Chapter XIV
ABSORPTION BY SOILS OF SOLIDS FROM SOLUTIONS. ABSORPTION OF GASES. AIR OF THE SOILS.
ABSORPTION OF SOLIDS FROM THEIR SOLUTIONS.
Just as solids have the power of condensing gases upon their surfaces, to an extent proportional to that surface, and therefore to the state of fine division: so fine powders have the power of withdrawing from solutions solids held in solution, to an extent varying with the nature of the substance dissolved, and the absorbing solid. The most commonly-known manifestation of this principle is that sea-water filtering through the sands of the shore, will at a certain distance become sensibly less brackish, and finally so nearly fresh as to be capable of domestic use.[98] The extent to which this occurs is in a measure proportional to the fineness of the sand, and to the amount of clay present in it. This is a clearly physical effect, independent of any chemical action whatever; for it occurs equally with quartz sand, charcoal, glass, limestone, or other rock powders having no chemical effect upon the substance dissolved or upon the liquid dissolving it. Very large amounts of water are often required to remove all the soluble matter thus “adsorbed.”
[98] In many cases this decrease of salinity is probably due to a slow influx of fresh water from landward; but very often it cannot be thus explained.
Decolorizing Action.—One of the commonest applications of this principle is the decolorization of colored solutions by means of finely pulverized charcoal. This property of charcoal, as is well known, is extensively utilized in the arts, and particularly in the refining of sugar; the charcoal used in this case being preferably bone charcoal (“bone black”), which on account of its state of extreme fineness, and separation by the earthy particles with which it is associated, is more effective than any other form. It is rendered still more effective, however, by the extraction of these earthy particles (calcic carbonate and phosphate) by means of acid; for by removal of the earthy particles, the surface of the charcoal is greatly increased, and its decolorizing as well as its absorbing power increases accordingly.
While in one and the same substance the decolorizing effect is more or less directly proportional to the fineness of the particles, corresponding to increased surface, it is nevertheless true that in this case, as in that of the absorption of gases, there are specific differences between different powders; so that for example no other substance can replace charcoal in the decolorizing effect which it produces upon colored solutions. It must not, however, be supposed that there is any special reason why coloring matters, as such, should be taken up by preference. Coloring matters are of all kinds of chemical composition, and have in common only the fact that a relatively small amount produces a very strong coloring effect; hence their name, and hence also the apparently extraordinarily strong effect produced upon them by charcoal.
This effect is not, however, by any means greater than it is in the case of many other compounds which are colorless.
Complex Action of Soils.—The powdery ingredients of soils, of course, share this power with all other powders. In the case of soils, however, the action is almost always much more complex than in that of charcoal, because solutions that are passed through the soil are apt to act chemically upon one or the other of its ingredients, usually resulting in a partial exchange of ingredients between the soil and the solution; one or more of the constituents of the solution being retained by the soil, while one or more of the (basic) soil constituents pass into the solution, in combination with its acidic ingredients.
Thus when a very dilute (½ or 1%) solution of potassic chlorid is filtered through almost any soil, the first portions passing through will be practically free from potash, but will contain the chlorids of calcium and magnesium. But as more of the solution is passed through, potash passes also ultimately without absorption. In addition to the zeolitic and clay portions of the soil, the humus is very effective in absorbing mineral ingredients from solution, and retaining them in such manner as to be readily available to plant growth. (See chap. 8, p. 124.)
In view of the almost invariable conjunction of physical and chemical effects, it may be fairly said that no solution, at least of mineral salts, can pass through the soil without being changed in its concentration and chemical composition. It is sometimes difficult to decide to which of the two classes of effects the several changes may be due.
Purifying Action of Soils.—The disinfecting action of dry soil, absorbing offensive gases from manure piles and from earth closets, has already been alluded to. Similarly it is a matter of common experience that the colored and otherwise offensive drainage from manure piles, tanneries, dyeworks, etc., is not only deodorized but also decolorized when passed through a sufficiently thick layer of clay soil. The filtration through fine sand by which the drinking waters of cities are so commonly purified before delivery to the consumer are familiar examples of the same effects.
Equally familiar, however, is the fact that this power of decolorization and retention of offensive compounds is limited; that after a while the filtering earth or sand becomes saturated, and afterwards the water or drainage will pass through without any sensible purification.
It is therefore clear that this purifying effect of earth cannot be relied upon for the permanent protection of wells from the surface-drainage from barnyard or house refuse. Even if fissures or layers of sand or gravel should not intervene so as to permit of the direct communication of surface-drainage with wells, it is certain that in the course of a few years at most, the intervening earth will become so far saturated with the noxious ingredients that the latter will pass through unhindered, and may contaminate to a considerable extent the domestic supply of drinking water.
Waste of Fertilizers.—The same, of course, holds true in regard to manure-water, or soluble fertilizers of any kind used on the soil of a field. The soil will retain them to a certain extent; but beyond that limit any surplus added will be quickly washed through into the country drainage by the rains. Moreover, a soil once so saturated will yield to rain water filtering through it, notable amounts of all the ingredients absorbed in it; and, at least so far as the physically condensed soluble ingredients are concerned, long-continued leaching with pure water will inevitably result in the withdrawal of additional amounts of absorbed ingredients, apparently dividing themselves up pro rata between the water and the soil.
It is obviously of the utmost importance to the farmer to know to what extent the soil will retain manurial ingredients against the influence of leaching rains; for unless this is taken into consideration, it may readily happen that the fertilizer supplied before a rainy season will be washed through beyond the reach of plant-roots, and so practically become a dead loss.
Absorptive Power Varies.—So far as the mere physical absorption is concerned, it will readily be understood that a coarse sandy soil exercises less retentive influence upon dissolved substances than clay or humous soils. In the humid region, where sand is substantially nothing but granular silica (see above, chap. 6, page 86), the same may be measurably true as regards the chemical absorption also. In the arid region, on the contrary, a great many sandy or silt soils, very poor in clay, exert fully as much chemical absorption as clay soils, and are no more liable to the washing-out of soluble fertilizers introduced than are the latter. For the chemical absorption lies chiefly in the zeolitic portion of the soil (see above chap. 3, p. 37), which in the humid region accumulates in the clay, while in the arid it remains encrusting the sand and silt grains.
Generalities regarding Chemical Absorption and Exchange.—In regard to the leaching-out and absorption or retention of substances important to agriculture, the following general statement may be made:
The substances most likely to be leached out of soils are, of bases: soda, magnesia and lime; of acidic constituents: chlorine, sulfuric acid and nitric acid. Lime sometimes passes off with either of the above acidic ingredients, and also in the form of carbonate.
Substances rather tenaciously retained in soils are: potash and ammonia among the bases, and phosphoric acid among the acids.
Thus (as stated above) when a weak (one or two per cent) solution of potassic chlorid or sulfate is poured upon a column of good soil several inches thick, it will be found that the first portions passing through are free from potash, but contain the chlorids or sulfates of magnesium and calcium. If potassic nitrate be used, lime and magnesia will pass off as nitrates; while in the case of potassic phosphate, both ingredients will be retained. A solution of gypsum (calcic sulfate) will usually cause the passing-off of some of the magnesia, soda and potash contained in the soil, in the form of sulfates; but the amount of potash thus dissolved soon diminishes to a mere trace. Solutions of potassic or amnionic phosphates will be absorbed and retained by the soil to a very considerable extent, before the soil becomes saturated.
While it is true that the degree to which the soil retains the several ingredients may serve in a very general way to indicate their richness or poverty in the same, the attempt to make such experiments serve to determine the agricultural needs of soils has met with but little practical acceptance.
Drain Waters.—The table on p. 22, chapter 2, illustrates forcibly the working of the above principles, which are verified by the composition of drain-waters. In all, the chief nutritive ingredients of plants, except nitrogen, are present in traces only; chlorids, nitrates and sulfates of sodium and magnesium form the bulk of the permanently soluble matter, with usually a considerable proportion of calcic (and magnesic) carbonate, depending upon the amount of the earth-carbonates present in the soil, as well as upon that of oxidizable organic matter from which carbonic acid can be formed. That calcic carbonate filters readily through the soil has already been somewhat elaborately discussed (see chap. 3, p. 41); one of the results being that the surface soil is sometimes almost completely depleted of this important substance, while it accumulates at a greater or less depth in the subsoil, or in underdrains, as the case may be.
Of the ingredients appearing in the above list, the one of greatest agricultural importance is nitric acid, since chlorine and sulfuric acid, as well as soda, are required only in very small quantities by most culture plants; so that they rarely need to be supplied in fertilizers. Nitric acid, however, is not only one of the most important fertilizers, but also the most expensive; hence the passing-off of nitrates in drainage-water is of such serious concern to the farmer, that the causes of its occurrence, and the means of preventing such loss, should be fully understood. This subject will, however, be more fully considered farther on.
The above Distinctions not Absolute.—It should, however, be also understood that while the above statements hold good in a general way, yet the line drawn is by no means an absolute one. For just as in the case of physical adsorption the long passing-through of distilled water will gradually abstract the substances condensed on the surface of the soil-grains, so an overwhelming amount of a solution of any one kind will have a tendency to substitute its own ingredients for those already present in the soil, removing the latter to a greater or less extent, even in the case of potash and phosphoric acid.
As an example in point, may be cited the case of the natural minerals Analcite and Leucite, which Lemberg was able to reciprocally transform from their natural condition of soda- and potash-alumina silicates merely by alternate treatment with solutions of potassium and sodium chlorids respectively. (See chap. 3, p. 37). The same is true in the case of the zeolitic matter of the soil. There is nevertheless a distinct preference in the direction of the retention of potash as against soda; so that in the case of alkali soils, a large excess of potash is found to be present in the zeolitic form, notwithstanding the presence of sometimes very large amounts of the chlorid, sulfate and carbonate of soda. This preferable retention of potash is, of course, of material advantage in the case of the use of soluble potash-fertilizers, as well as in preventing the waste of the potash of the soil itself.
ABSORPTION, OR CONDENSATION, OF GASES BY SOILS.
Like all bodies in a state of fine division, soils are capable of absorbing a not inconsiderable amount of various gases. It may be said that in general, other things being equal, the amount thus condensed on the surface of the soil-grains is more or less directly proportional to the facility with which the gas is condensed by either pressure or cooling. Hence the very large amount of water-gas or vapor which may be absorbed by soils, as shown in a preceding chapter. But excepting perhaps the case of ammonia, moist soils are less absorbent of gases than dry ones.
Oxygen and nitrogen, the main constituents of the atmosphere, being difficultly condensable by either pressure or cold, are absorbed by soils only to a relatively small, yet by no means unimportant extent. The condensation of oxygen within the soil-mass is doubtless of considerable importance in the processes of oxidation, as is shown by its partial replacement by carbonic gas in the free air of the soil (see chap. 2, p. 17). The intensifying of oxidizing action caused by surface condensation is well illustrated in the case of finely divided platinum, in which hydrogen is brought to rapid combustion when mixed with oxygen; as well as by the effect of bedding tainted meat in charcoal powder, when all odors of decay disappear, both by absorption and oxidation, ammonia and carbonic gas alone ultimately escaping through the powder.
Carbonic dioxid and ammonia gases, both normal constituents of the atmosphere, and of high importance to plant nutrition, are more readily condensable than either oxygen or nitrogen, and consequently may be taken up by the soil in larger relative proportions. Especially is this the case with ammonia gas, which is not only readily condensed by pressure, but is also extremely soluble in water; so much so that it rushes into a tube filled with this gas almost as quickly as though it were a vacuum. Water will absorb at the ordinary temperature, under normal pressure, about 700 times its volume of ammonia gas; but inasmuch as the proportion of the latter in the atmosphere amounts to only a few millionths, the actual amount taken up can only (as in the case of all gases) be proportional to its proportion (or “partial pressure”) multiplied into its coefficient of absorption. Consequently, water exposed to the ordinary air can absorb at best only a small fraction of a per cent of ammonia. Its presence in soils can be readily demonstrated by passing through the warmed soil a current of purified air, which is made to bubble through Nessler’s reagent (potassio-mercuric iodid) solution.
Absorption of gases by dry soils.—Perfectly dry soils are powerful absorbers of ammonia, and their absorption of this gas, as well as of carbonic gas, can readily be shown by the arrangement shown on the page opposite.
The two tubes shown to the left are filled with carbonic gas, those to the right with ammonia gas. After being immersed in a mercurial trough, there are introduced into each tube through the mercury small cylinders (conveniently one cubic centimeter in volume) consisting respectively of a very sandy soil or loose hardpan, a gray plastic clay, a gray clay soil or adobe, a very black “adobe” clay, and a highly ferruginous and humous soil (from Hawaii), which gives the highest absorption of all; next brown peat, and pine charcoal. The latter, and the ferruginous soil, were also exposed for the absorption of carbonic gas. All the absorbing cylinders are first heated for an hour to 110°C. (218°F) for the purpose of expelling from them moisture, air, and other absorbed gases. They are then quickly introduced into the tubes through the mercury and allowed to absorb the gases enclosed until the mercury columns cease to show any farther rise; in which condition they are shown in the figure.
It will be seen that this absorption is a different one, not only for each of the different substances used, but is also differently proportioned for the two gases. For it will be noted that while the clay soil has absorbed a very much larger amount of ammonia than the charcoal, and the sandy soil has remained far behind both: yet the charcoal has absorbed a considerably larger proportion of carbonic gas than either the clay or the sandy soil, proving that charcoal has a strong specific absorptive power for carbonic gas, independently of the relative size of clay and charcoal particles respectively. The sandy soil shows, by its low absorption even of ammonia gas, the coarseness of its particles and the scarcity of clay in its composition. The highest absorption of all is shown by the ferruginous soil from Hawaii, containing nearly 40% of ferric oxid together with 3⅓% of humus. The moisture-absorption of this soil at the ordinary temperature is 19.7 per cent. The difference in the absorbing power of the (non-humous) gray clay and gray adobe soil indicates the strong influence of humus upon the absorption; which is still farther emphasized by the difference between the gray and black adobe, the latter containing 1.2% of humus. As to the peat, since its weight was only .5 grams against an average of 2 grams for the soils employed, its absorptive power by weight doubtless exceeds all other substances.
While the experiment shown in the figure serves as a convenient and striking demonstration for lecture purposes, it is of course not adapted to a direct comparison of the absorbing powers of the several substances, because of different heights of the mercurial columns counteracting the atmospheric pressure. For direct comparative measurement the tubes must be sunk in mercury so as to equalize the levels inside and outside, since the corrected volumes obtained by calculation would not serve the purpose.
According to special measurements made under normal atmospheric pressure, the writer found that a black clay soil (“adobe”) absorbed (at 60°F) over two hundred times its bulk of ammonia gas, while under the pressure of one-fifth of an atmosphere (as shown in the photograph) the absorption was one hundred and twenty-three times its bulk. This energetic absorption of ammonia and related gases explains the marked disinfecting effects which a covering of dry earth exerts in the case of cemeteries, manure piles, and earth closets. But the difference between the sandy soil and the clay soil in the amount of absorption admonishes us that in all these cases, to secure disinfection the earth to be used should contain as much clay as possible, and should not be mere sand, as is sometimes the case. It also shows that the addition of charcoal to such materials does not increase their efficacy, as has been supposed, but that an equal bulk of clay would be more efficient.
Of course, so soon as the absorbing cylinders used for this experiment are exposed to the atmosphere, the principle above stated in regard to “partial pressure” asserts itself. The absorbed gases quickly begin to be given off, and in some hours the equilibrium with the ordinary conditions of the atmosphere is re-established. That the strong absorptive power of soils for ammonia is to some extent effective in maintaining the supply of this substance by absorption from the atmosphere, cannot be doubted.
