Decayed.| | and | | | +--------+------+ Humic | | | | Light | Dark | Acid. | | | | Brown. |Brown.| | --------+----------+------+--------+------+------------| Carbon | 44.44 | 50.60| 53.60 | 56.20|49.4 to 59.7| Hydrogen| 6.17 | 6.00| 5.20 | 4.90| 2.5 “ 4.5| Oxygen | 49.38 | | | |35.8 “ 47.3| | | 43.40| 41.20 | 38.90| | Nitrogen| | | | | .3 “ 18.7| --------+----------+------+--------+------+------------+
========+======================+================================ | Peat.[43] | Coals. |--------+-------------+--------+-----------+----------- | Brown | Black. | Lignite| Scotch | Penn’a |Surface.+------+------+ Brown | Splint |Anthracite. |(Ulmin.)|40 in.|80 in.| Coal. |Bituminous.| | | | |(Bovey).| | --------+--------+------+------+--------+-----------+----------- Carbon | 57.80 |62.00 |64.10 | 69.50 | 84.20 | 94.80 Hydrogen| 5.40 | 5.20 | 5.00 | 5.90 | 5.80 | 2.60 Oxygen | 36.00 |30.70 |26.80 | 24.00 | 8.80 | } | | | | | | } 2.60 Nitrogen| .80 | 2.10 | 4.10 | .60 | 1.20 | } +-------+--------+------+------+--------+-----------+-----------
[43] Detmer, Landw. Versuchst., Vol. 14, 1871.
The steady increase of carbon and nitrogen, together with a corresponding decrease of oxygen, are well illustrated in the analyses, especially in the strictly comparable series of peat samples from various depths. In this case there is also a steady decrease of hydrogen, and an increase of ash from 2.72% in the surface layer, to 9.16 at 80 inches depth. This increase is due in the main, of course, to the progressive volatilization of the organic matter in the forms of carbonic dioxid and marsh gas (methan, CH₄).
In considering this table it should not be forgotten that while normal humus stands very close to peat, and the latter when compressed in certain stages would be undistinguishable from lignite or brown coal; yet both peat and lignite are known to be formed under conditions permitting much less access of air or oxygen than occurs in the formation of normal black soil-humus. Hence even black peat cannot at once stand in place of soil-humus when removed from its watery bed, but requires considerable time and aeration (oxidation), and in most cases neutralization with lime or marl, before it can serve the purposes of humus in the soil.
Lignite and the progressively more carbonaceous coals are and have been formed under the conjoined action of submergence and pressure, sometimes also aided by heat; and thus they cannot perform the function of soil-humus, any more than the fire-clays or shales underlying them can resume their original soil-functions without prolonged weathering.
Amounts of Humus and Coal Formed from Vegetable Matter.—Only very general and indefinite estimates can be given of the amount of humus or coal formed from a given quantity of vegetable matter, since these must vary according to the conditions under which the transformation occurs. The greater or less access of air and of moisture, the temperature and pressure under which the process occurs, will modify very materially the quantitative as well as the qualitative result. In the hot arid regions the fallen leaves may wholly disappear by oxidation on the surface of the ground, while under humid conditions they are mostly incorporated with the surface soil. If we assume that in the humification of plant debris (estimating their average nitrogen content at 1%), no nitrogen is lost, it would seem that in the humid region one part of normal soil-humus might be formed from 5 to 6 parts of (dry) plant debris; while in the extreme regime of the arid regions, from 18 to 20 parts of the same would be required. But as most probably some nitrogen also is lost in the process of humification, a considerably larger proportion of original substance may be actually required.
As to coal, it is usually assumed that it requires about 8 parts of vegetable matter for one of bituminous coal. Much higher estimates are made by some, and an observation made by the writer at the Port Hudson bluff, Mississippi, in 1869, would seem to justify such estimates. The above figure, from a sketch made at the time, shows the proportions to which a pine log about eight inches in diameter had shrunk in drying into a small sheet of lignitized wood; the original trunk, projecting from a bed of sand some forty feet below the surface, being so porous and spongy that when wet it flattened somewhat by its own weight; it was connected with the little sheet of lignite by a spirally twisted, tapering stipe.
Here evidently the proportion of lignite formed was a very minute one, doubtless because of the long leaching to which the trunk had been subjected. It thus seems impossible, as in the case of humus, to assign any definite proportion as between woody matter and coal formed from it.
Normal humification takes place only under the influence of moderate temperature. When the temperature is too low, bacterial and fungous growth are repressed or arrested; when too high, the fungous vegetation assumes a different phase, the result of which is the almost total oxidation of the organic matter, sometimes so accelerated as to initiate rapid combustion “fire-fanging” of dung; leaving in any case but a trifling organic residue of very high ash contents.[44]
[44] A striking illustration of this is afforded by Naegeli’s experiment of enclosing several loaves of bread in a loosely closed tin-box. After eighteen months there remained only seventeen per cent of air-dry mouldy matter, totally destitute of starch.
Eremacausis.—In the absence of a sufficient degree of moisture to co-operate with the other agencies of humification, the final result in the soil is practically the same as in the “fire-fanging” of dung. The organic matter is almost wholly destroyed by direct oxidation (eremacausis) with or without the aid of minute organisms; leaving essentially only the ash behind to be reincorporated with the soil. This is to a very great extent the predominant process in the arid regions of the Globe; most of the soils formed in these climates being, therefore, very poor in humus-substances, and deriving it almost entirely from the decay of roots only.
The extent to which the humus of a soil may be derived from the vegetable debris falling or growing upon the surface, varies greatly with the climatic conditions as well with the nature of the soil. In the forests of humid climates with loamy soils, not only does the autumnal leaf-fall, as well as decaying twigs and trunks, become obviously incorporated with the surface soil as decay progresses on the lower surface, but active animal agencies (see below) carry the organic remnants bodily down. But where heavy clay soils prevail, these animal agencies are much restricted by the compactness of the material; only a light surface-layer of mold would be formed, and the humus of the lower soil layers must of necessity be derived from the decay of the roots only. This origin is claimed by Kosticheff[45] for the high content of black humus in the tchernozem or black earth of Russia. Following Hellriegel in determining the weight of roots contained in successive equal layers of soil from the surface downwards, Kosticheff gives for each six inches the following data as found in the tchernozem, taking as 100 the root-content of the surface layer:
===========+======+======+======+======+======+====== Number. | 1 | 1 | 2 | 2 | 3 | 3 Depth. |Roots.|Humus.|Roots.|Humus.|Roots.|Humus. -----------+------+------+------+------+------+------ 6 inches. |100. | 5.42 |100. | 8.11 |100. | 9.64 12 “ | 89.1 | 4.83 | 63.9 | 5.19 | 80.3 | 7.77 18 “ | 66.9 | 3.62 | 48.3 | 3.92 | 70.0 | 6.71 24 “ | 47.3 | 2.56 | 35.0 | 2.84 | 58.4 | 5.61 30 “ | 47.3 | 2.59 | 26.0 | 2.11 | 38.2 | 3.57 36 “ | 34.6 | 1.88 | 18.1 | 1.47 | 33.0 | 3.18 42 “ | 23.9 | 1.29 | 6.3 | .51 | 16.2 | 1.56 48 “ | 14.4 | .78 | | .70 | | 54 “ | 6.7 | .36 | | | | -----------+------+------+------+------+------+------
[45] Abstract in Ann. de la Science Agronomique, Tome 2, 1887.
It will be seen that there is a very close correspondence of the humus content with the root development in the several layers, and it seems as if though but little of the humus could be derived from the surface growth, which is that of the grasses of the steppe.
The climate of the black-earth country of Russia is, though not properly arid, yet one of rather deficient and uncertain rainfall. But as a consequence of extremely arid conditions, and in sandy lands, it may even happen that the immediate surface soil contains less humus than what, in the farmers’ habitual parlance, would be called the subsoil; because of the penetration of slow combustion for some distance into the porous soils. It will then be lower down that, in the presence of a favorable degree of moisture and lower temperature, the conditions of normal humification are fulfilled.
It is not always, then, that the commonly recognized distinction between surface soil and subsoil based upon humus content can be maintained. But the observation of everything bearing upon this point is of the utmost importance in determining both the agricultural value and the mode of treatment of the land.
Losses of Humus from Cultivation and Fallowing.—The fact that humus accumulates in woodlands and meadows, where no cultivation is given, would naturally lead to the converse conclusion, viz., that cultivation causes loss of humus and of its constituents. That this is actually the case is recognized and widely acted upon in practice, and there is no question that the general acceptance of stable manure as the most widely useful fertilizer, despite its usually low content of plant-food ingredients, is based upon the fact that it supplies vegetable matter, in a condition highly favorable to its conversion into humus. The most direct and cogent proof of the depletion of the soil of both humus and nitrogen by continuous cultivation of cereal grains has been given by Snyder,[46] who determined the loss both of humus and of nitrogen suffered by a Minnesota soil during eight years’ continuous cultivation of wheat. The total loss of nitrogen was 1700 pounds per acre, while only 350 pounds were utilized by the crop; about 1400 pounds being dissipated as gas or leached out as nitrates. A conservative estimate of the loss of humus suffered during the same period was about a ton per acre annually, and this loss seriously decreased not only the nitrogen-content, but rendered the soil more compact and less retentive of moisture. But by rotation of the wheat with clover in alternate years, very nearly an equilibrium of both humus and nitrogen-content was obtained. In addition, the amount of available mineral plant-food was decreased by continuous grain culture. Ladd has made similar observations in North Dakota, with similar results.
[46] Bull. No. 70 Minn. Exp’t Station, 1905.
That excessive aeration results in serious losses of humus as well as of nitrogen, is very obvious in the arid region, where it is the habit to maintain on the surface of orchards and vineyards during the dry, hot summers, a thick mulch of well-tilled soil, thus preventing loss of water by evaporation. In the course of years this surface soil becomes so badly depleted of humus that good tilth becomes impossible, the soil becoming light-colored and compacted; while the loss of nitrogen is indicated by the small size of the orchard fruits. Similar losses are of course sustained in the practice of bare summer-fallow, which at one time was almost universal in portions of the arid region. The complete extirpation of weed growth thus brought about, at first considered an unmixed benefit, has ultimately had to be made up for by the practice of green-manuring; since in the arid region the use of stable manure encounters many difficulties.
Estimation of Humus in Soils. It has been usual to determine the amount of humus in soils by means of (dry or wet) combustion, calculating the humus from the carbonic dioxid so formed, while measuring the nitrogen gas directly. But in this process the entire organic matter of the soil, humified and unhumified, is indiscriminately included; and it is wholly uncertain to what extent the latter will ultimately become humus, from the nitrification of which plants are presumed to chiefly derive their nitrogen.[47] In order to obtain definite results, the actual, functional humus must be extracted from the soil mass by some solvent which discriminates between the humified and unhumified organic matter. This cannot be done by direct extraction with caustic soda or potash, which inevitably dissolve unhumified matters and tend to expel ammonia from the humus; besides themselves acting as humifiers (see this chapter, p. 125.)
Grandeau Method: Matière Noire.—The only method now known which accomplishes this separation, practically excluding the unhumified while fully dissolving the humified matter—is that of Grandeau: the extraction of the soil, first with dilute acid, in order to set the humic substances free from their combinations with lime and magnesia; and their subsequent extraction with moderately dilute solutions of ammonia (or other alkali hydrates). Upon the evaporation of the ammonia solution the humus is left behind in the form of a black lustrous substance (“matière noire” of Grandeau) much resembling the crust of soot formed in flues from wood fires. As it contains a variable amount of ash, it must be burnt and the ash subtracted from the first weight.
[47] The humus determinations thus made, which include nearly all those made by German chemists, give the humus-content from 40 to 50% too high. The French determinations are mostly made by the method of Grandeau.
Amounts of Humus in Soils.—While in peat, marsh and muck lands the humus-content may rise above twenty per cent, in ordinary cultivated lands it rarely exceeds about five per cent, and very commonly falls below three per cent, even in the humid regions. In properly arid soils we find a very much lower average, rarely exceeding one per cent, and frequently falling to .30 and even less. This scarcity of humus manifests itself plainly in the prevalently light gray tint of the arid soils.
Meadows and woodlands generally show the highest humus-content in their surface soils, gradually increasing while in that condition; while when taken into cultivation the humus-content gradually decreases, owing to the free aeration and consequent “burning-out” caused by tillage. Hence the humus must be from time to time replaced by the use of stable manure, or green-manure crops, to prevent injurious changes in the tilling qualities of the land. Not only humus as such, but according to Schloesing also the insoluble colloid humates, produce in the soil a loosening effect or tilth (Germ. Bodengare), which apparently cannot be brought about by any other substance.[48]
[48] The decrease of humus from wheat culture in the soils of Minnesota and North Dakota has been studied by H. Snyder and E. F. Ladd, respectively. In the prairie lands of the latter State the total organic matter in the first six inches of soil ranges from 15 to as much as 26%, and the humus alone from 4 to 7.8%.
Humates and Ulmates.—That the insoluble humates of lime, magnesia, iron, manganese and alumina are present in most soils is conclusively shown by the composition of the solution obtained by the extraction of soils with weak acid, as above mentioned in connection with the quantitative determination of humus according to Grandeau; since these bases are almost always extracted by the weak acid. When the brown solution of alkali humate obtained in this process is carefully neutralized with sulfuric or hydrochloric acid, or is mixed with solutions of the above bases, flocculent, insoluble precipitates are formed, while the solution is discolored. Similar precipitates may be obtained with other metallic solutions, notably with that of copper, which precipitates the humus-acids most completely. Doubtless these compounds contribute greatly to the conservation of the humus-content of soils, protecting it to a certain extent from oxidation, and also preventing excessive acidity. The brown tint of certain subsoils in the northern humid regions have been shown by Tollens and others to be due not to ferric hydrate, as had been supposed, but to calcic, magnesic and aluminic humates. None of the mineral bases or acids present can be detected in the humic solution by the usual reagents.
Mineral Ingredients in Humus.—That the mineral plant-food ingredients present in the humus extracted by the Grandeau process, and which remain as ash when the matière noire is burned, are capable of nourishing plant growth, was directly shown by Grandeau, Snyder and others. The former was inclined to consider that those substances were mainly thus taken up by plants, under natural conditions. This theory, however, has not been sustained by subsequent investigations; the mineral plant-food thus extracted is not a measure of the immediate productiveness of the soils, as demonstrated by Snyder, and the residual soils are not sterile. It is also still doubtful to what extent the mineral bases and acids are naturally combined with the humus-substances, it being contended by some that they are brought into organic combination by the acid and ammonia extraction. The investigations of Snyder and Ladd, above referred to, prove however to some extent at least that the humus-substances are naturally combined with them, and that probably they are largely made available to plants through the direct and indirect action of the humus compounds. This subject is farther considered in chapter 19.
The nature and amounts of these mineral substances are well exemplified in the subjoined full analysis by Snyder, of the ash of the humus and humates extracted from a compound sample of prairie soils of Minnesota, which had been thrown down from the ammonia solution by simply neutralizing the liquid:[49]
ASH OF HUMUS FROM MINNESOTA PRAIRIE SOILS.
Insoluble matter[50] 61.97 Potash (K₂O) 7.50 Soda (Na₂O) 8.13 Lime (CaO) 0.09 Magnesia (MgO) 0.36 Peroxid of Iron (Fe₂O₃) 3.12 Alumina (Al₂O₃) 3.48 Phosphoric acid (P₂O₅) 12.37 Sulfuric acid (SO₃) .98 Carbonic acid (CO₂) 1.64
[49] Precipitation with an excess of acid does not greatly change the results.