Boussingault, and later Stenhouse, determined the absorptive power of wood charcoal for ammonia to be 90 and 98 volumes respectively.
THE COMPOSITION OF GASES ABSORBED FROM THE ATMOSPHERE BY VARIOUS SOLIDS.
In 1864 and 1865 Reichardt and Blumtritt[99] investigated elaborately the composition of gases driven off by heat from various powders, including soils, exposed to the atmosphere. All the substances examined were therefore “air-dry,” therefore to a certain extent moist; and the presence of this aqueous vapor of course modifies in a measure the results that would have been obtained had the materials used been exposed to dry air only. They found that, as had already been stated by previous observers, the presence of capillary water diminishes materially the absorption of gases, especially of those not as easily absorbed by water as are carbonic gas and ammonia. Contrary to what might have been expected from the more ready condensation of oxygen by pressure or cold, in nearly all cases nitrogen is absorbed to a greater extent than oxygen, and sometimes exclusively so; so that in some cases the latter was found to be present only in traces, as will be perceived from the subjoined table:
COMPOSITION OF GASES ABSORBED FROM THE ATMOSPHERE BY VARIOUS POWDERS ================================+========+======= | 100 | 100 | Grms | Vol’s Substance. |gave cc.| gave | Gas. | Vol’s. | | Gas. ------------------------------=-+--------+------- Charcoal, coniferous, air dry | 16.21 | “ moistened and air-dried | 140.11 | 59.0 “ Lombardy Poplar | 466.95 | 195.4 Peat | 162.58 | Garden Earth, moist | 13.70 | 19.9 “ “ air-dried | 30.28 | 53.6 River Silt, air-dried | 40.53 | 48.07 “ “ slightly moistened | 24.12 | 29.2 “ “ air-dried | 26.52 | 30.05 Clay, long exposed | 25.58 | 39.05 “ slightly moistened | 28.62 | 35.08 --------------------------------+--------+------- Ferric Hydrate, commercial | 251.59 | 275.0 “ “ freshly | 375.54 | 308.6 precipitated, | | air-dried | | --------------------------------+--------+------- Ferric Oxid, ignited | 39.4 | 52.4 Aluminic Hydrate, air-dried | 69.02 | 82.0 “ “ dried at 100°C. | 10.83 | 13.6 Prepared Chalk, 1864-65 | 43.48 | 52.4 “ “ 1865 | 38.98 | 48.0 --------------------------------+--------+------- Calcic Carbonate, precipitated, | 65.09 | 1864-65 | | “ “ precipitated, | 51.53 | 52.0 1865-66 | | --------------------------------+--------+------- Magnesic Carbonate | 729.21 | 124.9 Gypsum, finely powdered | 17.26 | --------------------------------+--------+------- ================================+================================== | 100 vol’s gas contained +--------+------+--------+--------- Substance. | | | | |Nitrogen|Oxygen|Carbonic| Carbon | | | Dioxid |Monoxid --------------------------------+--------+------+--------+--------- Charcoal, coniferous, air dry | 100.00 | 0.0 | 0.0 | 0.0 “ moistened and air-dried | 85.60 | 2.12 | 9.15 | 3.13 “ Lombardy Poplar | 83.60 | 0.0 | 16.50 | 0.0 Peat | 44.44 | 4.60 | 50.96 | 0.0 Garden Earth, moist | 64.34 | 2.85 | 24.06 | 8.75 “ “ air-dried | 64.70 | 2.04 | 33.26 | 0.0 River Silt, air-dried | 67.69 | 0.0 | 18.61 | 13.70 “ “ slightly moistened | 67.34 | 0.0 | 30.56 | 2.10 “ “ air-dried | 67.40 | 9.09 | 16.07 | 7.44 Clay, long exposed | 70.17 | 4.71 | 25.12 | “ slightly moistened | 59.59 | 6.39 | 34.02 | --------------------------------+--------+------+--------+--------- Ferric Hydrate, commercial | 33.26 | 1.43 | 65.31 | 0.00 “ “ freshly | 26.29 | 3.85 | 69.86 | 0.00 precipitated, | | | | air-dried | | | | --------------------------------+--------+------+--------+--------- Ferric Oxid, ignited | 82.87 |13.41 | 3.72 | 0.00 Aluminic Hydrate, air-dried | 40.60 | 0.00 | 59.40 | “ “ dried at 100°C. | 83.09 |16.91 | 0.00 | Prepared Chalk, 1864-65 | 100.00 | 0.00 | 0.00 | “ “ 1865 | 74.49 |15.49 | 10.02 | Calcic Carbonate, precipitated, | 80.81 |19.19 | 0.00 | 1864-65 | | | | Calcic Carbonate, precipitated, | 77.37 |15.09 | 7.54 | 1865-66 | | | | Magnesic Carbonate | 63.92 | 6.72 | 29.36 | Gypsum, finely powdered | 80.95 |19.05 | 0.00 | --------------------------------+--------+------+--------+---------
[99] Journal für praktische Chemie, Vol. 98, p. 167.
Discussion of the Table.—It will be observed that in this table, the largest amount of total gas given off by equal weights of any one substance was in the case of carbonate of magnesia; but it is quite probable that in part, at least, this large amount of gas was due to the evolution of carbonic gas from the easily decomposable carbonate; the more as the analysis of the gases shows over 29% of carbonic gas. But the highest absorption by equal volumes of any substance is shown by the ferric hydrate; next to this by the light poplar charcoal, and next by the carbonate of magnesia. The high absorptive power here shown by the ferric hydrate is of great interest in connection with the facts already stated regarding the absorption of moisture and ammonia by ferruginous soils (see page 274, this chapter); and the fact that the larger proportion of the gas—as much as 70% in one case—consisted of carbonic gas, is particularly interesting in the same connection. Both in the amount of gas contained, and in the proportion of carbonic gas therein, the ferric hydrate exceeds even peat, the representative of humus in soils. It will, however, be noted that in the garden soil, also, the proportion of carbonic gas is very large, while that of oxygen is very low. It is curious to note that in very few cases the proportion of oxygen to nitrogen is the same as in the atmosphere; in most cases the nitrogen predominates considerably beyond its normal proportion, and in two cases, that of charcoal and of calcic carbonate (whiting), the gas was found to consist of pure nitrogen.
We are forced to conclude that the substances here enumerated, as a rule, condense oxygen in smaller proportions than they do nitrogen, or carbonic gas. As regards the carbon monoxid mentioned in the table, it is doubtful that it was contained as such in the substance originally examined; it may readily have been formed under the influence of the heat required in expelling the gases from the substances containing organic matter. Among the important results shown in the table, is the comparative determination of the gases in moist, and in dry garden earth, showing that in the moist earth the amount of gas absorbed ranged from less than one-half down to almost one-fourth that absorbed by the dry. The importance of these differences in the case of the fallow can readily be appreciated.
The changes in the absorptive power brought about by wetting and drying, as shown in the above table, are very insignificant. In the case of the charcoal, soil and silt the diminution may fairly be assumed to be caused by the deposition of soluble salts on the surface, partly clogging the pores. In the case of the clay as well as in that of the river silt, the inevitable content of organic matter in process of decomposition has doubtless influenced the result, as is suggested by the increase of carbonic gas. That prepared chalk should in one case contain exclusively nitrogen gas, in the other case mixed gases, seems to indicate a difference in the air to which it is exposed, or in the water employed in its preparation; the latter case agreeing substantially with the results obtained from the precipitated carbonate. In both (as well as in the carbonates of barium and strontium), the absorption of carbonic gas is very small, or nil.
It thus appears that for the condensation of carbonic dioxid gas, ferric and aluminic hydrates are prepotent among mineral substances; while clays, river silts and soils may always be expected to contain relatively large proportions of this gas in absorption.
THE AIR OF SOILS.
The Empty Space in Soils.—In dry soils the empty space, usually amounting to from 35 to 50 per cent of its volume, is filled with air;[100] in moist or wet soils the space unoccupied by water is similarly filled. Hence when soils are in their best condition for the support of vegetation (chap. 11, p. 202), about one half of their interstices is filled with water, the other half with air. Actual measurements of the amount of air contained in well-cultivated garden soil have been shown by Boussingault and Levy to range between 10,000 and 12,000 cubic feet per acre, substantially agreeing, therefore, with the above statement. In uncultivated forest soil, on the contrary, they found only from somewhat less than 4000 to 6000 cubic feet of air per acre. Extended observations since carried out by Wollny, Ebermayer, and others have in general confirmed the earlier observations, while adding greatly to their significance in respect to their relations to plant growth, and to the process of humification and soil-formation.
[100] The normal composition of atmospheric air is given on p. 16, chap. 2.
As a matter of course, when water evaporates from the soil in drying, its place is taken by air so far as it is not filled by capillary water drawn from below.
Functions of Air in Soils.—That roots require for the performance of their vegetative functions the presence of oxygen, has already been discussed; but there can be no question that the higher productiveness of well-cultivated soils is largely due to the greater and readier access of air to the roots. Apart from this direct function, however, the presence of oxygen in the soil serves other important purposes, and among these doubtless the most dominant is the promotion of the oxidation of the organic matter of the soil through the agency of micro-organisms; and more particularly that of nitrification, which chiefly governs the supply of nitrogen to non-leguminous plants. In the case of leguminous plants, the presence of air as a furnisher of nitrogen as well as oxygen is absolutely essential.
The injurious effects of insufficient aeration of the soil have been repeatedly referred to already (pp. 45, 76). In water-logged soils reductive fermentations are soon set up, and the nitrates of the soils are reduced partly with the evolution of nitrogen gas, partly to ammonia; while their oxygen is consumed to supply the demands of the roots. Ferric oxid is reduced to ferrous carbonate, sulfates to sulfids; thus deranging the whole process of plant-nutrition and absorption of plant-food. If continued for any length of time these conditions end in the death of the plant. Too much importance cannot therefore be attached to the proper aeration of the soil and subsoil.
Excessive Aeration; Compacting the Soil.—On the other hand, excessive aeration of the soil may be injurious in causing a serious waste of moisture; especially in arid climates, where the hot, dry winds may readily destroy the germinating power of the swollen seed when the seed-bed is too loose and open, and later may injure or destroy the feeding roots. The abundant growth of grain often seen in the tracks of a wagon carrying the centrifugal sower, when the stand in the general surface is very scanty, is usually due to the consolidation of the seed-bed, and suggests at once the well-known efficacy of light rolling to insure quicker germination and a better stand. Similarly, the rolling of grain fields in spring is often the saving clause for a crop in dry years. But such needful consolidation must not, of course, be carried to the extent of creating a surface crust which would subsequently serve to waste the subsoil moisture. Hence, the soil-surface should be rather dry when rolling is resorted to.
The pressing of the earth around transplanted plants, similarly, is a needful precaution, not only with respect to the drying-out of the soil, but also to insure close contact between the roots and the soil.
The Composition of the Free Air of the Soil usually differs from the air above, in that besides being saturated with moisture, its nitrogen-content is slightly increased (by one-half to over one per cent); the oxygen-content on the other hand, is diminished, being in part (sometimes nearly to the extent of one-half of its volume) replaced by carbonic gas, derived partly from its secretion by the roots, partly from the oxidation of organic substances. It naturally follows that the richer the soil in the latter, the more carbonic gas will be formed under favoring conditions; so that in freshly-manured land the amount of oxygen transformed into carbonic gas will be greatest, while in the surface-soil of ordinary fields, carbonic gas rarely reaches to as much as one per cent. In all cases, however, the content of carbonic gas in the air of the soil is materially higher than that of the air above it, and thus serves to intensify greatly the solvent and disintegrating effect of the soil water upon the soil materials (see chap. 2, p. 17). The soil-mass itself, however, retains carbonic dioxid with considerable tenacity, so that it is not possible to wash it out completely by filtering water through it. When water containing carbonic gas in solution is filtered through the soil, the gas is sometimes completely absorbed, the water passing off free from gas.
The presence of free carbonic gas in soils is readily demonstrated by passing through the warmed soil a current of air, which is then made to bubble through lime water; a clouding of the latter, and the ultimate formation of a precipitate of calcic carbonate, proves the presence of the gas, and may also serve to measure its amount.
From the fact that the free air in normal soils may contain as much as one-fortieth of its bulk of carbonic gas, besides what may be contained in the condensed form, we may conclude that this gas is formed within them with considerable rapidity; for otherwise, in view of the free communication and diffusion with the outer air, such large amounts could not be maintained in the surface-soil. Doubtless a considerable proportion of the carbonic gas normally contained in the atmosphere is thus supplied from within the soil itself.
Relation of Carbonic Gas to Bacterial and Fungous Activity.—It has been fully demonstrated by the researches of Koch, Miquel, Adametz, Fuelles, Wollny and others, that the formation of carbonic gas in the soil is not a purely chemical oxidation process, but is essentially dependent upon the presence and life-activity of numerous kinds of organisms, bacterial as well as fungous. The crucial proof of this fact is that the presence of any antiseptic diminishes, and if exceeding certain proportions completely suppresses, the formation of carbonic gas; while on the other hand all conditions known to be favorable to the life of such organisms, viz., the proper conditions of temperature and moisture (varying with different kinds), increase the formation of the gas. Such formation is of course, however, conditioned upon the presence of oxygen. In the case of most bacteria, there is a certain limit beyond which the presence of their own product exerts an injurious or repressive effect upon their activity; so that if the gas accumulates beyond that limit, the rate of its formation decreases despite of otherwise favorable conditions.
It follows that the best life-conditions of these organisms (even when anerobic) cannot be fulfilled below a certain limited depth in the soil; and all observations show that their number decreases very rapidly with increasing depth (see chap. 9, p. 142), varying with the perviousness of the soil, but rarely exceeding four or five feet in the humid regions; though doubtless found at greater depths in the arid climates. It is also obvious that the use of any antiseptic or poisonous materials on the field or in the manure pile will tend to disturb and restrain the useful activity of these organisms.
Putrefactive Processes.—Carbonic gas is formed also, but to a much more limited extent, in putrefactive processes, occurring in the absence, or with only limited access, of air or oxygen. These processes likewise are conditioned upon the presence or activity of (largely anerobic) bacteria; but they should not occur in normally constituted, and especially in tilled soils, being as a rule inimical to the growth of cultivated plants (see chap. 9, p. 145).
XV. THE COLORS OF SOILS
The natural coloration of soils forms a prominent part of the characters upon which farmers are wont to base their judgment of land quality; hence the origin and value of soil-colors deserve consideration.
Black Soils.—From the oldest times down to the present a “rich, black soil” has commanded attention and approval. The black and brown-black colors being almost invariably due to the presence of much humus (very rarely to an admixture of carbon [graphite], of magnetic oxid of iron, or sesquioxid of manganese), it is obvious that the farmers’ judgment coincides with a high estimate of the agricultural value of humus. A discussion of this point will be found in another place; but the popular judgment is based quite as much upon the experience had in the advantages that usually accompany the presence of humus. It largely characterizes low grounds, and therefore alluvial lands, whose richness is due to far more general causes. But the shade of the blackness seen in the soil deserves and usually receives close consideration. If tending toward brown, acid humus or “sour” land is indicated; unless indeed the surface soil should be bodily derived from decayed wood, as in the primeval forests. Forest soils in general are usually dark-tinted for some inches near the surface, owing to the presence of leaf mold, and mostly have an acid reaction.