[50] In California soils this is mostly silica soluble in carbonate of soda.
The large amounts of the soluble alkalies potash and soda thrown down with the humic matters are very striking, as is the very large proportion of phosphoric acid. Lime and magnesia had, of course, been mainly eliminated by the preliminary acid treatment.
Functions of the Unhumified Organic Matter.—The unhumified plant debris in the soil are not to be regarded as useless, even aside from their potential conversion into active humus. Not only do these remnants of vegetation lighten the soil, rendering it more pervious to air and water, but in their progressive decay they give off carbonic gas, which is active in soil-decomposition; and they serve as nourishment to the soil bacteria upon which its thriftiness so greatly depends. See below, chapter 9.
The Nitrogen-Content of Humus.—Since soil-humus is doubtless the chief depository of soil-nitrogen, and the main source from which, through the process of nitrification, the nitrogen-supply to plants is usually derived, its content of that element is a matter of great interest. It has been customary to estimate approximately the nitrogen-content of soils by the proportion of humus-substance present; and as the light tints of the soils of the arid region indicate a small humus-content, a scarcity of nitrogen seemed to be also indicated for these lands. As this in a number of cases did not seem to accord with actual experience, an investigation of the subject was made at the California experiment station,[51] with the results shown in the subjoined table. In considering these results it must be kept in mind that while arid conditions can rarely be fulfilled in the humid region, humid conditions are quite frequently locally represented in the arid, in lowlands and on high mountains; while moderately moist benchlands represent the semi-arid regime.
[51] Hilgard and Jaffa. On the Nitrogen-content of Soil-humus in the Humid and Arid regions. Rep. Cal. Exp’t Station for 1892-4; Agric. Science, April, 1894; Wollny’s Forsch. Geb. Agr. Phys., 1894.
HUMUS PERCENTAGE AND NITROGEN CONTENT IN SOILS OF THE ARID AND HUMID REGIONS. =====+==============================================+=====+========+======== | |Humus|Nitrogen|Nitrogen Station| Soils arranged in order of nitrogen | in | in | in Number| percentages in humus. |soil,| Humus, |soil, | | per | per | per | |cent.| cent. |cent. -----+----------------------------------------------+-----+--------+-------- | SOILS OF THE ARID REGION (California). | | | | | | | 2061 |Dark clay loam, Arroyo Grande Valley, | 3.06| 22.00 | .670 | San Luis County | | | 2291 |Red soil, Orland, Glenn Co. | .71| 21.10 | .150 1904 |Sediment Soil, Porterville, Tulare Co. | .90| 19.50 | .180 1901 |Sandy soil near Ceres, Stanislaus Co. | .64| 18.75 | .120 704 |Sandy soil of plains, near Fresno, Fresno Co. | .60| 18.66 | .112 6 |Black adobe soil, Stockton, San Joaquin Co. | 1.05| 18.66 | .196 1679 |Black adobe soil, Berkeley, Alameda Co. | 1.20| 18.58 | .203 2324 |Clay soil of desert, Imperial, San Diego Co. | .38| 18.40 | .070 1167 |Black clay loam soil, near Tulare, Tulare Co. | 1.66| 18.19 | .302 1536 |Brown loam soil, Windsor Tract, Riverside, | .20| 18.00 | .036 | Riverside County | | | 1126 |Sandy loam soil, Paso Robles, | .55| 17.27 | .095 | San Luis Obispo Co. | | | 2301 |Red hill soil, Upper Lake, Lake Co. | .81| 16.90 | .137 1607 |Plateau soil of desert, Lancaster, | .25| 16.80 | .042 | Los Angeles Co. | | | 1159 |Sandy plains soil, Tulare, Tulare Co. | .37| 16.75 | .062 1900 |Sandy soil, near Modesto, Stanislaus Co. | .84| 16.65 | .140 1113 |Clay loam soil (slate), Jackson, Amador Co. | .54| 16.60 | .090 1149 |Adobe clay soil, near Paso Robles, | .47| 16.18 | .074 | San Luis Obispo County | | | 1538 |Mesa soil, Chino, San Bernardino Co. | .65| 16.08 | .105 1147 |Sandy loam soil, Paso Robles, | .66| 16.06 | .106 | San Luis Obispo Co. | | | 2403 |Valley Soil, Wheatland, Yuba Co. | 1.50| 16.00 | .240 1281 |Red Mesa soil, Pomona, San Bernardino Co. | .58| 15.50 | .090 1117 |Sandy granitic soil, near Jackson, Amador Co. | .80| 15.27 | .123 1406 |Red loam soil, Arlington Heights, Riverside, | .30| 15.00 | .045 | Riverside County | | | 1172 |Red clay loam soil, east of Tulare, Tulare Co.| .72| 14.75 | .106 1958 |Sandy Mesa soil, Nipomo, San Luis Obispo Co. | .85| 14.45 | .122 1423 |Chocolate-red soil, Carisa plain, | .39| 14.36 | .056 | San Luis Obispo County | | | 1291 |Sandy hill land, near Jackson, Amador Co. | .76| 14.34 | .109 585 |Wire-grass loam soil, Visalia, Tulare Co. | 1.00| 14.10 | .146 863 |Red ridge loam soil, Grass Valley, Nevada Co. | 2.89| 13.91 | .402 1907 |Dark loam soil, near Chino, San Bernardino Co.| .92| 13.26 | .121 1115 |Sandy granitic soil, near Jackson, Amador Co. | .85| 13.20 | .112 332 |Plateau desert soil, Mojave, Los Angeles Co. | .28| 12.50 | .035 2126 |Gravelly soil, East Highlands, | .62| 11.75 | .070 | San Bernardino Co. | | | 1910 |Ojai Valley soil, Nordhoff, Ventura Co. | 1.64| 11.21 | .183 2187 |Sandy loam soil, Soledad, Monterey Co. | .97| 11.10 | .110 1759 |Sandy soil, Perris Valley, Riverside Co. | .53| 11.04 | .059 774 |Bench slope soil, Ontario, San Bernardino Co. | 1.29| 10.85 | .140 1984 |Red soil, East Highlands “ “ “ | .58| 10.50 | .060 2325 |Silt soil of desert, Imperial, San Diego Co. | .65| 10.70 | .070 1906 |Light sandy soil, Pomona, San Bernardino Co. | .95| 9.80 | .093 2430 |Hillside adobe, Berkeley, Alameda Co. | 1.85| 8.70 | .160 | +-----+--------+-------- | Average of arid uplands | .91| 15.23 | .135 | | | | | SUB-IRRIGATED ARID SOILS (California). | | | | | | | 586 |Sandy plains soil, Tulare, Tulare Co. | 1.14| 10.79 | .123 1466 |Loam soil, Miramonte, Kern Co. | .60| 10.66 | .064 1284 |Moist land loam soil, Chino, | 1.99| 10.20 | .203 | San Bernardino Co. | | | 1148 |Swale soil, near Paso Robles, | 1.16| 9.65 | .112 | San Luis Obispo Co. | | | 1714 |Bench soil, Santa Clara River, Piru, | .78| 9.56 | .074 | Ventura Co. | | | 77 |Alluvial soil, Tulare Lake bed, Tulare Co. | .47| 9.37 | .045 1880 |Creek bench soil, Niles, Alameda Co. | 1.19| 8.90 | .109 1903 |Sediment soil, Porterville, Tulare Co. | 1.12| 8.50 | .140 168 |Alluvial soil, Santa Clara river, Santa Paula,| .84| 7.99 | .067 | Ventura Co. | | | 1760 |Green-sage land, Perris Valley, Riverside Co. | .91| 7.70 | .070 506 |Alluvial soil, Colorado River, Yuma, | .75| 7.47 | .050 | San Diego Co. | | | 1636 |Red soil, Manton, Tehama Co. | 2.00| 6.86 | .137 1758 |Alkali soil, Perris Valley, Riverside Co. | .60| 6.83 | .071 1963 |Sandy loam soil, Willows, Glenn Co. | .36| 6.05 | .022 2080 |Sandy soil, Santa Maria Valley, | 1.64| 5.36 | .090 | Santa Barbara Co. | | | | | | | | Average of sub-irrigated arid soils | 1.06| 8.38 | .099 | | | | | HUMID SOILS FROM ARID AND HUMID REGIONS | | | | (California). | | | | | | | 207 |Eel River Alluvial soil, Ferndale, | 1.25| 6.96 | .085 | Humboldt Co. | | | 2319 |Alluvial soil, Hupa Valley, Humboldt Co. | 7.83| 6.70 | .514 213 |Marsh soil, Novato, Meadows, Marin Co. | 1.54| 6.36 | .089 1704 |Valley soil, Hollister, San Benito Co. | .94| 5.21 | .049 2295 |Tule soil, Upper Lake, Lake Co. | 1.70| 4.50 | .077 110 |Alluvial soil, Putah Creek, Dixon, Solano Co. | 1.71| 4.25 | .072 37 |Redwood Valley soil, Pescadero, San Mateo Co. | 2.28| 3.07 | .070 | | | | | Average for California | 2.45| 5.29 | .135 | | | | | OTHER STATES. | | | | | | | 26 |Bog soil, Michigan[52] |33.02| 6.08 | 2.012 |Back-land clay loam, Houma, Louisiana | 5.07| 4.20 | .218 |Duff soil, Oregon |13.84| 3.49 | .483 |Sandy prairie soil, Harris Co., Texas | 2.13| 3.66 | .184 | | | | | Average for other States | 7.01| 3.78 | .295 | | | | 23 |Red soil, Oahu Island, Hawaii (maximum) | 1.57| 5.07 | .078 27 |Guava soil, Hawaii Island (minimum) | 9.95| 1.71 | .170 |Average of 5 soils, Oahu Island | 3.01| 6.07 | .237 |Average of 2 soils, Maui Island | 9.07| 2.13 | .286 |Average of 4 soils, Hawaii Island | 6.17| 2.54 | .146 | | | | | Average for Hawaiian Islands | 5.26| 3.69 | .169 | | | | | Total for Humid soils, average | 4.58| 4.23 | .166 -------+----------------------------------------------+-----+--------+-------
[52] Introduced only for comparison of the nitrogen percentage in Humus and not included in the average.