But the black tint is equally welcome to the land-seeker when seen outside of alluvial and forest areas. Belts of “black lands” appear on hillsides and plateaus; and these lands, though clearly not alluvial, are also found to be preëminently productive; witness the upland prairies of the western and southern United States. These black soils are always characterized by the presence of a full supply of lime in the form of carbonate, under the influence of which the most deeply black humus is formed. In other words, the jet black tint is indicative of calcareous lands; and these, as will be more fully shown below, are almost always highly productive.
From both points of view, then, the favorable judgment passed upon black soils by practical men is justified.
But it is not necessarily true that soils showing no obvious black tint are poor in humus; for in strongly ferruginous or “red” soils its tint is frequently wholly obscured, though when still visible it gives rise to the laudatory name of “mahogany land,” which every farmer considers a prize.
Of course then it would be wholly incorrect to judge of the agricultural value of land from its humus-content alone; for its color may be entirely imperceptible and yet its amount and nitrogen content be fully adequate to the requirements of thrifty vegetation. Gray and even whitish soils very frequently fall within this category in the arid region.
The black tint is also favorable to the absorption of the sun’s heat, and is therefore conducive to earlier maturity than is to be looked for in light-tinted lands similarly located.
Wollny (Forsch. Agr. Phys. Vol. 12, 1889, p. 385), discusses the influence of color on soils in relation to moisture and content of carbonic acid. The results show in general simply the effects due to increase of temperature when the soils are either darker-colored throughout, or made so superficially.
“Red” Soils.—Next to a black soil, a “red” one will usually command the instinctive approval of farmers. The cause of this preference is not as obvious as in the case of the black tints; but the general consensus of opinion requires an examination of its claims. It is of course easy enough to adduce examples of very poor “red” soils, derived from ferruginous sandstones that supply little else than quartz and ferric hydrate; the Cotton States supply cogent examples in point, as do also the lower Foothills of the Sierra Nevada of California. It is not, therefore, the iron rust or ferric hydrate that renders the land productive; but its presence is a sign of some favorable conditions. First among these is, that ferric hydrate cannot continue to exist in badly drained soils; a “red” soil is therefore a well-drained one, and this is probably one of the chief causes of the popular preference. The “white land” sometimes seem in tracts otherwise colored with iron, is distinctly inferior in production to the red lands; and examination will generally show that from some cause, such white lands have been subjected to the watery maceration which proves so injurious (see chap. 3, p. 46, chap. 12, p. 231).
That finely-diffused ferric hydrate has a very high power of absorbing moisture as well as other gases of the atmosphere, has been shown in the preceding chapter; it stands in this respect next to humus itself, and hence highly ferruginous soils need not contain as much humus as “white” soils from this point of view. Like humus, also, it renders heavy clay soils more easily tillable.
Origin of Red Tints.—Where crystalline rocks prevail, the red tint usually indicates the derivation from the weathering of hornblende; implying also, outside of the tropics, the presence of sufficient lime in the land. Such lands are naturally preferred to those of lighter tints derived from purely feldspathic rocks (see chap. 3, p. 32), although they may be poorer in potash than the latter.
But the red tint has also its intrinsic advantages in the more ready absorption of the sun’s heat by the colored than by a white surface. This is probably the chief cause of the higher quality of wines grown on red hillsides in the middle and northern vine districts of Europe, where everything that aids earlier maturity is of the greatest importance. The function of ferric oxid as a carrier of oxygen (chap. 4, p. 45) probably also aids nitrification.
“Yellow” lands owe their tint, of course, to smaller amounts of ferric hydrate, but share more or less in the advantages of the “red.”
White soils, or more properly those having very light gray tints, are not usually looked upon with favor, especially in the humid region. The causes of the unfavorable judgment current among farmers in respect to white soils has already been partially explained in the discussion of the black and red tints. The light color means the scarcity or absence of both humus and ferric hydrate, and usually implies that the soil has been subject to reductive maceration through the influence of stagnant water; reducing the ferric hydrate to ferrous salts, oxidizing away the humus, and accumulating in the form of inert concretions most or all of the lime, iron and phosphoric acid of the soil mass (see chap. 3, p. 46, chap. 10, p. 184). The term “crawfishy,” so commonly applied to white soils in the eastern United States, expresses well the usual condition of the white soils of that region; which are very commonly inhabited by crayfish, whose holes reach water a few feet below ground, and are surrounded on the outside by piles of white subsoil mixed with “black gravel” or concretions of bog iron ore. It is needless to say why such lands cannot command the favorable consideration of the farmer; they cannot as a rule be cultivated without previous drainage, and even after that will usually prove unthrifty, “raw,” and in immediate need of fertilization by green-manuring, and the use of phosphates.
In the arid region, lands of this character are of rare occurrence, while (as has been explained above, chap. 8, p. 135), the light gray or “white” tints are there a very common characteristic of even the very best soils. It is true that they are poor in humus and in finely diffused ferric hydrate; but their “light” texture renders the presence of humus for this purpose less needful, and as stated elsewhere (see chap. 8, p. 135), the high nitrogen-content of arid humus renders a smaller supply adequate for vegetative purposes. As to iron, its presence being more important as a sign of good drainage and aeration than directly, its absence from soils of great depth and loose texture is of no consequence; especially when the heat-absorption which it favors is not only not needed, but is usually already in undesirable excess during the hot summers.
White Alkali Spots.—In the valleys of the arid region, however, very white spots commonly indicate the prevalence of alkali salts, and to that extent are an unfavorable indication; especially when coupled with the occurrence of black rings or spots, which indicate the presence of black alkali or carbonate of soda (see chap. 22).
XVI. CLIMATE
Heat and Moisture are the main governing conditions of plant growth. In a preceding chapter the relations of soils and plant growth to water have been considered; in the present one the relations of both moisture and heat to soils and plants will be discussed; and to do this intelligibly to those not making a specialty of such studies, it becomes necessary to introduce, first, a summary consideration of the subject of climate.
Climatic conditions control, and to a great extent determine, the industrial pursuits of every country; all the more so as the rapid communication and transportation by means of modern appliances now brings every part of the globe in competition with every other. The question is not now what it may be intrinsically possible to do under certain climatic and geographical conditions, but whether these things can be done with a reasonable prospect of profit and commercial success, in competition with other countries offering more or less of similar possibilities. While it is true that the cost of labor frequently enters most heavily into such problems, yet favorable or unfavorable climatic or soil-conditions may in many cases turn the scale. Thus the high price of labor and fuel on the Pacific coast of the United States would at first blush seem to render competition with Europe and the East in the production of beet sugar commercially impossible; yet exceptionally favorable climatic and soil-conditions concur to turn the scale in favor of California at least, so as to have placed that state at the head of the sugar-producing states of the Union. A general understanding of the climatic conditions which concern the United States more or less directly, is therefore needful to an appreciation of their agricultural possibilities.
Climatic Conditions.—The factors usually mentioned as constituting climate are temperature, rainfall and winds. Since the latter two factors, however, are themselves merely the result of heat conditions, it is proper to discuss from the outset the origin and mode of action of heat.
TEMPERATURE.
The temperature of stellar space outside of the atmosphere is known to be very low. The increasing cold as we ascend to greater heights, is a fact familiar to all. Langley has calculated upon the basis of observations made at the summit and foot of Mount Whitney in California, that the temperature of space lies near 200° Cent. (360° F.) below the freezing point of water; and this would be the temperature near the Earth’s surface, were it not for the surrounding atmosphere. The latter absorbs but a small amount of the sun’s direct heat rays (which are of high intensity), as they penetrate it to the Earth’s surface. But as the earth’s surface is warmed, the heat rays of low intensity which it emits cannot pass back through the atmosphere to the sun or to outer space; they are “trapped,” as it were, by the dense air resting near the earth’s surface, which is then warmed partly by the radiation from, partly by direct contact with, the soil. It is to the existence of our atmosphere, then, that the possibility of our animal and vegetable life in their present form is due; and a decrease of the trapping effect on the sun’s heat rays makes itself quickly felt when ascending, either in balloons or on high mountains. Moreover, it is well known to mountain climbers that at great elevations the sun’s heat is extremely intense at noon; even though the temperature may fall to the freezing point at night, owing to the failure of the thin air to prevent the radiation back into space of the heat absorbed during the day. On the high plateaus of the Andes and of Asia, therefore, very extreme climates prevail, on account of the great range of temperature between day and night.
Ascertainment and Presentation of Temperature-Conditions.—The proper understanding of the temperature conditions of any locality or region is by no means a simple matter. Shall we study the daily, monthly, or annual changes of temperature, or the means deduced from either or all of them, in order to gain a clear insight into the climatic conditions that control crop production and health conditions?
The Annual Mean Temperature not a Good Criterion.—Since one and the same figure may result equally from the averaging of two widely divergent data, and from such as are close together, it is clear that the mean annual temperature cannot be a proper criterion of the agricultural adaptations of a country. Thus an average temperature of 60° F. might result, equally, from the averaging of 65 and 55 degrees, or from taking the mean of 15 and 105 degrees; yet the respective cultural adaptations would be widely different.
Extremes of Temperature are Most Important.—It is, on the contrary, rather the extremes of temperature, more particularly of cold, but frequently also of heat, together with the total amount of heat available during the growing season, that determines such adaptation so far as temperature is concerned; for no culture plant can be successfully grown where the temperature during winter even occasionally falls for more than a few hours below the point which it can resist; and for each plant there is a certain aggregate requirement of heat to carry it from germination to fruiting. Even different varieties of one and the same plant differ materially in the latter respect, so that it is very important that in the selection of varieties to be grown, this factor should be taken into consideration. It cannot be too strongly urged that the comparison of annual means of temperature, so commonly made by promoters of colonization schemes, must not be taken as a guide either in the estimate of cultures in which the immigrant may desire to engage, or by those in search of a climate adapted to their health-conditions.
Seasonal and Monthly Means.—The statement of the mean temperatures of the conventional four seasons—spring, summer, autumn and winter—afford a much clearer conception of climatic adaptations; provided always that the extreme temperatures be considered at the same time. With the same understanding, the monthly means are still more instructive; but here again, it is most essential that the distribution and amount of rainfall in each be regarded at the same time, since the most desirable temperature is of no avail without the moisture required for vegetation.
In some cases, e. g., that of California, it becomes necessary for practical purposes to regard the “season,” and not the calendar year, as the unit or reference for crop production. There the crops depend upon what rainfall may occur from October to May, there being no summer rains of agricultural significance, and outside of irrigated lands, almost all vegetation save that of trees being in abeyance. In India, there are two distinct growing seasons (“kharif” and “rabi”), corresponding to the two “monsoon” seasons; and no matter how much rain may fall during one, almost total failure may occur in other tropical and arid sections of that country.
The Daily Variations are of interest chiefly with respect to health conditions, since most plants are more adaptable in this respect than the average man.
RAINFALL.
Distribution Most Important.—The summary statements of the annual rainfall are almost equally as deceptive as are those of annual mean temperature, since quite as much depends on the manner in which it is distributed through the year, as upon its absolute amount; and also upon the manner of its fall. Thus Central Montana has the same aggregate annual rainfall as the country surrounding the Bay of San Francisco, viz. about 24 inches; but while in the Franciscan climate this amount of rain falls during one-half of the year, and that the growing season, enabling crops to be grown without irrigation, in Montana the rainfall is distributed over the entire season, so that irrigation is absolutely essential for the successful production of crops. This so much the more as, while the winter snowfall is very light, the rains of summer are largely torrential, running off the surface in muddy floods and giving little time for absorption into the soil. Farther west, in Washington, where grain crops are largely grown without irrigation, the sowing of winter grain is impracticable because the dry summer is immediately followed by the very light snowfall of winter, which falls on dry ground. Fall-sown grain would thus simply lie dormant in the ground through the winter, with great liability to injury from stress of weather in early spring, apart from the depredations of birds and rodents. Hence grain is always sown there in spring only.
These examples may suffice to show that summary statements of either temperature or rainfall by yearly means are of little practical interest to the farmer. What he needs to know is whether or not sufficient rains to mature a full crop are likely to fall during the time that the growing temperature prevails; and what are the minima and maxima of temperature—heat and cold—that his crops will be called upon to endure.
WINDS.
The third climatic factor mentioned, the winds, though proverbial for their unreliability and inconstancy, are not only very incisive in their action, but also to a considerable extent of very definite local or regional occurrence and significance. Moreover, their occurrence, direction, temperature and moisture-condition can, in regions whose climatology has been reasonably well studied, be foretold with sufficient accuracy to be of great use to the farmer.
Heat the Cause of Winds.—As already stated, the primary cause of all winds is heat, substantially on the principle according to which draught is created in our domestic fires. The hot air rising creates an indraught from all directions, especially from that which it can most readily come; viz., from the ocean,[101] or from level lands, rather than across mountain chains. Hence the sea-breeze in the after part of the day, when the land has become heated; while the sea, requiring a much larger amount of heat to change its temperature to a similar extent, remains relatively cool. But at night the earth cools more rapidly than the sea, by radiation; hence toward evening the sea-breeze dies down, and toward and after sunset the land-breeze takes its place.
[101] A striking case in point is the regular wind which in summer blows through the “Golden Gate,” a gap in the Coast Range connecting the Pacific Ocean directly with the great interior valley of California, along the bays of San Francisco, San Pablo, and Suisun. The great interior valley and adjacent mountain slopes becoming intensely heated during the rainless summer, the ascending air is replaced by a steady indraught from the sea, which is bordered by a belt of cold water causing fogs along the coast. The fogs are quickly dispelled on reaching the edge of the valley near the middle of its length; whence steady breezes blow northward and southward, up the valleys of the Sacramento and San Joaquin respectively. These winds, popularly often, but erroneously, called trade-winds, are really “monsoons” similar in their origin to those of India, which, when coming from the sea cause rains, but when from the heated land itself are hot and dry; as in the case of the sirocco of Italy and North Africa, the terral of Spain and the northers of California.
The principle of this local change of winds, together with the rotation of the earth, the absorption of moisture by air, and the fact that the latter becomes cooler when it expands on rising and warmer when it is compressed by descending, serves to explain all the major phenomena usually observed in connection with winds. The air of the equatorial belt, heated throughout the year, necessarily rises and creates an indraught from both north and south; but since the air thus flowing in has a lower rotary velocity than the earth’s surface at the Equator, it lags behind and so gives rise to northeast and southeast winds, respectively, between the two tropics and the equatorial belt. These regular winds, from the aid they give to commerce in passing from continent to continent, are known as the trade winds. On the other hand, the air that has risen from the hot equatorial belt, cooling by expansion as it rises and flowing northward and southward from the Equator, on descending as it mainly does into the temperate zones, has a higher rotary velocity than the land-surface and so tends to give rise to southwest and northwest winds in the northern and southern hemispheres respectively. At sea, on coasts and in level inland regions to windward of mountain chains, such winds are often quite regular during a portion of the year.
Cyclones.—But local disturbances arising from heated land areas or mountain slopes, as well as wide atmospheric changes whose causes are not fully understood, give rise to waves of alternating high and low barometric pressure, largely converting rectilinear or slightly curved wind-motion into whirls or “cyclones”[102] ranging from a thousand to over two thousand miles in diameter. These in the case of low-pressure waves or centers, toward which the air flows from the outside, revolve in the direction contrary to the movement of the hands of a clock, and commonly produce rain in their east portion. A high-pressure wave or center, from which the air naturally flows toward the outside, will usually bring about an “anti-cyclone” area with fair, and in winter cold (“blizzard”) weather, the direction of the whirl being, in this case, the reverse, or in the same direction as the hands of a clock. Both cyclones and anti-cyclones move in North America from west to east, mostly entering from the Pacific Ocean off the northwest coast and traversing the continent with a slight southeast (or in the case of cold weather almost south) trend, with a velocity of twenty to thirty miles an hour; until upon reaching the region of the Great Lakes they generally turn northeastward and pass into the Atlantic Ocean from the New England and Canada coasts.—It is upon these general facts, roughly outlined here, that the weather forecasts are in the main based; taking into consideration, of course, the local or regional conditions, topography, etc., which modify the application of the general rules.