It thus appears that on the average the humus of the arid soils contains about three and a half times as much nitrogen as that of the humid; that in the extreme cases, the difference goes as high as over six to one (see Nos. 37 and 704); and that in the latter cases, the nitrogen-percentage in the arid humus considerably exceeds that of the albuminoid group, the flesh-forming substances.
It thus becomes intelligible that in the arid region a humus-percentage which under humid conditions would justly be considered entirely inadequate for the success of normal crops, may nevertheless suffice even for the more exacting ones. This is more clearly seen on inspection of the figures in the third column, which represent the product resulting from the multiplication of the humus-percentage of the soil into the nitrogen-percentage of its humus; as appears in comparing the respective averages, or Nos. 1167 and 110 and others. An additional consideration is the probable greater ease with which the nitrifying bacteria can act upon a material so rich in nitrogen.
We must not, then, be misled by the smallness of many humus-percentages in the arid region, into an assumption of a deficiency in the supply of soil-nitrogen.
Decrease of Nitrogen-Content in Humus with Depth.—Since the oxidation of the carbon and hydrogen in the humus-substance, and the consequent increase of its relative nitrogen-content, are manifestly dependent upon the presence of air and heat, it is reasonably to be expected that the nitrogen-percentage of the humus should decrease with the depth of the soil. That this is really the case is plainly shown in the subjoined table, which gives the humus-percentages and the nitrogen-content of the humus from the surface foot down to twelve feet, in a soil on the bench of the Russian River, Cal., which is sub-irrigated, and liable to more or less rainfall during the summer. It will be seen that not only does the absolute humus-percentage decrease quite regularly down to seven feet, at which point there evidently was at one time a strong root development, causing a notable increase of the humus-content; from which again there is a regular decrease down to the twelfth foot. It will be noted that the nitrogen-percentage in the humus, while not decreasing with the same regularity as the humus-content itself, yet exhibits a general recession from 5.30 to 1.15 in the ninth foot, to which direct oxidation doubtless never penetrates.
HUMUS AND NITROGEN-CONTENT OF RUSSIAN RIVER SOIL. ==============+==============+=================+================ Depth in feet.| Per cent | Per cent | Per cent | Humus | Nitrogen | Humus-Nitrogen. | in soil. | in Humus. | in soil. --------------+--------------+-----------------+---------------- 1 | 1.21 | 5.30 | .064 2 | 1.16 | 4.32 | .054 3 | 1.14 | 3.87 | .044 4 | 1.17 | 3.76 | .044 5 | .74 | 2.16 | .016 6 | .60 | 2.66 | .016 7 | .47 | 2.54 | .012 8 | .78 | 1.54 | .012 9 | .54 | 2.24 | .012 10 | .52 | 1.15 | .006 11 | .53 | 1.51 | .008 12 | .44 | 1.81 | .008 --------------+--------------+-----------------+----------------
Influence of the Original Materials on the composition of Humus.—The great variability of the composition of humus formed from different substances is well shown in the subjoined table, representing the results of experiments made by Snyder,[53] who caused various substances to humify by mixing the pulverized material intimately with a soil poor in humus, and allowing the process to continue for a year. At the end of that time the humus formed was extracted by the method of Grandeau, outlined above, and analyzed, with the following results.
========+======+======+=======+======+========+=======+========== |Sugar.| Oat | Green |Wheat |Sawdust.| Meat | Cow | |Straw.|Clover.|Flour.| |Scraps.|Manure.[54] --------+------+------+-------+------+--------+-------+---------- Carbon | 57.84| 54.30| 54.22| 51.02| 49.28 | 48.77| 41.93 Hydrogen| 3.04| 2.48| 3.40| 3.82| 3.33 | 4.30| 6.26 Nitrogen| 0.08| 2.50| 8.24| 5.02| 0.32 | 10.96| 6.16 Oxygen | 39.04| 40.72| 34.14| 40.14| 47.07 | 35.97| 45.63 --------+------+------+-------+------+--------+-------+---------- |100.00|100.00| 100.00|100.00| 100.00 | 100.00| 100.00 --------+------+------+-------+------+--------+-------+----------
[53] Bull. No. 53, Minn. Exp’t Station, p. 12, Chem. of Soils and Fertilizers, p. 94.
[54] The figures for cow manure are so far out of range with any others thus far observed, that it seems reasonable to suppose that they are influenced by unchanged substances present in the excreta.
While it may be questioned whether the process of humification had in these materials really reached the point of sensible completion in all cases (notably in those of sawdust and cow manure), the great variability of the products from different materials is very striking. When the nitrogen-content is deducted the percentage composition of the products agrees more nearly. Considering that the nitrogen is probably present in the amid form, it is natural that hydrogen should in a measure vary with it, as in the case of the clover, flour and meat humus. Nitrogen being the most variable ingredient of humus, it seems probable that the variation of the proportion of the humus-amids present is the most potent factor in the variability of the composition of natural soil-humus.
Arranging these results in the order of their nitrogen-content as in the table below, we see that the latter approximately corresponds to the original protein-content of the humified substances.
Humus from meat scraps 10.96 % Nitrogen. “ “ green clover 8.24 “ “ cow manure 6.16 “ “ wheat flour 5.05 “ “ oat straw 2.50 “ “ sawdust .32
While the above data prove the correlation between the first products of humification and the original substance, it must be remembered that subsequently, under proper conditions, the nitrogen-percentage in humus may, in the course of time, increase very greatly, even to a proportion considerably above that contained in flesh itself. When we consider that ordinarily, the latter, and the albuminoid substances generally, decompose in contact with air with an abundant evolution of ammonia compounds, sometimes leaving only a little fat (adipocere) behind, it is surprising that the decomposition within the soil should have exactly the opposite result, viz., an accumulation of the nitrogen. The causes of this marked difference are not yet well understood, but it is probably due to the differences in the kinds of bacteria that are active in the two cases.