[102] This designation is popularly and incorrectly applied to the comparatively limited, but very violent and destructive rotary storms or whirlwinds which originate locally on the heated plains of the Middle West of the United States, and are almost always accompanied by violent electric phenomena. These should properly be called tornadoes. At sea such whirlwinds give rise to waterspouts, in deserts to sand storms.
In the southern hemisphere, the air-movements substantially correspond to those observed in the northern, so far as not modified by mountain chains; as is especially the case in South America.
INFLUENCE OF TOPOGRAPHY.
Rains to Windward of Mountain Chains.—The surface features or topography of the regions traversed by the air currents or winds may materially modify both their direction and their physical condition, especially as to moisture and temperature. Mountain chains may deflect them, or, causing the air currents to rise on their slopes, and thus to cool by expansion, the moisture these bring with them from the sea may be partially, or sometimes almost wholly, deposited in the form of rain or snow; chiefly on the windward slopes. Then, continuing across the range, the air deprived of most of its moisture cannot readily yield up more; hence the scarcity of rain—“arid climate”—under the lee of mountain chains; as in the Great Basin between the Sierra Nevada and Cascade ranges on the one hand and the Rocky Mountains on the other, and also on the Great Plains under the lee of the latter. The abundant rainfall between the Mississippi river and the Atlantic coast is due to the moist winds coming from the warm waters of the Gulf of Mexico and Caribbean sea, whose access is not interfered with by any cross-ranges of mountains. But the Great Plains lying between the Mississippi and the Rocky Mountains are not within the sweep of the Gulf winds, whose trend is SW to NE; while they are equally out of reach of moisture from the Pacific, all that having been successively deposited on the intervening mountains; hence their deficient rainfall.
Northward of the temperate zone the rainfall generally decreases as we approach the arctic regions; except where the influence of warm ocean currents to windward creates comparatively local exceptions, as in the case of Norway and Alaska.
The general Distribution of Rainfall on the globe is well shown in the annexed diagram, which is copied by permission of the author from his treatise on the “Evolution of Climates,”[103] and represents the mean deduced from data given in the Atlas of Meteorology by J. G. Bartholomew. It is a composite curve derived from the consolidation of four curves showing the distribution of rainfall, viz., on the meridians of 20°E.L.; the west coasts of Europe and Africa; the 30th meridian W.L., in the Atlantic Ocean; and the west coasts of North and South America, projected on the plane of the 100th meridian W.L. The latter curve corresponds with remarkable closeness to the mean curve here given. “It is not intended that these curves should include the rainfall upon meridians on which the distribution in belts is interrupted by continental influences, and by the irregular oblique belts of rain on the east coasts.” But it presents an admirable generalization upon which, as a basis, the local disturbances may be studied.
[103] “The Evolution of Climates”; by Marsden Manson, July, 1903; also Amer. Geologist, Aug.-Oct. 1897.
It will be noted that the maximum of rainfall in the tropical rain-belt lies several degrees to northward of the equator, owing doubtless to the greater land area in the northern hemisphere. There is thus, on the whole, a narrower belt of deficient rainfall or aridity between the tropical and northern temperate rain-belts, than in the southern hemisphere. The southern temperate rain-belt touches only the extreme ends of South America, Africa and New Zealand; elsewhere on the ocean it has not been sufficiently observed as yet. The zones of rainfall and aridity are, however, known to be subject to seasonal oscillations of several degrees in latitude, owing to the obliquity of the plane of the ecliptic, which shifts its position upon that of the equator.
Ocean Currents.—Since water as a fluid is subject to the same circulatory motions which cause winds, it is to be expected that ocean currents should exist corresponding to those of the air, as characterized in general above. But as water warms so much more slowly than air, its circulation would be comparatively insensible were it not for the effects produced by the air currents upon the surface of the sea, combined, as in the case of the winds, with the effects of the rotation of the earth. Without going into the details of the ocean currents in the tropics, it may suffice to say that owing partly to the moving and warming effects of winds, partly to the natural circulatory motion of the water, two great warm currents flow from the tropics northward, materially modifying what would otherwise be the climates of the coasts they touch.
The Gulf Stream.—The current most generally known is the Gulf Stream, flowing partly from the Gulf of Mexico and the Caribbean Sea, partly from outside of the same along the chain of the Lesser Antilles, along the southeast coast of the United States (Florida, Georgia and South Carolina); but owing to its greater rotational velocity it is soon, like the winds of the same latitudes, deflected from a northward to a NE. course, which carries it away from the American coast, to impart some of its warmth, (probably mainly through the winds that blow over it), to Great Britain and Ireland, Scandinavia, and Western Europe generally; while the northern American coast is left to be bathed by the icy polar current flowing from the Arctic through Baffin’s Bay, which carries icebergs far to the south in the way of the transatlantic traffic between the Eastern States and Europe, and causes a difference in climate that is well exemplified in the comparison of the climate of New York with that of Naples, both lying in the same latitude; and similarly of the bleak coast of Labrador with that of Great Britain.
The Japan Stream.—On the eastern Asiatic Coast, a warm current originating in the Sunda seas, flows off the coasts of the Philippines and of China and bathes the Japanese islands; hence it is known as the Japanese Current, or Kuro-siro. It is partly this current which, failing to pass into the Arctic through the shallow waters of Behring strait, renders the coast climates of the northwest coast of America so much milder and moister than is that in corresponding latitudes on the east coasts of both continents. Alaska corresponds to Norway in its moist, foggy and relatively mild coast climate; British Columbia, Washington and Oregon participate in the benefits of the tempering influence of the return current of the Kuro-siro. But as this return (“Alaska”) current passes southward into the warmer seas off the California coast, its influence is reversed; it becomes a cold current in the warm waters of the Pacific, and the warm, moist air of the ocean being carried by the westerly winds across this cold stream which flows along the shore of California, in summer dense fogs are formed, which render navigation difficult and produce a coast climate whose average summer and winter temperatures (e. g. at San Francisco) may differ by only a few degrees, viz., 15.5 and 13.0° C. (60 and 56° F.); so that a change of clothing from season to season is hardly called for. The Alaska Current leaves the immediate coast of California off Pt. Conception near Santa Barbara, gradually losing itself southwestward, but still tempering the tropical heat in the Hawaiian Islands. Hence the coast climate is much warmer and less foggy in southern California; but throughout the State in the interior valleys, screened from the coast winds by the Coast ranges, the temperature in summer may rise several degrees above 100° F. for days together; although, owing to the dryness of the air, the heat is not oppressive.
Contrasting Climates in N. W. America.—An even more striking contrast, showing the effects of the warm ocean and air currents, when intercepted by mountain chains, exists on the Pacific coast farther northward, as already mentioned. In Oregon and Washington first the low Coast ranges, and then the higher Cascade mountains, obstruct the eastward progress of the westerly ocean winds. The result is a very heavy rainfall to coastward of and within the Coast ranges, and an almost equally heavy precipitation on the western slope of the Cascades. Standing on the crest of the latter in summer, one may see to westward a rolling sea of clouds, causing almost daily rains; while to eastward the eye ranges over brown or whitish, dusty plains or rolling lands, almost destitute of tree growth and quivering with heat, under a deep blue sky untroubled by clouds for months.
A somewhat similar contrast is seen in the Hawaiian islands, which are in the sweep of the subtropical northeast trade winds, and on their windward (eastern) slopes have abundant rains; while on the leeward slopes an almost arid climate prevails, calling for extended irrigation.
Continental, Coast and Insular Climates.—From what has been said above, the striking differences of climate caused by the position of any region with reference to the sea or other large bodies of water on the one hand, and to mountain chains on the other, can be readily understood; provided of course that the direction of the winds and the trend of the mountain chains be properly taken into consideration. Western coasts in the temperate and subtropical regions will have a relatively even, temperate and moist climate as compared with the interior of continents, from which the tempering influence of the sea is cut off by mountain chains. Where no such chains intervene the coast climate may extend far inland. The latter case is that of Europe, where the prevailing westerly winds, warmed by the Gulf Stream, temper the climate as far east as the borders of Russia, and northward to Norway; while to southward the warm waters of the Mediterranean and Black seas temper both heat and cold in Spain, southern France, Italy and the Mediterranean border generally. But to eastward, in Russia and Siberia, the climate becomes “continental” to an extreme degree, with very cold winters and very hot summers. The same is true of interior North America, wherever the continental divide cuts off the tempering influence of the westerly winds; Montana, the Dakotas and the Great Plains states generally being examples. The climate of the Mississippi valley, as stated before, is tempered by the winds blowing from the Gulf of Mexico, but with occasional irruptions of the continental climate (sometimes reaching as far east as the South Atlantic coast) in the forms of cold “blizzards,” from which the coast climates of the Pacific and of western Europe are practically free. The Atlantic coast of North America (including the coast of the Gulf of Mexico), moreover, not unfrequently suffers from the violent cyclonic storms that originate in the Antilles and follow more or less the direction of the Gulf Stream.
Islands, differing from continents mainly in their extent, and having a relatively large proportion of coast, naturally have climates controlled essentially by the surrounding ocean. The insular or oceanic climates are therefore, as a rule, more temperate and even than are those of the nearest mainland. It is often said that the climate of western Europe is “insular”; and owing to its position under the lee of the Gulf Stream, this is eminently true of Great Britain.
Subtropic Arid Belts.—Where the surface features of the land in relation to the ocean and prevailing winds do not interpose special obstacles, we find to poleward of both tropics a climatic belt of greater or less width, in which the annual, or at least the summer rainfall is too small to maintain annual herbaceous vegetation throughout the season, even when the temperature is favorable. These two “arid” belts are best defined in Africa, where the northern one is represented by the Sahara desert, lower Egypt and Arabia, while the southern one is exemplified in the Kalahari desert, to northward of the Cape of Good Hope. The northern belt is continued into Asia Minor, Palestine, Syria and Persia, and is again manifest in northwestern India; but to eastward is stopped by the influence of the great Himalaya range. The plateau countries beyond, in Central Asia, are extremely arid, largely by reason of their high elevation.
In Australia the southern arid belt is very strongly defined. In North America, the arid belt is characteristically defined on the Pacific Coast. It embraces all but the southernmost point of the peninsula of Lower California, with about two-thirds of the State of California; thence eastward across Sonora and Arizona to New Mexico and western Texas. But here the influence of the mountain ranges and high plateaus obscures the subtropical belt as such, the arid climate continuing, east of the great Pacific ranges, through Nevada, Utah, Wyoming, Montana, Idaho, and eastern Oregon and Washington nearly to the line of British Columbia on the north, and with gradually decreasing aridity, into Colorado, Kansas, Nebraska, and the Dakotas.
In South America the rainless seaward slopes of southern Peru and northern Chile indicate the southern arid belt; but here, the great chain of the Andes intervening, the dry pampas of Argentina, and the Gran Chaco of southwestern Brazil, like the Nevada basin, though arid would naturally be referred to the moisture-condensing influence of the Andes chain, under the lee of which they lie. From this cause the region of deficient rainfall, which on the western coast ends to northward of Santiago de Chile, is east of the Andes continued much farther poleward, as in North America; reaching into Patagonia.
Utilization of the Arid Belts.—While, as already explained, the distribution of the rainfall through the year is nearly as important as its total amount, yet it is evident that even with the minimum of twenty inches of total precipitation as the measure for crop production, a very large proportion of the land of the arid region cannot, even with the most elaborate system of water conservation, be supplied with sufficient water for ordinary crops, and must be otherwise utilized, mainly for pasture purposes. This is rendered practicable to a much greater extent than might be expected, because the rapid transition from the rainy to the permanent dry season cures the standing herbage into hay, which affords good grazing during the rainless season. Moreover, the use of drought-resistant, browsing forage plants, both shrubs and trees, serves to supplement materially any deficiency in the supply of “standing hay,” especially in case the rains should toward the end be unduly delayed. The same is true of the dried pods and seeds of native herbage, which in some cases (bur clover, lupins, etc.,) afford highly nutritious additions to the leafy forage.[104]
[104] See Rept. of the U. S. Commissioner of Agriculture for 1878, pp. 486-488; Bull. Nos. 16 and 42, Wyoming Expt. Station; Bull. No. 150 Calif. Expt. Station; Bull. No. 51, Nevada Expt. Station; South Dakota Station Bulletins Nos. 40, 69, 70, 74; Kansas Expt. Station, Bulletin No. 102; New Mexico Expt. Station, Bulletin No. 18; Montana Expt. Station, Bulletin No. 30; and others.
XVII. RELATIONS OF SOILS AND PLANT GROWTH TO HEAT
The Temperature of Soils.—The rapid germination of seeds, as well as the development of plants to maturity, is essentially dependent upon the maintenance of the appropriate temperature. The temperature most favorable to germination or growth, as well as the degree of tolerance of high and low temperatures, varies greatly with different plants, governing mainly what is known as their climatic adaptation. A knowledge of these points with reference to the several crops is therefore of no mean importance to the farmer; for, to a certain extent, he can control the temperature in the soil itself, and he can mostly choose for sowing and planting, the time when the soil shall have the proper temperature for rapid germination or maturity. As a rule, it is not desirable to have either seeds or seedling plants in the ground for any length of time when the temperature is too low for active vegetation; for while they rest, other, lower organisms (fungi and bacteria), adapted to low temperatures, may continue in full activity at the expense of the vitality of the crop plant.
Water exerts controlling Influence.—Since the capacity of water for heat is approximately five times greater than that of the average soil, equal weights being considered, it follows that the temperature of soil-water must exert a controlling influence over that of the soil. Taking the case of a cubic foot of loamy soil, fully saturated with water, in which one-third of the volume may be assumed to be water: the weight of the dry soil being about eighty pounds per cubic foot, calculation shows that the amount of heat required to raise the temperature of the water contained, one degree, will be fully twice as great as for that required for the soil itself. It is thus obvious that the control of soil-temperature is largely dependent upon the control of the water-content of the same, which has been discussed in a former chapter. Even in the condition of moisture known to be most favorable to plants, viz., one-half of the maximum water capacity, the influence of the water-content upon the temperature will still be as great as that of the entire soil mass. This consideration emphasizes the importance of such control.
Cold and Warm Rains.—It is not surprising then that the occurrence of cold or warm rains or the use of cold or warm irrigation water at critical periods, may largely determine the success or failure of the crop. It is well known that the occurrence of a cold rain after vegetation has started actively in early spring, may not only destroy the season’s fruit crop by preventing the setting, or thereafter causing the dropping, of the fruit, but may even, if the suppression of vegetative action be continued for some length of time, result in serious injury to, or death of trees. Widely extended disastrous experience of the kind was had in California in February and March, 1887, resulting in the death of tens of thousands of fruit trees and vines during that and the following season. It is obvious that in such a case as this the rapid draining-off of the cold water through underdrains would have materially mitigated, if not wholly prevented, such injury.
Solar Radiation.—Aside, however, from such overwhelming influences as the above, the soil temperatures are measurably controlled by the extent to which they receive and absorb the sun’s heat rays, whether directly or through the mediation of the air. The direct effect of the sun’s rays upon the surface is, upon the whole, the most generally potent, although warm winds may occasionally exert a very strong influence. The varying influence of the sun’s rays depends primarily upon the change of seasons, which themselves result from the varying angles at which the sun’s rays strike the surface; as well as upon the duration of the day. The greater or less cloudiness or fogginess of the sky, of course, exerts a decided effect in this connection.
The Penetration of the Sun’s Heat into the Soil.—In the temperate regions of the earth the daily variations of temperature cease to be felt at depths ranging from two to three feet, according to the nature of the soil material and its more or less compacted condition. The monthly variations, of course, reach to greater depths; while the annual variations do not disappear in the temperate zone, e. g., at Paris, Zürich and Brussels, at a less depth than seventy-five feet. At these depths of constant temperature we find approximately the same temperature as that which we can deduce from the thermometric observations as the annual mean. From similar causes the mean annual temperature of any place may be approximately deduced from the observation of the water of wells and springs derived from moderate depths. For below the level of constant annual temperature the latter begins to ascend steadily as we progress downward, owing to the interior heat of the earth.
Change of Temperature with Depth.—The following table of observations made at Brussels illustrates the decrease of annual range of temperature with depth:
Average Annual At feet: temperature. range. (°C.) (°C.) 3.25 7.2 10.5 15.6 13.5 4.5 30.8 16.4 1.3 75.0 17.0 0.0
It is interesting to compare with this record that of a well sunk by Ermann at Yakutzk, Siberia, where the mean annual temperature is—9.7 C. (14.6 F.). This temperature was found a few feet below the surface. At 50 feet the temperature was—7.2 C. (19° F.); at 145 feet—5° Cent. (23° F.) at 350 feet—.9° C. (30.8° F.) showing that the ground was below the temperature of freezing water for some distance farther down; so that the search for liquid water was abandoned.
We thus see that in the Arctic regions, owing to the presence of water in the form of ice, the melting of which impedes the access of solar heat, the level of no variation is found at the distance of a few feet below the surface, despite the great variations in temperature between the short but hot summer and the extremely cold winter. In the tropics, also, the annual temperature-variation disappears at a less depth than 2 feet, in consequence of the very slight difference between the two seasonal extremes of temperature.
Surface—Conditions that influence Soil-Temperature. Among these, color has already been mentioned, and to a certain extent discussed. While it is true that, broadly speaking, dark-colored soils absorb more of the sun’s heat than light-colored ones, other things being equal, it must still be understood, that the nature of the color-giving substance exerts a very material influence upon the amount of heat absorbed. Thus charcoal is among all known substances the one absorbing and radiating the sun’s heat rays most powerfully, and all kinds alike; so much so, that its absorbent power is taken as 100. But other substances which to the eye appear equally black, have by no means the same absorbing power. The heat absorption by black humus is high, though not quite equal to that of charcoal; and many gray soils, though appearing to the eye of rather light tint, really absorb more heat than others which, to our perception, have the darker tint, but are colored by other substances. Gardeners and especially vine growers in the colder portions of Europe often take advantage of the powerful absorbing power of carbon by spreading charcoal or black slate powder over the surface of the soil where early maturity is specially desired; and slate powder is similarly used by the peasants at Chamouni to hasten the melting of the snow.
Heat of High and Low Intensity.—It must also be kept in view that the surfaces, and especially the colors that favor absorption of the intense rays of the sun, may comport themselves quite differently toward heat rays of low intensity, such as those thrown back from the soil at night when it cools. Were this otherwise, a soil that absorbs much heat in the daytime would lose it with corresponding rapidity at night. But this is true only of charcoal; in the case of most other substances, there is a material difference in favor of the retention of the heat, of low intensity, by slower radiation into a “heat-trapping” atmosphere.
Reflection vs. Dispersion of Heat.—Theoretically, a smooth surface reflects more heat than a rough one, and warms much more slowly by absorption; as is strikingly shown by the use of polished metal screens placed on walls to prevent their being overheated by a flue near by. In the case of soils, also, the condition of the surface affects materially the absorption of heat, but not in accordance with the above rule so far as the result is concerned. For it is found that, other things being equal, a loose or cloddy surface disperses in many directions the heat it receives, and does not permit it to penetrate by conduction to so great extent as would a more compact soil, whose smooth surface would waste less of the heat received by radiation.
King has called special attention to the difference of temperature existing between soils smoothed and compacted by a roller, and the unrolled soil having a loose surface. He found that the former at a depth of one and a half inches was as much as 5.5°C. (10°F.) warmer than the loose soil, and that even at a depth of three inches a difference as high as 3.5°C. (6.5°F.) existed between the two. He observed at the same time that the temperature of the air over the unrolled ground was considerably warmer than above the rolled, thus corroborating the differences observed in the soil itself. But at night the heat is given out more rapidly from the rolled than from the unrolled surface, the latter acting as a non-conductor and keeping the soil warmer than that of the more compact rolled land. King gives as the average difference observed between rolled and unrolled land on eight Wisconsin farms, 1.6°C. (3°F.) in favor of the rolled land between 1 and 4 p.m.
It will thus be seen that the loose tilled layer, while impeding the penetration of the sun’s heat into the deeper portions of the soil during the day, on the other hand serves to retain it at night better than a more compact soil. This obviously places it within the power of the farmer to exert considerable control over the soil-temperature at critical times; restraining or favoring the access of the sun’s heat in accordance with the requirements of the climate or season, as the case may be.
Influence of a Covering of Vegetation, and of Mulches.—A cover of either living or dead vegetation depresses the temperature of the soil as compared with the bare land, as elaborately shown by Wollny and Ebermeyer. In the monthly averages these differences rarely exceed .8° C. (1.5 degrees F.), and are mostly below .50° C. (1° F.), but during different parts of the day they may rise to 2.2 to 2.5° C. (4 to 4.5° F.), at 4 inches depth. In summer they are greater than at other seasons. Of course the density of the vegetation or the thickness of the mulch influences them materially. Forests exert the greatest cooling influence upon the soil, and next to these the dense herbaceous crops, such as clover, and the legumes generally.
Influence of the Nature of the Soil-Material.—Aside from the surface condition, the nature of the material itself exerts a certain influence, not only upon the rate of introduction of heat, but also upon the amount taken up. Thus quartz sand having the highest density (greatest weight per cubic foot) and also the highest capacity for heat among the usual mineral soil-ingredients, will, mass for mass, experience a smaller rise of temperature than would clay or loam soil, of less density or volume-weight, and also of lower heat-capacity. While this holds good theoretically, differences corresponding to this consideration rarely occur in nature, for the reason that the much greater influence of the mechanical condition of the soil mostly overbalances these effects. Thus Wollny has shown that while quartz is a better heat-conductor than clay, quartz cobbles or gravel will materially increase the temperature of the soil in which they are imbedded. Yet compact clay is a better conductor of heat than loose sand; hence the latter, when exposed to the intense heat of the summer sun in the desert, becomes intensely hot on the surface, yet allows of the existence of abundant moisture at a depth of ten or twelve inches; while clay in the same region, being usually in a compacted condition, will show a lower surface-temperature and will be warmer and drier at a depth at which sand will still retain abundant moisture, and be comparatively cool (See chap. 13, p. 257.) So much indeed depends upon the state of mechanical division and flocculation in which the several soils may happen to be, that a hard-and-fast statement in regard to their relations to heat cannot and should not be given, as it would only lead to disappointments and practical mistakes; the more as in all cases the moisture-condition exerts an influence predominating by far over that of the dry material itself, and this moisture-condition is subject to rapid changes, owing to intrinsic differences in the several classes of soils. Wollny states as the result of his experiments, that in summer sandy soils are warmest; then humous, lime and loam soils; while in winter humous soils are warmest, then loams; and sand coldest.
Influence of Evaporation.—In treating of the Conservation of Soil Moisture (chapter 13), the effects, conditions and control of evaporation from the soil have already been discussed from several points of view; so that a summary review of the subject must suffice in this place.
It has been stated above that in the case of an average loam soil saturated with water, the heat required to raise the temperature of the water one degree would be about twice that needed to so change the dry soil material itself. But if it is required to evaporate the same amount of water from the soil, nearly ten (9.667) times that amount of heat will be required; or in the case assumed, twenty times as much as would suffice to raise the temperature of the dry soil through an equal interval of temperature. While in a few cases the cooling of the soil by evaporation is desirable, in the vast majority of cases it is injurious to the progress of vegetation, and should be restricted as much as possible by the means outlined in a former chapter.
Formation of Dew.—There is, however, another aspect of evaporation from the soil which has been long misunderstood, although the true state of the case was partially recognized long ago. Dew is in common parlance said to “fall,” it being supposed that, like rain, it is derived from the atmosphere. While this is partially true, inasmuch as from very moist, and notably from foggy air dew is frequently deposited on grass and foliage generally, as well as on wood and other strongly heat-radiating surfaces; yet as a matter of fact, in by far the majority of cases, as shown by H. E. Stockbridge[105] and confirmed by everyday observation, dew is formed from the vapor rising from the warmer soil into a colder atmosphere, and condensed on the most strongly heat-radiating surfaces near the ground, such as grass, leaves both green and dry, wood, and other objects first encountering the rising vapor. In manifest proof of this it will be noted that very heavy dews may be seen on the ground, when the roofs of houses as well as the higher shrubs and trees remain perfectly dry. In winter this may be most strikingly seen in the deposition of hoar-frost in and immediately around the cracks of plank sidewalks, whose surface remains dry.
[105] “Rocks and Soils,” pp. 175-189.
Dew rarely adds Moisture.—Candid observations will convince any one, therefore, that in most cases the supposed addition to the moisture of the soil from dews is an illusion. Whatever dewdrops fall on the ground are in general simply the return to the soil of a part of what came from it; while the dew that evaporates from the bedewed leaves or other objects represents simply a delayed outgo of moisture from the soil, which for a time retards evaporation direct from the soil, and thus effects a slight saving of moisture.
But while this is measurably true of inland and especially of continental areas like the great plains of North and South America, it is also true that in deep moist valleys, and on the rainy and foggy coast regions of continents, dews are found to both fall and rise, not uncommonly to such an extent as to be equivalent to a not inconsiderable aggregate precipitation. Thus in the moist coast belt of Oregon and Washington lying west of the Cascade range of mountains, the morning dews of summer are frequently so copious that the water falls in showers from the lower trees and shrubs, so as to necessitate the use of water-proof clothing when traversing the woods in the morning, quite as much as though rain was actually falling. In hilly and more especially in mountainous regions the cold air descending from above and flowing down in the ravines will often cause a heavy condensation of dew in these, while the bordering ridges, which rise above the cold currents, remain free from dew. These descending currents as a rule not only bring no surplus moisture with them, but in their downward course become warmer by contraction and therefore relatively drier. In these cases also, therefore, the dew is purely moisture derived from the ground, which in rising encounters the cold air and is thus condensed.
The fact that dew is most commonly derived from the soil could have been foreseen from the other fact, long ascertained and known, that during the night the soil is as a rule warmer than the air above it; as has been shown by the earlier observers, as well as more specifically by Stockbridge.
Dew within the Soil.—It is obvious that whenever dew is formed above the surface of the soil, the air within the latter must be at or near its point of saturation with vapor, as in fact is usually the case a few inches below the surface. It follows that when a depression of temperature occurs within the soil, e. g., at night, dew must be deposited within the soil down to the depth to which the nightly variation reaches, increasing at that depth as the vapor from the warmer soil below rises, to be in its turn condensed. There is thus formed at that level a zone of greater moisture, which may sometimes be noted in digging pits, by a deeper tint, without any corresponding variation in the nature of the soil. The daily repetition of this process, at varying depths, and its greater or less recurrence at or near the limit-levels of monthly and even annual variations, must exert a not inconsiderable influence upon the vertical distribution of moisture in the soil; which instead of being usually found in horizontal bands or zones of varying moisture-contents, is usually remarkably uniform for considerable depths, despite the fitfully recurrent additions from rains. It is at least probable that this process of dew-formation within the soil materially assists capillarity in effecting a measurably uniform vertical distribution of moisture. (See also page 207, chapter 11).
Plant-development under different Temperature Conditions.—In the arctic regions the ground, frozen in winter to unknown depths, may thaw to only three to five feet during the summer, notwithstanding the great length and continuous sunshine of the arctic day. The shallow-rooted arctic flora develops very rapidly under the influence of the continuous daylight and heat, in the course of from five to eight weeks. The seeds of these plants must, of course, be capable of germinating at very low temperatures; and as a matter of fact, we find that both in the arctic regions and in the higher mountains, certain plants are found growing and blooming on slopes flecked with snow; each plant surrounded by a small circle of bare ground, where the snow has been melted under the influence of the dark-tinted earth and leaves. It is clear that here germination has occurred, the foliage has been formed, and the roots have been exercising their vegetable functions, in ground soaked with water practically ice-cold.
Germination of Seeds.—While wild plants of special adaptation may thrive in very low (or high) temperatures, it is also true that few of our cultivated plants will germinate, and still less grow thriftily, at such low temperatures. The limit below which most cultivated plants may be considered as remaining practically inactive lies between 4.4 and 7.2° C. (40 and 45° F.). Few tropical plants will germinate much below 23.8° (75° F.) and in some cases not below 35° Cent. (95° F.). Even maize and pumpkins, according to Haberlandt, germinate most rapidly between 35 and 38.3° C. (95 and 101° F.), while for wheat, rye, oats and flax the best temperature for germination lies between 21.1 to 26.1 (70 to 79°). Under the most favorable conditions of temperature and moisture, some small seeds which readily absorb moisture will germinate in from twenty-four to forty-eight hours, while at a lower temperature they may require from three days to two weeks. Thus Haberlandt found that while oats would germinate in two days at a temperature of 17.2 to 17.5° C. (63° to 63.5°), it took a full week for germination when the temperature was only 5° C. (41° F.). It is obvious that seeds remaining inert in the soil for such lengths of time will be subject to a variety of vicissitudes that may injure or destroy their vitality. There are many bacteria and fungous parasites which at low temperatures are perfectly capable of attacking and destroying the water-soaked seed. There is thus for each plant, from the lowest to the highest, a certain temperature most favorable to development; and both above and below this, the vegetative activity is seriously interfered with or wholly checked. A knowledge of these limits is manifestly of the utmost practical importance.
The influence of too high a temperature in preventing the germination of cinchona seed from India, was curiously exemplified when it was subjected to a supposedly favorable steady temperature of 23.8° C. (75° F.) under otherwise most favorable conditions. Not a single one came up in the course of six weeks, and the box in which it had been sown was put away outside of the hothouse as a failure. Within two weeks a full stand of seedlings was obtained, at temperatures ranging between 12.7 and 15.5° C. (55° and 60° F.). The fact that the cinchona is a tree of the lower slopes of the Andes (three to five thousand feet) although at home strictly within the tropics, explains the apparent anomaly.
PART THIRD.
CHEMISTRY OF SOILS.
Chapter XVIII
THE PHYSICO-CHEMICAL INVESTIGATION OF SOILS IN RELATION TO CROP PRODUCTION.
The chemical constituents of soils have been incidentally mentioned and discussed above, both in connection with the processes of soil-formation, and with the minerals that mainly participate therein. The manner of their occurrence and their relations to plant life, so far as known, must now be considered more in detail.
HISTORICAL REVIEW OF SOIL INVESTIGATION.
While the obvious importance of the physical soil-conditions has long ago rendered them subjects of close study by Schübler[106] Boussingault and others, the chemistry of soils was very generally neglected for a considerable period, after the hopes at first entertained by Liebig that chemical analysis would furnish a direct indication and measure of soil fertility, had been sorely disappointed in respect to the only soils then investigated, viz., the long-cultivated ones of Europe. The results of chemical analysis sometimes agreed, but as often pointedly disagreed, with cultural experiences; so that after the middle of the nineteenth century, but few thought it worth while to occupy their time in chemical soil analysis.