Snyder has also shown that the richer the organic matter humified is in nitrogen, the more energetically it acts in rendering available the mineral matters of the soil for plant nutrition. Correspondingly, Ladd[55] has shown that with the increase of humus in the soil, there is also a corresponding increase in the amounts of mineral plant-food extracted from the soil by a four per cent solution of ammonia, such as is employed in the Grandeau method of humus-determination.
[55] Bull., S. Dakota Station, Nos. 24-32, 35, 47.
IX. SOIL AND SUBSOIL (Continued)
ORGANISMS INFLUENCING SOIL CONDITIONS; BACTERIA, ETC.
MICRO-ORGANISMS OF THE SOIL.
Intimately correlated with the humus-substances of the soil, as well as with its temporary contents of the carbohydrates (cellulose, gums and sugars) from which humus is formed, is the multitudinous flora of micro-organisms always present and exercising important functions in connection with the growth of the higher plants. Extended researches by Adametz, Schloesing and Müntz, Miquel, Koch, Fraenkel, Winogradsky, Frank and many others, have thrown light upon the immense numbers and great variety of minute organisms, especially of the bacterial group, present in soils, and upon their distribution and activities in the same. It has been shown that their numbers are greatest near (although usually not at) the surface, decreasing rapidly downward and generally disappearing wholly at depths between seven and eight feet; the latter depth varying of course according to the nature and porosity of the soil, and both depth and numbers being greatest in summer.
Numbers of Bacteria in Soils.—Adametz found in one gram of soil, 38,000 bacteria at the surface, 460,000 at ten inches depth; in a loam soil at the surface 500,000, at ten inches 464,000 in each gram of earth. Of mould and similar fungous germs there were only 40 to 50 in the same, 6 species being true molds, while four were ferments, including the yeasts of wine and beer. Fraenkel found in virgin land from near Potsdam, a sudden, marked decrease at depths of from three to five feet; while in earth from inhabited places within the city of Berlin, considerable numbers were still present at eight and even ten feet, in some cases.
In the researches lately made by Hohl at the bacteriological station at Liebefeld, near Bern, it was found that in cultivated soils the number of bacteria greatly exceeds the figures given by Fraenkel. He found a gram of moist soil to contain from three to fifteen millions of bacteria. In the cultivated soil of Liebefeld he found 5,750,000, in meadow land 9,400,000, in a manure pile 44,500,000 per cubic centimeter. These figures seem high for so small a quantity of material, but taking the average size of a bacterium, a cubic centimeter might readily contain six hundred millions. (Grandeau, Ann. Sci. Agronomique, vol. 1, p. 461, 1905).
Mayo and Kinsley (Rep. Kansas Exp’t Station for 1902-3) have made elaborate investigations of the numbers and kinds of bacteria found in various soils in Kansas, in connection with different crops. It is noteworthy that in most cases their figures exceed considerably those given by European observers, as they often reach high into the millions, in one case to over fifty millions, per cubic centimeter.[56]
[56] The mode of statement in the paper is not always quite clear as to the manner in which the averages given were calculated. It must be remembered that these data refer to cubic centimeters of soil, or about twice the amount (1 gram) used by European observers.
Five fields with different soils were investigated; the land being described as follows: “Field No. 1 is a black loam containing considerable humus; field No. 2 is similar to field 1 but contains more humus; field No. 3 is a thin soil with clay gumbo subsoil; fields Nos. 4 and 5 are black loams, but not as rich in humus as either No. 1 or No. 2.”
The average bacterial contents of the several fields are given as follows:
Field No. 1 33,931,747 per cubic centimeter. “ No. 2 53,596,060 “ “ “ “ No. 3 78,534 “ “ “ “ No. 4 8,643,006 “ “ “ “ No. 5 3,192,131 “ “ “
“The crop records of these fields for the past ten years indicate that the crop yield has been (more or less?) directly proportional to the bacterial content of the soil of each field; field 2 has produced the largest yield, field 3 the least.”
Unfortunately no chemical analyses of any of these soils are communicated; but at the request of the writer samples of the soils of the first three fields were sent from the Kansas station for humus determinations (courteously made by Dr. H. C. Myers), which gave the following results:
Field No. 1 2.19% of Humus. “ No. 2 3.07% “ “ “ No. 3 1.85% “ “
While these humus-percentages are not directly proportional to the bacterial content, a favoring effect of high humus-content is clearly shown. The bacterial and the humus-content of these soils are sensibly, even if not directly, correlated; which might reasonably be expected, since the organic matter and the humus are the bacterial food.
The investigation also showed wide differences in the bacterial content of the same soil when different crops were growing on it. Thus in samples taken on Aug. 15, there were found in the first twelve inches of a black loam soil bearing timothy and clover, 1,380,000, in the same with alfalfa and clover, 21,091,000, with maize from one to over two millions. In soils from the western part of Kansas, the bacterial content of the same crops was much less (as doubtless is the humus-content), and it is noteworthy that the prairie buffalo grass shows throughout a relatively high bacterial content in the first foot of the soil, ranging next to alfalfa. The root bacteria living on the legumes will naturally increase the bacterial content of the soils on which they grow, more than plants which, like maize, do not directly utilize bacterial action.
Multiplication of the Bacteria.—Marshall Ward and Duclaux have made some special observations in regard to the rapidity with which certain bacteria multiply. Duclaux summarizes the final conclusion thus: taking as a basis the time of 35 minutes for the subdivision into two, which has been frequently observed by Ward, there would be four millions of bacteria produced in twelve hours. The first filaments had plenty of room in a drop culture of one cubic millimeter; but at the end their total volume amounted to the tenth part of the total volume of the drop. At the above rate, making 48 generations in 24 hours, 281,500 billions of organisms would be produced. (Grandeau, Ann. Sci. Agron. Vol. 1, 1905, p. 456).
Aerobic and Anaerobic Bacteria.—As may readily be inferred, the cultural and other surface conditions exert a potent influence both upon the kinds and abundance of the bacteria and molds; since the life-functions of some are dependent upon the presence of free oxygen (“aerobic”), while others flourish best, or only, in the absence of air (“anaerobic”), or are able to avail themselves of the presence of combined oxygen, by reduction of oxids present. Their number is found, in general, to be greatest in cultivated lands, and bacteria are there by far predominant over the moulds. On the other hand, the moulds gain precedence in woodlands and meadows, at least so far as air can gain access; while in the deeper layers of the same, as well as in peaty lands, bacterial life is always scanty. This holds particularly in respect to the nitrifying organisms, and others whose life-functions are dependent upon abundant access of oxygen (aerobic).
Food Material Required.—All bacteria, like the fungi, are dependent for their development upon the presence of adequate amounts of some organic food-material, best apparently in water-soluble form. In the soil it seems to be chiefly compounds of the carbohydrate group, especially various gums derived from the decaying plant substance, or from stable manure; in artificial cultures, glucose is mostly found to be a highly available food. When the decaying substance reaches the state of humus, the latter seems to be available as food only to comparatively few bacteria. The very abundant development of bacterial life seems to be among the most important effects produced by stable manure upon the surface soil, in establishing good tilth (“Bodengare” in German).
Functions of the Bacteria.—While there is still much uncertainty as to the exact functions performed by most of these bacteria in respect to soil-formation and plant growth, there are several kinds whose activity has been proved to be of the utmost importance in one or both directions; it having been shown that when the soil is sterilized either by heat or antiseptic agents, certain essential processes are completely suppressed until the soil is re-infected and the conditions of bacterial life restored.
Probably the chief in importance are those connected with the processes of nitrification and denitrification, bearing as they do upon the supply to plants of the most costly of the three substances furnished by fertilizers. These organisms have been first extensively studied by Winogradsky, while the conditions of their activity have been largely developed by R. Warington.
Nitrifying Bacteria.—The conversion of ammonia into nitrates is accomplished under proper conditions by two organisms, or groups of organisms; the first stage being the formation of nitrites by the round, often flagellate cells of nitrosomonas (or nitrosococcus). The second, the oxidation of the nitrites into nitrates by very minute rod-shaped bacilli, named nitrobacteria. The conditions under which these bacteria can act are quite definite in that, aside from a supply of the nitrifiable substance, a fairly high temperature (24° C. or 75° F.) and a moderate degree of moisture, there must be a free access of oxygen (air); and there must be present a base (or its carbonate) with which the acids formed by oxidation can immediately unite. In an acid medium (“sour” soils) nitrification promptly ceases; as it also does whenever the amount of base present has been fully neutralized. The bases most favorable to nitrification are lime and magnesia in the form of carbonates, an excess of which does no harm; while in the case of the carbonates of potash and soda, the amount must be strictly limited.
Conditions of Activity.—Dumont and Crochetelle found that up to .25 per cent, potassic carbonate acted favorably on the process; which was, however, completely stopped by as much as .8 per ct. Warington has shown that ammonic carbonate similarly prevents nitrification when exceeding about .37 per ct. Ammonia salts in general appear to be antagonistic to the transformation of nitrites into nitrates.
Aside from the carbonates, some neutral salts favor nitrification very markedly; while others tend to depress it. Deherain found that .5 per cent of common salt suffices to prevent nitrification altogether, while smaller amounts retard it proportionally. According to Dumont and Crochetelle, potassium chlorid acts favorably up to .3 per cent, but at .8 per cent suppresses nitrification. Earthy and alkaline sulfates, on the contrary, seem to act favorably throughout, at least up to .5 per cent. This is especially true of gypsum, which, according to Pichard, accelerates the process more than any other substance known. Taking the effect of gypsum as the maximum, he found that, other things being equal, the amounts of nitrates formed were as shown in the table below, the effect of gypsum being taken as 100:
Gypsum 100 Sodic Sulfate 47.9 Potassic Sulfate 35.8 Calcic Carbonate 13.3 Magnesic Carbonate 12.5
The above estimates are markedly confirmed by the observations of the writer in the alkali soils of California. In these, nitrates exist most abundantly when the salts contained in the soil are mainly sulfates; while wherever common salt or sodic carbonate are present in considerable amounts, the amounts of nitrate found are notably less. In saline seashore lands nitrates are usually present in traces only. Wollny has moreover shown that the nitrates themselves exert a repressive influence on nitrification.
Effects of Aeration and Reduction.—While the fostering effect of sulfates upon nitrification is very energetic in well aerated soils, they become injurious whenever by a reductive process in ill-drained lands, the sulfates are reduced to sulfids. Under such conditions the process will in any case be much impaired. On the other hand, the favoring effect of abundant aeration was strikingly shown in the experiment made by Deherain, in which a cubic meter of soil was left unmoved for several months, while a similar mass was thoroughly agitated once a week during the same time. The proportion of nitrates formed in the latter case was as 70 to 1 formed in the quiescent soil mass. It follows that the intensity of nitrification is essentially dependent upon the porosity of the soil; and that it is thus greatly favored in the pervious soil-strata of the arid regions. It also follows that thorough and frequent tillage and fallowing greatly favor nitrification; thus explaining one of the beneficial results of these operations. At the same time, it is true that we may thus in a short time seriously diminish the reserve stock of nitrogen contained in the soil in the form of humus-amids; and since nitrates are exceedingly liable to be lost from the soil in several ways, such excessive nitrification is to be avoided.
Unhumified Organic Matter does not Nitrify.—There can be little doubt that the formation of ammonia from the amido-compounds in humus is also the work of bacteria; but this, really the initial phase of the nitrogen-nutrition of plants, has not yet been fully elucidated. That, however, it is essentially only the ready-formed humus and not the unhumified debris of the soil which participate in nitrification was shown by the experiments of the writer, see chapter 19.
Denitrifying Bacteria.—Among the sources of loss of nitrates in the soil is the action of denitrifying bacteria; some of which cause merely the reduction of nitrates to nitrites and progressively to ammonia, while others cause gaseous nitrogen to be given off from nitrites and nitrates, resulting in their complete loss to the soil. While there are probably several kinds of the latter class, the most rapidly effective is an organism contained abundantly in fresh horse dung, and also on the surface of old straw. This can readily be shown by subjecting a very dilute solution (1-3 per cent.) of Chile saltpeter to the action of fresh horse dung in a close flask, when nitrogen and carbonic dioxid gases are evolved, and in a few days the nitrate has totally disappeared. In the course of time this power of horse-manure disappears; so that “rotted manure” is practically free from it and under proper conditions serves nitrification so effectively, that in the past it has served extensively for the production of saltpeter in the “niter-plantations” for the industrial purposes; the material of which was loose earth, marl and manure, kept moist and frequently forked over for better aeration. Saltpeter is similarly produced in stables, corroding the mortar of brick foundations. Nevertheless, it is necessary to avoid the use, either together or at short intervals apart, of Chile saltpeter and fresh manure; the manure if used first should be allowed to remain at least two months in the soil before saltpeter is applied.
The reduction of nitrates to nitrites and ammonia is brought about by quite a number of bacteria, mostly anaerobic, and such as consume combined oxygen in their development. Thus the butyric ferment, which in the absence of readily reducible compounds evolves free hydrogen, will in presence of nitrates reduce the latter to nitrites, or form ammonia by addition of hydrogen to nitrogen just set free by reduction. Such reductive processes of course occur chiefly in soils rich in organic matter, or ill-aerated. The ammonia so formed, while at first simply combining with any humus acids present, may in the course of time be itself reduced to the amidic condition, being thereby rendered relatively inert, until again brought into action by ammonia-forming bacteria.
Ammonia-forming Bacteria.—A large number of different bacteria appear to be concerned in the formation of ammonia from compounds of the albuminoid group, (and probably from humus). Among these is one of the most common in soils (Bacillus mycoides, root bacillus), which while forming ammonia carbonate in solutions of albumen, is also capable of reducing nitrates to nitrites and ammonia in presence of a nutritive solution of sugar.
The “hay bacillus” (B. subtilis), so abundantly developed in hay infusions, and one of the most abundant in cultivated soils, has together with B. ellenbachensis, B. megatherium, B. mycoides, and others, by some been credited with important action in favoring vegetation; so that a fairly pure culture of B. ellenbachensis has been brought out commercially in Germany under the name of “Alinit.” Rigorous culture experiments made by Stutzer and others have, however, failed to show any general benefit from the use of alinit in infecting either land or seeds. But there is no doubt of the Effects of Bacterial Life on Physical Soil Conditions.—It is apparent that all conditions favoring the life of aerobic (air-needing) bacteria tend also to produce the loose, porous state (tilth) of the surface soil so conducive to the welfare of culture plants, designated by German agriculturists as “Bodengare.” Whether or not this condition is directly due to bacterial processes, as is thought by Stutzer (Landw. Presse, 1904, No. 11) it is assuredly a highly important point to be gained, and is essentially connected with the presence of humus in adequate amounts, which is also a favoring condition of abundant bacterial life. It seems that the preference given to the shallow putting-in, or even surface application of stable manure, existing in Europe, is largely based upon the marked effect upon the looseness of the surface soil, generally credited to the physical effect of the manure substance itself, but apparently largely due to the intensity of bacterial action thus brought about.