[106] The early work of Schübler on soil physics, published at Leipzig in 1838 under the title of “Grundsätze der Agrikulturchemie” and now almost inaccessible outside of old libraries, is remarkable as having anticipated very definitely much that has since been brought forward and elaborated anew. He is really the father of agricultural physics.
Popular Forecasts of Soil Values.—In newly-settled countries, and still more in those yet to be settled, the questions of the immediate productive capacity, and the future durability of the virgin land are the burning ones, since they determine the future of thousands for weal or woe. This need has long ago led to approximate estimates made on the part of the settler, by the observation of the native growth, especially the tree growth; and where this consists of familiar species, normally developed, such estimates on the part of experienced men, based on previous cultural experience, are generally very accurate; so much so that in many of the newer states they have been adopted in determining not only the market value, but also the tax rate upon such lands, their productiveness, and probable durability being a matter of common note.
Thus in the long-leaf pine uplands of the Cotton States, the scattered settlements have fully demonstrated that after two or three years cropping with corn, ranging from as much as 25 bushels per acre the first year to ten and less the third, fertilization is absolutely necessary to farther paying cultivation. Should the short-leaved pine mingle with the long-leaved, production may hold out for from five to seven years. If oaks and hickory are superadded, as many as twelve years of good production without fertilization may be looked for by the farmer; and should the long-leaved pine disappear altogether, the mingled growth of oaks and short-leaved pine will encourage him to hope for from twelve to fifteen years of fair production without fertilization.
Corresponding estimates based upon the tree growth and in part also upon minor vegetation, are current in the richer lands also. The “black-oak and hickory uplands,” the “post-oak flats,” “hickory bottoms,” “gum bottoms,” “hackberry hammocks,” “post-oak prairie,” “red-cedar prairie,” and scores of other similar designations, possess a very definite meaning in the minds of farmers and are constantly used as a trustworthy basis for bargain and sale, and for crop estimates. Moreover, experienced men will even after many years’ cultivation be able to distinguish these various kinds of lands from one another.
Cogency of Conclusions based upon Native Growth.—Since the native vegetation normally represents the results of secular or even millennial adaptation of plants to climatic and soil-conditions, this use of the native flora seems eminently rational. Moreover, it is obvious that if we were able to interpret correctly the meaning of such vegetation with respect not only to cultural conditions and crops, but also as regards the exact physical and chemical nature of the soil, so as to recognize the causes of the observed vegetative preferences; we should be enabled to project that recognition into those cases where native vegetation is not present to serve as a guide; and we might thus render the physical and chemical examination of soils as useful practically, everywhere, as is, locally, the observation of the native growths. To a certain extent, such knowledge would be useful in determining the salient characters of cultivated soils, also; and would be the more useful and definite in its practical indications the more nearly the cultural history of the land is known, and the less the latter has been changed by fertilization. For, so soon as the first flush of production has passed, the question of how to fertilize most effectually and cheaply demands solution.
It was from this standpoint, suggested by his early experience in the Middle West and subsequently most impressively presented to him in the prosecution of the geological and agricultural survey of Mississippi, that the writer originally undertook, in 1857, the detailed study of the physical characters and chemical composition of soils. It seemed to him incredible that the well-defined and practically so important distinctions based on natural vegetation, everywhere recognized and continually acted upon by farmers and settlers, should not be traceable to definite physical and chemical differences in the respective lands, by competent, comprehensively-trained scientific observers, whose field of vision should be broad enough to embrace concurrently the several points of view—geological, physical, chemical and botanical—that must be conjointly considered in forming one’s judgment of land. Such trained observers should not merely do as well as the “untutored farmer,” but a great deal better.
“Ecological” studies.—Yet thus far we vainly seek in general agricultural literature for any systematic or consistent studies of these relations. We do find “ecological” lists of trees and other plants, or “plant associations,” growing in certain regions or land areas, described in some of the general terms which may refer equally well to lands of profuse productiveness, or to such as will hardly pay for taxes when cultivated. Or when the productive value is mentioned, the probable cause of such value is barely alluded to, even conjecturally, unless it be in describing the “plant formations” as xerophytic, mesophytic or hydrophytic, upon the arbitrary assumption that moisture is the only governing factor; wholly ignoring such vitally important factors as the physical texture of the soil, its depth, the nature of the substrata, and the (oftentimes abundantly obvious) predominant chemical nature of the land. And on the other hand, we find even public surveys proceeding upon the basis of physical data alone, practically ignoring the botanical and chemical point of view, and inferentially denying, or at least ignoring, their relevancy to the practical problems of the farm.[107]
[107] Bull. 22, Bureau of Soils, U. S. Dept. of Agriculture.
Early Soil Surveys of Kentucky, Arkansas and Mississippi.—Among the few who during the middle of the past century maintained their belief in the possibility of practically useful results from direct soil investigation, were Drs. David Dale Owen and Robert Peter, who prosecuted such work extensively in connection with the geological and agricultural surveys of Kentucky and Arkansas; and the writer, who carried out similar work in the states of Mississippi and Louisiana, with results in many respects so definite that he has ever since regarded this as a most fruitful study, and has later continued it in California and the Pacific Northwest. This was done in the face of almost uniform discouragement from agricultural chemists until within the last two decades; with occasional severe criticisms of this work as a waste of labor and of public funds.
Investigation of Cultivated Soils.—All this opposition was largely due to the prejudices engendered by the futile attempts to deduce practically useful results from the chemical analysis of soils long cultivated, without first studying the less complex phenomena of virgin soils; and these prejudices persisted longest in the United States, even though in Europe the reaction against the hasty rejection of chemical soil work had begun some time before; as is evidenced by the methods employed at the Rothamsted Experimental Farm in England, the Agricultural College of France, the Russian agronomic surveys, and at several points in Germany. In none of these cases, however, more than the purely chemical or physico-chemical standpoint was assumed; although in Russia at least, virgin soils were easily obtainable and their native growth verifiable; and were actually in part made the subject of chemical investigation.
In the course of their work, Owen and Peter always carefully recorded the native vegetation of the soils collected; but neither seems to have formulated definitely the idea that such vegetation might be made the basis of direct correlation of soil-composition with cultural experience. Owen repeatedly expressed to the writer his conviction that such a correlation could be definitely established by close study; but early death prevented his personal elaboration of the results of his work. Peter likewise stoutly maintained to the last his conviction that soil analysis was the key to the forecasting of cultural possibilities; but not being a botanist he did not see his way to put such forecasts into definite form.
Change of Views.
In the United States as well, the ancient prejudices have now gradually given way before the urgent call for more definite information than could otherwise possibly be given to farmers by the experiment stations, most of whose cultural experiments, made without any definite knowledge of the nature of the soil under trial, were found to be of little value outside of their own experimental fields. Even the multiplication of culture stations in several states, unaccompanied by soil research, is found to be a delusive repetition of the same inconclusive, random experimenting, since it takes into consideration only the climatic differences, but leaves out of consideration the potent factors of soil quality and soil variations. At most these were usually mentioned by them in such indefinite terms as “a clay loam,” “a coarse sandy soil,” “gray sediment land,” and the like; frequently not even with a statement of the depth and character of the subsoil and substrata, much less of their geological derivation or correlations. Thus any one not happening to be personally acquainted with the land in question would be wholly without definite data to correlate the results with his own case. It is quite obvious that even if only to make possible the identification of new lands with others that have already fallen under cultural experience, and can therefore afford useful indications to the new settler, a close physical and chemical characterization of lands should be made the special object of study by the experiment stations and public surveys, particularly in the newer states.
Advantages for Soil Study offered by Virgin Lands.—Among the special advantages, then, offered by virgin soils for the study of the correlations of soils and crops, the usual existence of a native flora, representing the results of secular adaptation, is of fundamental importance. As it is at this time still historically known of most lands west of the Alleghenies what was their original timber growth, it is clear that their original condition can still be ascertained by comparison with uncultivated lands of similar growth, usually not very far away; and as their cultural history also is largely within the memory of the living generation, the behavior of such lands under cultivation is known or verifiable. Foremost among the data thus ascertainable is the duration of satisfactory crop production, and its average amount. To ascertain these surviving data by inquiry among the farming population should be among the foremost duties of those connected with soil surveys; and persons temperamentally unable to enlist the farmer’s sympathy and interest in such inquiries must be considered seriously handicapped, no matter what their scientific qualifications may be. In no quest is it more literally true that there is no one from whom the earnest inquirer may not learn something worth knowing.
Practical Utility of Chemical Soil-Analysis; Permanent Value vs. Immediate Productiveness.—In many existing treatises so much emphasis is given to the alleged proofs of the inutility of chemical soil examination in particular, that a special controversion of these arguments seems necessary, in connection with a detailed statement of what can, and in part has been, done in that direction. Hence the often-repeated allusion, in the sequel, to points bearing on this question. Hence, also, the detailed discussion of many points which in most agricultural publications are given only passing notice.
In all these discussions the difference between the ascertainment of the permanent-productive value of soils, as against that of their immediate producing capacity, must be strictly kept in view. The former interests vitally the permanent settler or farmer; the latter concerns the immediate outlook for crop production, the “Düngerzustand” of the Germans. The methods for the ascertainment of these two factors are wholly distinct, even though the results and their causes are in most cases intimately correlated. The failure to observe this distinction accounts for a great deal of the obloquy and reproach that has in the past so often been heaped upon chemical soil-analysis and its advocates.
PHYSICAL AND CHEMICAL CONDITIONS OF PLANT GROWTH.
While it is true that plants cannot form their substance or develop healthy growth in the absence or scarcity of the chemical ingredients mentioned on page xxxi of this volume, it is also true that they cannot use these unless the physical conditions of normal vegetation are first fulfilled. Both sets of conditions are intrinsically equally important and exacting as to their fulfilment; and the farmers’ task is to bring about this concurrence to the utmost extent possible. The chemical ingredients of plant-food can, however, be artificially supplied in the form of fertilizers, should they be deficient in the soil; but as has been shown in the preceding pages, it is not always possible to correct, within the limits of farm economy, physical defects existing in the land. Hence, however important is the natural richness of the soil in plant-food, the first care should always be given to the ascertainment of the proper physical conditions in the soil, subsoil and substrata. Without these, oftentimes, no amount of cultivation, fertilization and irrigation is effective in assuring profitable cultural results.
Condition of the Plant-food Ingredients in the Soil.—But even the abundant presence of the plant-food ingredients, as shown by analysis, will not avail, unless at least an adequate portion of the same exists in a form or forms accessible to plants. Of course this condition would seem to be best fulfilled by the ingredients in question being in the water-soluble condition. But in the first place, plants are quite sensitive to an over-supply of soluble mineral salts, as is evidenced by the injurious effects produced by the latter in saline and alkali lands. Furthermore, substances in that form would be very liable to be washed or leached out of the soil by heavy rains or irrigation, and would be lost in the country drainage. It is therefore clearly desirable that only a relatively small proportion of the useful soil-ingredients should be in the water-soluble or physically absorbed condition, but that a larger supply should be present in forms not so easily soluble, yet accessible to the solvent action which the acids of the soil and of the roots of plants are capable of exercising. This virtually available supply we may designate as the reserve food-store.
Finally, there is practically in all soils a certain proportion of the soil-minerals in their original form, as they existed in the rock-materials from which the soil was formed. These minerals being usually in a more or less finely divided or pulverulent condition, they are attacked much more rapidly by the chemically-acting “weathering” agencies, viz., water, oxygen, carbonic and humus acids, than when in solid masses; and thus, transformation of the inert rock-powder into the other two classes of mineral soil-ingredients progresses in naturally fertile soils with sufficient rapidity to produce, in a single season, sensible and practically important results, known as the effects of fallowing.
The Reserve.—The nature of these processes has been discussed in chapters 1 to 4; and it will be remembered that two of their most prominent results are the formation of clay, and of zeolitic-compounds, the latter being, as heretofore stated (pp. 36 ff) hydrous silicates of earths and alkalies, easily decomposable by acids, and also capable of exchanging part or the whole of such basic ingredients with solutions of others that may enter the soil. These zeolitic compounds therefore fulfil two important functions in the premises, viz.: a ready yielding-up of part of their ingredients to acid solvents, and a tendency to fix, by exchange, a portion or the whole of the soluble compounds that may be set free in, or brought upon the land. The first-mentioned property is of direct avail in that the soil-humus forms, and the roots of plants exude, acid solvents on their surface, and can thus draw upon the reserve store of food; the second tells in the direction of preventing the waste of water-soluble manurial ingredients supplied to, or formed in the soil. (See above, chapter 3, page 38).
The reserve food-store may then be placed under the following heads:
Hydrous or “zeolitic” silicates, from which dilute acids can take up the bases potash, soda, lime and magnesia. These silicates may be in either the gelatinous or powdery form; in the former case they may also occlude water-soluble substances.
Carbonates of lime and magnesia, which are readily dissolved by carbonated water as well as by the vegetable acids.
Phosphates of lime and magnesia, not very readily soluble in carbonated water, but more readily attacked by the acids of the soil and of plant roots; thus supplying phosphoric acid to plants. The more finely divided they are the more readily they are dissolved; some soils containing only crystalline needles of apatite (see chap. 5, p. 63) only are nevertheless poor in available phosphoric acid.
The natural phosphates of iron and alumina are practically insoluble in all solvents at the disposal of vegetation and though present in considerable amounts in some soils, (see chapter 19, page 355), may be considered as being permanently inert, and therefore not to be counted among the soil resources for plant nutrition. As yet no artificial process by which their phosphoric acid can be made available within the soil, has been discovered.
Water-soluble Ingredients.—As regards these it has already been explained that they are largely retained in the condition of purely physical adsorption, as in the case of charcoal or quartz sand, through which sea water filters and is thereby partially deprived of its salts. But these can be gradually withdrawn by washing with pure water alone, and still more easily when stronger solvents are used. Since the soil-water is always more or less charged with carbonic acid, and the roots themselves secrete carbonic as well as stronger acids in their absorption of mineral plant-food, there is no difficulty about explaining the manner in which such physically condensed ingredients are taken up.[108]
[108] Whitney (Bull. 22, U. S. Bureau of Soils) claims on the basis of a large number of (three-minute) extractions of soils made with distilled water, that these solutions are essentially of the same composition in all soils; that all soils contain enough plant-food to produce crops indefinitely; and that the differences in production are due wholly to differences in the moisture supply, which he claims is, aside from climate, the only governing factor in plant growth. The tables of analytical results given in Bull. 22 fail to sustain the first contention; the second is pointedly contradicted both by practical experience, and by thousands of cumulative culture experiments made by scientific observers; the third fails with the second, except of course in so far as an adequate supply of moisture is known to be an absolute condition both of plant growth, and the utilization of plant-food. It is moreover well known that it is not water alone, but water impregnated more or less with humic and carbonic acids, that is the active solvent surrounding the plant root.
Recognition of the Prominent Chemical Character of Soils. In a former chapter the soils formed from the several minerals and rocks have been discussed in a general manner. We can as a rule obtain some insight into the nature of any soil which we can trace to its parent rock or rocks, if we are acquainted with the composition of the latter.
Similarly, but in a much more direct manner, we can obtain a strong presumption as to the nature of any soil by determining the undecomposed minerals present in it. In all ordinary cases the presumption must be that the decomposed portion of the soil has been derived from the minerals still found in it. Of course it may happen in the case of lands derived from widely distinct and distant regions that no such characteristic minerals can be found; this is very commonly true of the soils forming the deltas of large rivers, in which sometimes the only remaining recognizable mineral is quartz in its several forms, with occasional grains of such hardy minerals as tourmaline, garnet, etc. Apart from such cases, the hand lens or the microscope permits us to recognize in most soils the minerals that have mainly contributed to their formation, thus also gaining a clew to their prominent chemical nature.