ROOT-BACTERIA OR RHIZOBIA OF LEGUMES.—Among the most important bacteria, agriculturally, is that which enables plants of the leguminous order—(peas, beans, vetches, clovers, lupins, etc.),—to obtain their supply of nitrogen from the air independently of those contained in the soil. The source of nitrogen to plants was long a disputed question; it was at first supposed (by de Saussure) that it was obtained directly from the soil by the absorption of humus; but this was disproved, and Liebig then contended that it was derived directly from the atmosphere through the ammonia in rain water. This was then shown to be wholly inadequate; and Boussingault proved conclusively that plants do not take up nitrogen gas from the air. This was subsequently denied by Ville; but investigation at the Rothamstead agricultural station by Lawes and Gilbert definitely confirmed Boussingault’s results. At the same time they also proved very definitely that while grass and root crops deplete the soil of nitrogen, clover and other leguminous crops leave in the soil more nitrogen than was previously present, even when the entire, itself highly nitrogenous, leguminous crop is removed from the land. The improvement of lands for wheat production by rotation with clover had long ago become a practical maxim; but the cause was not understood until, in 1888, Hellriegel and Wilfarth announced that the variously-shaped excrescences or tubercles which had long been observed as frequently deforming the roots of legumes, are caused by the attacks of bacilli capable of absorbing the free nitrogen of the air and thus enabling the host-plant to acquire its needed supply by absorbing the richly nitrogenous matter thus accumulated in the excrescences. The minute rod-shaped organism was named Bacillus radicicola by Beyerinck; Rhizobium leguminosarum, by A. Frank, who has published an extensive treatise on the subject.[57]
[57] Uber die Pilzsymbcose der Leguminosen, Berlin, 1890.
[58] Original figure from drawing by O. Butler, Asst. in Agr. Dep’t Univ. of California.
Microscopic examination of the nodules shows their tissues to contain partly motile, free bacteria, partly others (bacteroids), which have assumed a quiescent condition, and are of much greater dimensions than those of the motile form. These relatively thick, and sometimes forked, forms, differing somewhat in each of the group adaptations mentioned below, constitute the bulk of the cell-contents of the nodules, and ultimately serve for the nutrition of the host-plant with nitrogen. When the growth of the excrescence is completed, the swollen, quiescent bacteroids gradually collapse and become depleted of their nitrogenous substance; and finally the apparently empty husk remains or drops off, carrying with it the minute cocci which in the soil become active bacteria again. The nodules are thus found mainly on the actively-growing roots, and at the time when vegetation and assimilation are most active in the plant. In autumn, or when the plants are in fruit, the roots may be wholly destitute of nodules.
The adhesion of the nodules to the roots is mostly very loose, and their falling-off when the seedlings are carelessly transplanted, doubtless accounts for much of the difficulty generally found in transplanting legumes when once established.
The figures annexed show the various forms assumed by the nodules in different plants, and with them also the corresponding forms of the bacteroids of each. The latter, here shown magnified about 1000 times, are taken from the inaugural dissertation of D. Brock on this subject, published at Leipzig in 1891. It appears that the forms of the bacteroids are quite as much varied as are those of the nodules they form.
Varieties of Forms.—While these bacilli seem to be normally present in most soils, it seems to be necessary that they should adapt themselves for this symbiosis[59] with each of several groups of the legumes in order to exert their most beneficial effects. In many soils there appears to exist a “neutral form”, which requires about a season’s time or more to adapt itself specially to the several leguminous groups so that a great advantage is gained by infecting either the seeds or the soil with the forms already adapted, when no similar plant has lately occupied the same ground. Thus the bacillus of the clover root is of little or no benefit to beans, peas or alfalfa, and the root-bacilli of each of the latter are relatively ineffectual when used to infect either of the other groups. The same is true of the bacilli of lupins and of acacias, as applied to leguminous plants of any other groups.[60]
[59] “Living together” beneficially; in contradistinction to parasitism, which is injurious to the host plant.
[60] It is asserted by some observers that the root-bacilli producing differently-shaped excrescences upon different legumes are distinct species; but this view is not sustained by the experiments of Nobbe and Hiltner, and seems intrinsically improbable.
Mode of Infection.—The infection is especially effectual when applied to the seeds before sowing; and for that purpose there may be used either the turbid water made by stirring up in it some earth of a properly infected field, or else water charged with a pure culture of the appropriate kind, commercially known under the name of nitragin, now manufactured for the purpose. Or else, the field to be sown may be infected by spreading on it broadcast, and promptly harrowing in, a wagon-load of earth per acre from a properly infected field. Such earth must not be allowed to dry, or to be long exposed to light.
Specially effective (“virulent”) and hardy forms of such bacteria have been produced under artificial culture by Dr. Geo. T. Moore of the U.S. Department of Agriculture. These cultures can be sent by mail on cotton imbued with them, for the infection of seeds.
It is very important that the bacillus should be present in the earliest stages of the growth of the seedlings; otherwise the latter will undergo a longer or shorter period of starvation, unless the soil contains, or is furnished with, a sufficiency of available nitrogen to supply their immediate wants. When such a supply is very abundant, the legume crop will sometimes develop no nodules at all; but the best crops appear to be the result of a thorough infection, and abundant formation of the excrescences.
Cultural Results.—The marked results obtained in certain soils by inoculation with the legume-root bacillus are exemplified in the following table, showing results of experiments by J. F. Duggar, at the Alabama Experiment station.[61]
TABLE SHOWING INCREASE OF PRODUCTION BY SOIL INOCULATION. ==============================+=======+========+================ PER ACRE. | TOPS. | ROOTS. | NITROGEN. | lbs. | lbs. | lbs. | Value. ------------------------------+-------+--------+-------+-------- Hairy vetch, not inoculated | 194 | 387 | 7 | $ 1.05 “ “ inoculated | 3045 | 1452 | 106 | 15.90 Crimson clover not inoculated | 106 | 266 | 4.3 | .65 “ “ inoculated | 4840 | 1452 | 143.7 | 21.25 ------------------------------+-------+--------+-------+--------
[61] Bull. Ala. Exp’t Station, No. 96, 1898.
Such marked increases from soil inoculation cannot of course be expected in cases where the soil has previously borne leguminous crops of similar nature and therefore already contains the root bacteria. Hence Duggar found no increase of production when inoculating for cowpea, land that had borne that crop two years before and already contained the root bacteria. In the arid region, where the almost universally calcareous soils usually bear a natural growth largely composed of various leguminous plants, inoculation is likely to be less commonly effective than in the humid region east of the Mississippi, where leguminous plants are much less generally present in the native flora.
The distinctive agricultural function of supplying nitrogen to the soils on which they grow, renders inexcusable the persistence of some writers and teachers in designating all forage plants as “grasses.” Whatever excuse there may have been for this practice so long as the nitrogen-gathering function of the legumes was unknown, disappears with this discovery, and the misleading misnomer should be banished from agricultural publications and lectures, at the very least.
Other Nitrogen-Absorbing Bacteria.—An increase in the nitrogen-content of some soils, aside from the action of leguminous root-bacteria, has long been observed. As already stated, this increase was at first ascribed to certain green algæ often seen to develop on the soil surface; but it has now been shown that the nitrogen-gathering function belongs to at least two bacteria, one of which (Clostridium pastorianum) was discovered by Winogradski, the other (Azotobacter chroococcum) by Beyerinck, and has since been farther investigated by Koch, Kröber, Gerlach and Vogel, and last by Lipman and Hugo Fischer. According to the latter it seems likely that Azotobacter chroococcum lives in symbiosis with the green algæ, all of which, like the Azotobacter itself, develop with special luxuriance on calcareous soils.
Lipman (Rep. Agr. Exp’t Station, New Jersey, 1903 and 1904) describes as Azotobacter vinelandii a form somewhat different from the A. chroococcum, the nitrogen-assimilating power of which he tested quite elaborately. He exposed to air pure cultures of A. vinelandii in nutritive solution containing the proper mineral ingredients, and glucose 20 grams per liter. 100 cub. centimeters of this solution was exposed in flasks of respectively 250, 500 and 1000 cc. content, therefore having greater surface in the larger flasks. After ten days, the amounts of nitrogen fixed were found to be respectively 1.67, 3.19 and 7.90 milligrams. When mannite solution was employed instead of glucose, a similar fixation was observed; and it was also shown that the presence of combined nitrogen in the forms of nitrates or ammonium salts discouraged the fixation by the bacillus.