Such recognition sometimes involves, of course, a somewhat intimate knowledge of mineralogy; yet a little practice will enable almost any one to identify the more important soil-forming minerals, under the lens or microscope, according to the degree of abrasion or decomposition they may have undergone. The details of such researches lie outside of the limits of this treatise, but some general directions on the subject are given farther on.[109]
[109] See Appendix B.
Acidity, Neutrality, Alkalinity.—A test never to be omitted is that of the reaction of the soil on litmus or other test paper, to ascertain its acid, neutral or alkaline reaction. Should the latter occur quickly (by the prompt blueing of red litmus paper), “black alkali” would be indicated; but a blueing after 20 to 30 minutes means merely that a sufficiency of lime carbonate is present. An acid reaction (the reddening of blue litmus paper) of course indicates a “sour” soil (see chap. 8, page 122).
Chemical Analysis of Soils.—When the observations mentioned above give no very decisive results or inferences as to the soil’s chemical character, the more elaborate processes of qualitative and quantitative chemical analysis may be called in. It would seem at first sight that these ought to yield very definite results to guide the cultivator; yet such is by no means always the case. Both the previous history of the land, and the method of analysis, influence materially the practical utility of the results of chemical soil analysis.
The cause of this uncertainty becomes obvious when we consider the three groups of ingredients outlined above, viz., the insoluble or unavailable, wholly undecomposed rock minerals; the “reserve,” consisting of compounds not soluble in water but soluble in or decomposable by weak acids; and the water-soluble portion, either actually dissolved in the water held by the soil, or held by the soil itself in (physical) absorption. While the latter portion is directly and immediately available to plants, the amounts thus held are usually quite small, and (outside of alkali lands) would rarely suffice for the needs of a crop during a growing season.[110] This demand must be materially supplemented by what can be made available from the soil minerals and the “reserve” by weathering, conjoined with the direct action of the acids secreted from the plant’s root-hairs upon the soil particles to which they are attached. It is obvious that the greater or less abundance of the plant-food in the soil-material upon which these processes may be brought to bear, must essentially influence the adequacy of the plant-food thus supplied. Moreover, the greater or less extent to which these sources may have been drawn upon previously in the course of cultivation, will similarly influence that adequacy, on account of the diminution of the readily available supply.
[110] The investigations of King (On the Influence of Soil Management upon the Water Soluble Salts in Soils and the Yield of Crops, Madison, 1903) show that from some soils at least, a sufficiency of plant-food ingredients for a season’s crop may be dissolved by distilled water alone, if the soil be repeatedly leached and dried at 110°. Whether such a supply can be expected under field conditions, remains to be tested.
Water-soluble and Acid-soluble Portions most Important.—It thus seems that while the undecomposed rock minerals are indicative of the nature of the soil, but not directly concerned in plant nutrition, the most direct interest attaches to the water-soluble portion, and the acid-soluble reserve. Both of these can, of course, be withdrawn from the soil by treatment with acids of greater or less strength; and it would seem that if we knew just what is the kind and strength of the acid solvent employed by each plant, we could so imitate their action as to determine definitely whether or not the soil contains an adequate or deficient supply of actually available food for the coming crop.
We Cannot Imitate Plant-root Action.—In this, however, we encounter serious difficulties. The acids secreted by the plant roots are not the only solvents active in the dissolution of plant-food; as yet we know the nature of only a few; and even these, instead of acting for a long time (season) on a relatively small number of soil particles touched by the root-hairs, can in our laboratories only be allowed to act for a short time on the entire soil-mass. Clearly, the results thus obtained cannot be a direct measure of the amount of plant-food which a plant may take up in a given time; we can only gain comparative figures. These, however, can be utilized by comparison with actual cultural experience obtained in similar cases.
Cultural experience must, of course, be the final test in all these questions; and it is generally more fruitful to investigate the causes underlying such actual practical experience, than to attempt to supply, artificially, the supposed conditions of plant growth. The latter are so complex and so difficult of control, that the results obtained by synthetic, small-scale experiments are constantly liable to the suspicion that they are partly or wholly due to other causes than those purposely supplied by the experimenter.
Analysis of Cultivated Soils.—It is also clear that in view of the inevitable complexity of the conditions governing vegetable growth, we should whenever feasible proceed from the more simple to the more complex. The failure to conform to this rule in soil investigation has been the cause of an enormous waste of energy and work bestowed, at the very outset, upon the most complex problem of all, viz., the investigation of soils long cultivated and manured; lands which, having been subject perhaps for centuries to a great and wholly indefinite variety of crops and cultural practices, had thereby become so beset with artificial conditions that without a previous knowledge of what constitutes the normal regime in natural soils, the correlation of their chemical constitution, as ascertainable by our present methods, with their production under culture, became as complex a problem as that of motions of three mutually gravitating points in space. Neither can be solved by the ordinary processes of analysis, chemical or mathematical. Nevertheless, though it was at one time contended that the minute proportion of plant-food ingredients withdrawn from soils by cultivation could not be detected by quantitative analysis, numerous examples have shown that with our present more delicate methods this can in most cases be done, though not always after a single year’s crop.
Methods of Soil Analysis.—The more or less incisive solvent agents used in extracting a soil for analysis will of course produce results widely at variance with each other. When fusion with carbonate of soda, or treatment with fluohydric acid is resorted to, we obtain for each soil-ingredient the sum of all the amounts contained in each of the three categories—the unchanged minerals, the zeolitic “reserve,” and the water-soluble portion. It was early recognized that the results of such analyses bear no intelligible relation to the productive capacity of soils; for pulverized rocks of many kinds, or volcanic ashes freshly ejected and notoriously incapable of supporting plant growth, might be made to give exactly the same composition. The amounts of plant-food ingredients thus shown might be several hundreds or thousands of times greater than what one crop would take from the soil, and yet not an ear of grain could be produced on the material. The only case in which any useful information could be thus obtained would be that of the absence, or great scarcity, of one or more of the plant-food ingredients.
The next step was to use in soil analysis acids of such strength as to dissolve all the zeolitic (and water-soluble) portion, leaving the unweathered soil minerals behind; it being assumed that the prolonged action of the roots and soil-solvents would in the end act similarly to the acids employed, such as chlorhydric or nitric acids.
But here also the results of analysis very commonly failed to correspond to cultural experience in the case of cultivated soils; which frequently failed utterly to produce satisfactory crops even when the acid-analysis had shown an abundance of plant-food ingredients. Upon this evidence, this method of soil investigation was also condemned as being of little or no practical utility; and this has ever since been a widely prevalent view.
The preferable investigation of cultivated soils was due to the fact that they are practically the only ones available in the countries where the study of agricultural science was then being prosecuted; and the paucity of useful results there achieved discouraged the undertaking of similar researches where, as in the United States, the materials for the investigation of the simpler cases—those of unchanged, natural or virgin soils—were readily accessible. It was not apparent on the surface that the indefinitely varied conditions introduced by long culture would inevitably cause this lack of definite correlation between the immediate productive capacity of a soil and the composition of its acid-soluble portion, and that yet the same might not be true of natural, uncultivated soils, which have all been subjected, alike, only to the natural processes of weathering, and to the annual return of nearly the whole of the ingredients withdrawn by plant growth.
Following the failure of the treatment with strong acids to yield with cultivated soils results definitely correlated with cultural experience, numerous attempts were made to gain better indications by the employment of weaker acid solvents. The pure arbitrariness of such diluted solvents was equaled by the total indefiniteness and irrelevance of the results with different soils. Only two rational alternatives seem to remain, viz., either to push the extraction to the full extent beyond which action becomes so slow as to clearly exclude any farther effective action of plant acids; or else to use the latter themselves at such strengths as by actual experiment is found to exist in their root sap. The first alternative aims to ascertain the permanent productive values of soils; the latter to test their immediate productive capacity. Both alternatives are purely empirical, and derive their only claim to practical value from their accordance with practical experience (see chapter 19).
THE SOLVENT ACTION OF WATER UPON SOILS.
The almost universal solvent power of pure water has already been alluded to in chapter 2 (see p. 18), and illustrated by the analyses of drain and river waters. While these convey a general idea of the chief substances dissolved and carried off, the direct investigation of the solutions actually obtainable from the soil by longer treatment and with no more water than is compatible with the welfare of ordinary crops, necessarily gives somewhat different results. For when drains flow during or after heavy rains the water has not time to become saturated. The following data afford a clearer insight into the actual and possible solvent effects of water in the soil, and its possible adequacy to plant nutrition unaided by acid solvents.
Extraction of Soils with Pure Water.—Eichhorn and Wunder treated soils from Bonn, and from Chemnitz (Saxony) respectively for ten days and four weeks with about one-third of their weight of water; the solutions thus obtained contain in 1,000,000 parts:
==========================+=======+=========== | Bonn. | Chemnitz. --------------------------+-------+----------- Silica | 48.0 | 25.7 Potash (K₂O) | 115.4 | 7.5 Soda (Na₂O) | 11.0 | 30.4 Lime (CaO) | 128.0 | 83.6 Magnesia (MgO) | 38.4 | 37.4 Peroxid of Iron (Fe₂O₃) | Trace | 11.7 Alumina (Al₂O₃) | ? | ? Phosphoric acid (P₂O₅) | 31.0 | Trace Sulfuric acid (SO₃) | 100.2 | Chlorid of Sodium (NaCl) | 58.6 | 47.6 ---------------------------+-------+-----------
These figures differ widely in most respects from those given for drain and river waters. Potash especially is far more abundantly present in the Bonn-soil solution than in the drain water, and so is phosphoric acid; while lime is not widely different. Eichhorn therefore calculates that with a reasonably adequate supply of water, these ingredients would fully suffice for a full crop of wheat. The Chemnitz soil, on the other hand, does not yield enough plant-food for more than a very small crop upon the same assumptions.
Continuous Solubility of Soil-ingredients.—It seems to be impossible to exhaust a soil’s solubility by repeated or continuous leaching with water. This was demonstrated in 1863 and 1864 by Ulbricht[111] and by Schultze;[112] their general conclusions have quite lately been corroborated by King,[113] as the result of extended and very careful investigations.
Schultze experimented on a rich soil from Mecklenburg, by continuous leaching with distilled water for six days, one liter passing every twenty-four hours, with the following results.
[111] Vers. Stat. V. p. 207.
[112] Ibid. VI. p. 411.
[113] Proc. Ass’n Prom. Agr. Sci. 1904.
RICH SOIL FROM MECKLENBURG (Schultze.) 1,000,000 PARTS OF EXTRACTS CONTAINED: ==============+============+===========+==========+========== |Total matter|Organic and|Inorganic.|Phosphoric | dissolved. | volatile. | | acid. --------------+------------+-----------+----------+---------- First extract | 535.0 | 340.0 | 195 | 5.6 Second “ | 120.0 | 57.0 | 63 | 8.2 Third “ | 261.0 | 101.0 | 160 | 8.8 Fourth “ | 203.0 | 83.0 | 120 | 7.5 Fifth “ | 260.0 | 82.0 | 178 | 6.9 Sixth “ | 200.0 | 77.0 | 123 | 4.4 | ------- | ----- | --- | ---- Total | 1,579.0 | 740.0 | 839 | 41.4 --------------+------------+-----------+----------+----------
It thus appears that while the first extraction removed the main portion of the organic matter, the inorganic matters dissolved were not greatly diminished in subsequent leachings; and that phosphoric acid continued to come off to the last. The rich soil used in this case gave results corresponding in general to these from the Bonn soil, in the previous table. From a poorer soil similarly treated by Ulbricht, described by him as a ferruginous sand from Dahme, the leaching of which was continued for thirty days in periods of three days each, with a total of forty times its weight of water, the results were as follows:
SOIL OF LOW PRODUCTION FROM DAHME (Ulbricht). THE SEVERAL EXTRACTS CONTAINED IN 1,000,000 PARTS: ============+========+========+========+========+========+======== | First | Second | Third | Fourth | Fifth | Sixth |Extract.|Extract.|Extract.|Extract.|Extract.|Extract. ------------+--------+--------+--------+--------+--------+-------- Potash | 7 | 6 | 7 | 7 | | 3 Soda | 41 | 11 | 26 | 17 | | 8 Lime | 96 | 70 | 55 | 48 | 62 | Magnesia | 14 | 10 | 9 | 7 | 8 | Phosphoric | | | | | | acid | trace | 2 | trace | 1 | | + ----- + -- + ----- + -- | -- + -- Totals | 158 | 99 | 97 | 80 | 70 | 11 ------------+--------+--------+--------+--------+--------+--------
It will be seen that there is a considerable difference both in the total amounts of matters dissolved and in the phosphoric acid taken out by the water, as compared with the rich soil treated by Schultze. The uniformity of the amounts of potash removed at the successive leachings is remarkable.
King’s Results.—The same general features are again strikingly illustrated by King’s results, as given in the following table. King’s first leachings were always made by shaking up the soil with ten times its dry weight of water for three minutes, then after subsidence filtering the solutions through a Chamberland (porcelain biscuit) filter, and then (without evaporation) determining the ingredients dissolved, by very delicate, mostly colorimetric methods. Subsequent leachings were made by packing the soil around the filters and washing with five times the weight of water, taking about fifteen minutes each time; but drying the soil at 120 degrees C. between successive leachings.
WATER EXTRACTION OF SOILS OF LOW AND HIGH PRODUCTION, BY F. H. KING. PARTS PER MILLION. =======================+=======+=======+=========+=========== |Potash,| Lime, |Magnesia,| Nitric Extractions | K₂O. | CaO. | MgO. | Acid, | | | | N₂O₅. -----------------------+-------+-------+---------+----------- SOILS OF LOW PRODUCTION. Sassafras sandy 1 | 12.62| 74.39| 17.82 | 18.03 soil. 11 | 218.25| 135.35| 147.45 | 21.76 -----------------------+-------+-------+---------+----------- Norfolk, North 1 | 21.17| 58.30| 22.91 | 30.64 Carolina sandy soil 11 | 166.08| 162.98| 125.00 | 27.11 Average. | 192.60| 149.20| 136.23 | 24.44
SOILS OF HIGH PRODUCTION. Janesville, Wis. 1 | 25.35| 135.30| 51.72 | 55.10 Loam. 11 | 313.70|1120.30| 500.60 | 51.42 -----------------------+-------+-------+---------+----------- Hagerst’wn, Pa. 1 | 21.73| 165.25| 76.88 | 25.72 Clay loam. 11 | 301.55| 967.80| 463.15 | 96.04 Average. | 307.60|1044.05| 487.88 | 73.73 -----------------------+-------+-------+---------+-----------
=======================+==========+========+========+========+====== |Phosphoric|Sulfuric|Carbonic|Chlorin,|Silica Extractions | Acid, | Acid, | Acid, | Cl₂. | SiO₂. | P₂O₅. | SO₃. | CO₂. | | -----------------------+----------+--------+--------+--------+------ SOILS OF LOW PRODUCTION. Sassafras sandy 1 | 7.41 | 53.84 | 13.94 | 1 | 5.60 soil. 11 | 64.16 | 203.96 | 221.33 | 2 | 70.20 -----------------------+----------+--------+--------+--------+------ Norfolk, North 1 | 10.15 | 42.82 | 20.42 | 1 | 8.24 Carolina sandy soil 11 | 80.34 | 172.42 | 148.52 | 2 |122.20 Average. | 72.25 | 126.13 | 184.93 | 2.- |146.20
SOILS OF HIGH PRODUCTION. Janesville, Wis. 1 | 16.96 | 125.43 | 29.31 | 2.67 | 40.28 Loam. 11 | 418.85 | 592.75 | 472.95 | 0.00 |414.50 -----------------------+----------+--------+--------+--------+------ Hagerst’wn, Pa. 1 | 11.51 | 187.59 | 97.09 | 1.67 | 21.17 Clay loam. 11 | 136.21 | 502.82 | 620.00 | 0.00 |283.80 Average. | 277.03 | 547.79 | 546.48 | 0.00 |349.15 -----------------------+----------+--------+--------+--------+------
King’s observations show strikingly both the continuous solubility of the soil, and the differences between the solutions derived from soils of low and high productiveness; wholly negativing the contention of Whitney that the solutions from different soils are of practically the same composition.[114] King also calls attention to the fact, shown in other experiments made in the extraction of soils without intermediate dryings, that the amounts extracted were very much less in subsequent than in the first extraction; doubtless because the evaporation from the soil particles had carried a large proportion of soluble matters to the surface, whence it was readily abstracted by the first touch of the solvent water. At each drying not only are the soluble matters again drawn to the surface, but heating a soil even to 100° renders additional amounts of soil ingredients soluble both in water and in acids. It can scarcely be doubted that the intense heating which desert soils undergo during the warm season is similarly effective; and thus the great productiveness of these soils under irrigation, and the marvelously rapid development of the native vegetation when rains moisten the parched soil, is in part at least accounted for by this immediate availability of a large supply of plant-food.
[114] Bulletin No. 22, Bureau of Soils, U. S. D. A.
Composition of Janesville loam.—In connection with the above data given by King, it is interesting to note the composition of the soil in the above table yielding the highest proportions of soluble matter, when analyzed according to the method practiced by the writer (see chap. 19, p. 343). This analysis was made under the supervision of Professor Jaffa in the laboratory of the California Experiment Station by Assistant Charles A. Triebel.
Loam Soil from Janesville, Wisconsin; sample sent by Prof. F. H. King, Madison, Wis.
This soil is a light friable loam, resembling the northern Loess in color and texture; it is highly productive. It is underlaid at 5 feet by the drift gravel of that region, enclosing much calcareous material, which evidently has had a large share in the formation of this soil, just as is the case in southern Michigan.
The soil, when dried at 110° C, consisted of
CHEMICAL ANALYSIS OF FINE EARTH.
Insoluble matter 69.35 Soluble silica 10.89 Potash (KO₂) .59 Soda (Na₂O) .04 Lime (CaO) .83 Magnesia (MgO) .51 Br. ox. of Manganese (Mn₃O₄) .08 Peroxid of Iron (Fe₂O₃) 3.60 Alumina (Al₂O₃) 5.26 Phosphoric acid (P₂O₃) .06 Sulfuric acid (SO₃) .10 Water and organic matter 8.72 ------ Total 100.03
It will be noted that in accordance with the interpretation of analyses of soils as given in the next chapter, this is a high-class soil in every respect, except that its content of phosphoric acid is only just above the lower limit of sufficiency. But as is also shown below, in presence of a large supply of lime even lower percentages of phosphoric acid are adequate for long-continued production (see chap. 19, pp. 354, 365) by rendering the substance more freely available; and that this is true in this case is shown by the result of King’s leachings, in which this soil yields a maximum of 419 parts per million as against 80 and 64 parts in the poor soils, which at the same time yield only one fourth as much of lime. Unfortunately we have no full analyses of these other soils for comparison; although they have served as a basis of comparison for years in the Washington Bureau of Soils.
Solubility of Soil Phosphates in Water.—The solubility of the phosphate contents of soils has been elaborately investigated by Th. Schloesing fils.[115] He found in the case of a number of soils investigated by him that the amount of phosphoric acid P₂O₅ in the soil-solution ranged from less than one millionth (or one milligram per liter of water) in a poor soil, to over three milligrams in a rich one. He also found that for one and the same soil the amount so found was constant, if about a week’s time were allowed for saturation. He calculates that while in general the amount of phosphoric acid capable of being supplied to the crop during a growing season of twenty-eight to thirty weeks would suffice for but few crops, the supply so afforded is in no case a negligible quantity, frequently amounting to more than half of the crop-requirements. Experiments with various crops prove that these dilute solutions are utilized by all of them, sometimes to the extent of completely consuming the content of the solution. The much smaller content of phosphoric acid in drain waters is accounted for by the lack of time for full saturation during the time that the flow lasts. Whitney, (Bureau of Soils, Bulletin 22) has extracted the soil-solution by means of the centrifuge from several soils; the contents of phosphoric acid thus found are in general of the same order as those shown in the preceding table by King, but much in excess of Schloesing’s figures; notwithstanding the fact that Whitney’s soils had been in contact with water for only twenty-four hours. The cause of this wide discrepancy is not clear.
[115] Ann. de la Sci. Agron., 2de série tome I, pp. 416-349; 1899.
Practical Conclusions from Water Extraction.—As regards the practically useful conclusions to be drawn from the extraction of soils with pure water, the data given above, and especially the results obtained by King, seem to prove that there is a more or less definite correlation between the immediate productiveness of soils and the amount and kinds of ingredients dissolved; especially in the case of phosphoric acid, the adequacy of the supply of which for immediate production is assumed to be thus demonstrable by many French chemists. Moreover, a number of King’s results, tabulated in curves, exhibit a remarkable general parallelism of the curves showing totals of plant-food extracted by water, and actual crop production. This is the more remarkable since it is known to be, not pure water, but such as is more or less impregnated with carbonic acid at least, that is actually active in soil-solution and plant-nutrition. The farther development of this method may, it would seem, lead to definite conclusions at least in respect to the immediate productive capacity of cultivated, and perhaps also of virgin soils. But it is not likely to give any definite clew as to the durability of such lands.
ASCERTAINMENT OF THE IMMEDIATE PLANT-FOOD REQUIREMENTS OF CULTIVATED SOILS BY PHYSIOLOGICAL TESTS. PHYSIOLOGICAL SOIL-ANALYSIS.
As has already been stated, the quantitative analysis of cultivated soils by means of strong acids affords a presumptive insight into their immediate productiveness, and the kind of fertilizer needed to improve it, only in case of the extreme deficiency of one or several of the chiefly important plant-foods. The limits of deficiency of these in virgin soils have been discussed above; but since in cultivated soils amounts of soluble plant-food so small as to be beyond the limits of ordinary analytical determinations, when distributed through an acre-foot of soil may, when rightly applied, nevertheless produce very decided effects, the indications thus obtainable are not absolute. Thus a dressing of 150 lbs. of Chile saltpeter, containing only about 24 lbs. of nitrogen, is capable of causing the production of a full crop of wheat where otherwise, even under favorable physical conditions, only a fraction of a crop would have been harvested; provided, that all the other requisite ingredients were present to a sufficient extent and in available form. Yet the amount of nitrogen thus added would amount, in one acre-foot of soil to only .0008%, say eight ten-thousandths of one percent; which, with the amounts of substance usually employed in soil analysis, would be an unweighable quantity, and might easily be overlooked.
Since the amounts of potash and phosphoric acid actually taken out of the soil by one crop are in general of the same order of magnitude as the above, what is taken out by one or two crops will usually fall within the limits of analytical errors, especially of those incurred in sampling the soil. Yet that the changes caused by a number of successive crops can be proved, even by the ordinary methods, has been abundantly verified. For it seems that the losses of soil ingredients in cultivated lands exceed considerably those calculated from the actual drain represented by the crops.
Plot Tests.—There is, however, an obvious and apparently simple method by which every farmer might make his own fertilizer tests, on a small and inexpensive scale, the results of which may afterwards be put in effect on his entire land. It is to apply in proper proportions on plots (of say from one twentieth to one fortieth of an acre), the several plant-food ingredients usually supplied in fertilizers, singly as well as conjointly with each other, leaving check unfertilized plots around as well as among them. By comparison with these, the cultural results should at once determine which of the fertilizers can most advantageously be applied to the land. Such tests when carried out with all the proper precautions are often very decisive and practically successful. But they so frequently suffer from seasonal influences (such as scanty or excessive rainfall, cold or heat, etc.), inequality of soil conditions, failure to apply the fertilizers at the right time, or in the right way, the depredations of insects and birds, and other causes, that it generally takes several seasons’ trial to obtain any definite results. On level lands of uniform nature and depth, they are most likely to be successful; while on undulating or hill lands it is not only very difficult to secure uniformity of soil and subsoil on areas of sufficient size, but also to prevent the washing of fertilized soil, or fertilizer in solution, from one plot to the other by the influence of heavy rains or irrigation; thus wholly vitiating the experiments. In very many cases, especially in the arid region, the results of such trials have been practically nil, for the reason that physical defects of the soil, and not lack of plant-food, were the cause of unsatisfactory production.
A full examination of physical conditions, as outlined in previous chapters, should in all cases precede the application of fertilizers; such examination will at the same time serve to determine the greater or less uniformity of soil-conditions, which is of first importance to the cogency of fertilizer tests. As a matter of fact, few farmers possess the necessary qualifications to carry out such tests successfully, since their execution requires a certain familiarity not only with the principles and methods of experimentation, but also the faculty and practice of close and reasoning observation; which, unfortunately, is not as yet a part of instruction in our schools. The experience so often had in co-operative work between experiment stations and farmers is cogent on this point.
Those desiring to do such work, however, can make use of something like the plan given above; it being understood that in the case of clay soils, the unplanted paths left between the plots should be at least two feet in width; in the case of sandy soils the distance should be not less than three feet, and more if the plots are located on a slope. The crop from each plot should if possible be weighed as a whole; but if the plot be large and the crop measurably uniform, an aliquot part, such as one fourth, may be weighed instead. In regular experimentation the crops are weighed both in the green (freshly cut) condition, and after drying. Since the dry matter is the real basis of value in the case of most field crops, its weight is the most important; as the water-content of green crops may vary considerably. But in the case of vegetables as well as fruit crops, not only must the weight of the fresh crop be determined, but it should be sorted into the “marketable” and “unmarketable” sizes and qualities. Failure to do this may vitiate the entire experiment for practical purposes.
Pot Culture Tests.—The uncertainty attending plot culture tests on account of the difficulty of controlling seasonal and other external conditions, has resulted in the extended adoption of indoor culture tests, usually conducted in zinc or “galvanized” cylinders of a size sufficient to contain from twelve to twenty or more pounds of soil. These are kept in a green-house whose temperature and moisture-condition can be regulated at will, and where the soil-moisture is wholly under control. For investigations of the effects of various kinds of plant-food upon vegetable development, this method has served most satisfactorily and effectually, and striking photographs of results thus obtained are seen on all hands: for which reason, to save space, they have not been introduced into this volume. It seems at first sight that the same method should serve admirably to determine the manure-requirements of soils under controlled conditions.
It must, however, be remembered that the field conditions as regards subsoil, evaporation, ascent of moisture from below, penetration and spread of roots, etc., in other words, all the physical conditions so vitally concerned in crop production, except the temperature and moisture-condition of the soil, are wholly left out of consideration in this method. Hence the application of the results so obtained to actual field conditions can only be made with great caution, and are often widely discrepant with actual experience.
The method has of late been carried to an extreme by the U. S. Bureau of Soils in the proposition to supplant the large soil-pots heretofore used by small paraffined wire-cloth baskets, 3 × 3 inches in size, in which the soil to be tested is sown with seeds which are allowed to develop only for three to five weeks; it being claimed that the development occurring during that time is quite sufficient to indicate what will be the ultimate outcome in crop production. But practical experience has long ago demonstrated that these early stages of growth cannot be relied upon to show the crop results to be expected. Yet if this minute scale of pot-culture should, on further test, prove to give truthful forecasts even in a mere majority of cases, the facility with which it may be carried out will entitle it to favorable consideration. A great deal more proof is needed on this point than the confident claims of the Bureau indicate.
CHEMICAL TESTS OF IMMEDIATE PRODUCTIVENESS.
Testing chemical soil-character by crop analysis.—Another method for the determination of immediate soil requirements has been elaborated by E. Godlewski.[116] The principle upon which this method rests is that plants growing in a soil deficient in available plant-food of any one kind will in their ash show a corresponding deficiency, or at least a minimum proportion of the same; and that in many cases, the nature of the deficiency manifests itself in the form or development of the plant, so clearly as to render chemical analysis unnecessary (see below, chapter 22).
[116] Zeitschr. Landw. Vers. Oesterr., 1901.
To a certain extent the latter idea has been and is constantly being utilized in practice. It is essentially involved in the habit of judging of land by its natural vegetation; and by agricultural chemists and intelligent farmers, when they check excessive growth of stems and leaf (indicating excess of nitrogen) by the use of lime or phosphates; or prescribe the use of nitrogenous manures when a superabundance of small, unmarketable fruit is produced. From the coincidence of such indications with the results of the analyses of soils and ashes, very definite and permanently valuable indications as to the proper fertilization and other treatment of the land may be deduced.
Godlewski insists strongly, and with a good deal of plausibility, upon the importance of making such trials in the open field and not merely in pots. While this is true, it is also true that such field experiments suffer from the same liability to imperfection as the “plot fertilizer-test” plan just described; viz., that the season may exert a much more powerful influence than the fertilization, and the tests may lead to wholly erroneous conclusions unless the experiments are continued for a number of years, and under skilled supervision. But when once the normal ratio between the ash ingredients for a particular soil and climatic region have been ascertained, the data will be of lasting benefit to agriculture there, and perhaps, other things being equal, to the world at large.
H. Vanderyst has discussed the entire subject of physiological soil analysis elaborately in the Revue Génerale Agronomique of Louvain, 1902-3 (Exp’t St. Record, April 1904, Vol. 8, page 757) and shows in detail the conditions under which it may be successful. Among these he reckons as full a knowledge of the chemical characteristics of a soil as can be obtained by chemical analysis.
Chemical Tests of Immediately Available Plant-food.—It is scarcely doubtful that plants differ considerably in the energy of their action upon the “reserve” soil ingredients; hence no one solvent used by the analyst could represent correctly the action of plant-roots in general upon the soil, even if we could give that action the same time (a growing season) and opportunity afforded them in nature by the root-surface. We are forced to proceed empirically; and among the numerous solvents suggested for the purpose of soil extraction, that of Dyer, already mentioned, viz., a one per cent solution of citric acid, making allowance for such neutralization as may occur in the soil, has seemed to the writer to give results most largely in agreement with cultural experience. Walter Maxwell has recommended aspartic acid in lieu of citric, as approaching nearer to practical results, at least with sugar-cane.
According to the investigations of Dyer, on Rothamstead soils of known productiveness or manurial condition, it appears that when the citric-acid extraction yields as much as .005% of potash and .010% of phosphoric acid, the supply is adequate for normal crop production, so that the use of the above substances as fertilizers would be, if not ineffective, at least not a profitable investment. These figures refer to the ordinary field crops of England and to soils originally fertile and well supplied with lime. It can readily be foreseen that under other climatic and soil conditions, different figures may have to be established. So far as the writer’s experience goes, however, the above figures are very nearly valid for the arid climates as well; only the figures obtained for arid soils are usually far in excess of the above minimum postulates. Figures for lime and nitrogen are given in chapters 8 and 19. But the results obtained with the highly ferruginous soils of Hawaii show that under such conditions, figures far exceeding the minimum ones established by Dyer nevertheless coexist with need of phosphate fertilization.