It was thus clearly proved that A. vinelandii at least does not need symbiosis with algæ to fix atmospheric nitrogen; but experiments with mixed cultures of the above bacillus and another (designated as No. 30 by Lipman) proved that when these two co-operate the absorption of atmospheric nitrogen is nearly doubled. As it is probable that this is the case also with other soil bacteria, the importance of this source of nitrogen to plants is obvious; provided of course that the proper nutritive ingredients are present in available form. Lipman shows that among the organic nutrients, besides the sugars, glycerine and the salts of propionic and lactic acids, and probably also others of the same groups, can serve as nourishment to the nitrogen-fixing bacteria.
DISTRIBUTION OF THE HUMUS WITHIN THE SURFACE SOIL.
The uniform distribution of the humus-contents of the surface soil, as shown in sections of the same, is by no means easily accounted for. The roots from which its substance is so largely derived are not so universally distributed as to account for it; but least of all can the rapid disappearance of the leaf-fall and other vegetable offal from the surface be accounted for without some outside agencies. Of these, the action of fungous vegetation, and of insects and earthworms, are doubtless the chief ones.
Fungi.—When we examine a decaying root, we find radiating from it a zone of deeper tint, as though from a colored solution penetrating outward. But since under normal conditions humus is insoluble, this explanation cannot stand. Microscopic examination, however, reveals that the outside limit of this zone is also the limit to which the fungous fibrils concerned in the process extend; and as these fibrils are much more finely distributed and much more numerous than the roots of any plant, it is natural that the humus resulting from their decomposition should be more evenly distributed than the roots themselves.[62]
[62] Kosticheff, Formation and Properties of Humus; in abstract Jour. Chem. Soc., 1891, p. 611.
Such fungous growth is not, however, confined to dead and decaying roots only. A large number of trees and shrubs, among them pines and firs, beeches, aspen and many others, also the heaths, and woody plants associated with them, appear to depend largely for their healthy development, notably in northern latitudes, upon the co-operation (“symbiosis”) of fungous fibrils that “infest” their roots, enabling them to assimilate, indirectly, the decaying organic (and inorganic) matter which would otherwise be unavailable, and at the same time converting that matter into their own substance. Fungous growths thus mediate both the decomposition and rehabilitation of the vegetable debris.
The vegetative fibrils (mycelia) of several kinds of molds are constantly present in the soil, and while consuming the dead tissue of the higher plants, spread their own substance throughout the soil mass. The same is true of the subterranean or “root” mycelia of the larger fungi, toadstools, mushrooms, which are commonly found about dead stumps and other deposits of decaying vegetable and animal offal. All these being dependent upon the presence of air for their life functions, remain within such distance from the surface as will afford adequate aeration; the depth reached depending upon the perviousness of the soil and subsoil. In the humid region this will usually be within a foot of the surface, but in the arid may reach to several feet. Ultimately these organisms contribute their substance to the store of humus in the land.
On the surface of moist soils we frequently find a copious growth of green fibrils, which may be either those of algæ, such as Oscillaria, or the early stages (prothallia) of moss vegetation. This vegetation has been credited with absorption of nitrogen from the air, thus enriching the soil; but later researches have shown this effect to be due to symbiotic bacteria (see above p. 156).
Animal Agencies.—Darwin first suggested that wherever the common earthworm (Lumbricus) finds the conditions of existence, it exerts a most important influence in the formation of the humous surface-soil layer; and the limitation imposed upon these conditions by the subsoil has doubtless a great deal to do with the sharp demarcation we often find between it and the surface soil. Briefly stated, the earthworm nourishes itself by swallowing, successively, portions of the surrounding earth, digesting a part of its organic matter and then ejecting the undigested earth in the form of “casts,” such as may be seen by thousands on the surface of the ground during or after a rain. Darwin (The Formation of Vegetable Mold, 1881), has calculated from actual observation that in humid climates and in a ground fairly stocked with these worms, the soil thus brought up may amount to from one-tenth to two-tenths of an inch annually over the entire surface; so that in half a century the entire surface foot might have been thus worked over. Aside from the mechanical effect thus achieved in loosening the soil, and the access of air and water permitted by their burrows, the chemical effects resulting from their digestive process, and the final return of their own substance to the soil mass; also their habit of drawing after themselves into their burrows leafstalks, blades of grass and other vegetable remains, renders their work of no mean importance both from the physical and chemical point of view. The uniformity, lack of structure and loose texture of the surface soil, especially of forests, as compared with subsoil layers of corresponding thickness, is doubtless largely due to the earthworms’ work. It has frequently been observed that when an unusual overflow has drowned out the earthworm population of a considerable area, the surface soil layer remains compacted, and vegetation languishes, until new immigration has restocked the soil with them. Again, the humus formed under their influence is always neutral, never acid.
Wollny (Forsch, Agr., 1890, p. 382), has shown by direct experimental cultures in boxes, with and without earthworms, surprising differences between the cultural results obtained, and this has been fully confirmed by the subsequent researches of Djemil (Ber. Physiol. Lab. Vers. Halle, 1898). In Wollny’s experiments, the ratio of higher production in the presence of the worms, varied all the way from 2.6 per cent in the case of oats, 93.9 in that of rye, 135.9 in that of potatoes, 300 in that of the field pea, and 140 in that of the vetch, to 733 per cent in the case of rape. Wollny attributes these favorable effects in the main to the increased looseness, and perviousness of the soil to air, and diminished water-holding power. Djemil’s results all point in the same direction; and he shows, moreover, that the allegation that the roots penetrate more deeply in the presence of the worms by following their burrows, is unfounded, the descending roots often passing close to and outside of these.
The work of earthworms is especially effective in loamy soils and in the humid regions. In the arid region, and in sandy soils generally, the life-conditions are unfavorable to the worm, and the perviousness elsewhere brought about by its labors already exists naturally in most cases. It is stated by E. T. Seton (Century Mag. for June, 1904) that the earthworm is practically non-existent in the arid region between the Rocky Mountains and the immediate Pacific coast, from Manitoba to Texas. In the Pacific coast region, however, they are abundant, and do their work effectually.
Insects of various kinds are also instrumental in producing, not only the uniform distribution of humus in the surface soil, but also the looseness of texture which we see in forest soils especially. Ants, wasps, many kinds of beetles, crickets, and particularly the larvæ of these, and of other burrowing creatures, often form considerable accumulations, due directly both to their mechanical activity, and to their excrements.
The work of ants is in some regions on so large a scale as to attract the attention of the most casual observer. Especially is this the case in portions of the arid region, from Texas to Montana, where at times large areas are so thickly studded with hills from three to twelve feet in diameter, and one to two feet high, that it is difficult to pass without being attacked by the insects. The “mounds” studding a large portion of the prairie country of Louisiana seem also to be due to the work of ants, although not inhabited at present.
Larger burrowing animals also assist in the task of mixing uniformly the surface soils, and aiding root-penetration, as well as, in many cases, the conservation of moisture. Seton (loc. cit.) even claims that the pocket gophers (Thomomys) in a great degree replace the activity of the earthworms in the arid region, where they, together with the voles (commonly known there as field mice), exist in great numbers. Of course the work of these animals, as well as that of the prairie dogs, ground squirrels, badgers, etc., is incompatible with cultivation. But the effects of their burrows on the native vegetation, and the indications they give of the nature of the subsoil, are eminently useful to the land-seeker.
Thus in the rolling sediment-lands of the Great Bend of the Columbia, the observer is surprised to see the “giant rye grass,” usually at home in the moist lowlands, growing preferably on the crests of the ridges bordering the horizon. Examination shows that this is due to the burrowing of badgers, whereby the roots of the grass are enabled to reach moisture at all times, even in that extremely arid region.