Chapter XIX

THE ANALYSIS OF VIRGIN SOILS BY EXTRACTION WITH STRONG ACIDS.

As stated already, the analysis of soils by extraction with strong acids is intended to enlighten us, not in regard to their immediate productiveness (the “Düngerzustand” of German agricultural chemists), but as to their permanent value or productive capacity. As has been seen in the preceding chapter, the efforts to unite investigators upon a generally applicable and acceptable method for the testing of immediate productiveness have not been very successful, and the number of methods employed in different countries and by different chemists within the same country are widely at variance, with no immediate prospect of agreement. Moreover, in most cases the effort is to combine both problems—temporary and permanent productive capacity—in one method or operation; which still farther confuses the issue.

Convinced that the only way to unification lies in the direction of falling back upon a method that is based upon a natural limitation about which there can be no difference of opinion, the writer has, in following the lead of Owen and Robert Peter, endeavored to settle definitely the natural limit of the action of a suitable acid upon soils, and the time and strength of acid producing the maximum effect.

Loughridge’s Investigation.—Systematic work on these points was undertaken, at his suggestion, by Dr. R. H. Loughridge in 1871 and 1872. The results of this work were published in the succeeding year in the Amer. Journal of Science, and in the proceedings of the A. A. A. S. for 1873. They seem to be of sufficient general interest to be reproduced here.

The soil selected for this purpose was a very generalized one, representing large areas in the states of Kentucky, Tennessee, Mississippi and Louisiana, bordering on the east the immediate valley of the Mississippi river, and known locally as the “Table lands;” a noted cotton-producing upland region. The brown or yellow, moderately clayey loam is of great uniformity throughout its region of occurrence, and is evidently derived from such widely-spread sources that it represents no special rock or complex of rocks. Its natural growth is a mixture of oaks and hickories, strong and well-developed trees, such as any land-seeker would at once approve for settlement. Its cotton product when fresh was a 400-pound bale of cotton lint per acre. It may therefore well be considered a typical generalized soil of the humid upland of the Mississippi valley. Its physical analysis is given in chapter 6, it being No. 219 of the table on p. 98.

Strength of Acid used.—Three different strengths of acid were simultaneously employed, viz., chlorhydric of 1.10, 1.115 and 1.160 density. With these the soil was digested at steam heat in porcelain beakers covered with watch glasses for five days each, then evaporated and analyzed as usual. The results were as follows:

ANALYSIS WITH ACID OF DIFFERENT STRENGTHS. =========================+========================= Ingredients. | Sp. G. of Acid. -------------------------+--------+--------+------- | 1.10 | 1.115 | 1.160 +--------+--------+------- Insoluble residue | 71.88 | 70.53 | 74.15 Soluble silica | 11.38 | 12.30 | 9.42 Potash | .60 | .63 | .48 Soda | .13 | .09 | .35 Lime | .27 | .27 | .23 Magnesia | .45 | .45 | .45 Br. ox. Manganese | .06 | .06 | .06 Ferric Oxid | 5.15 | 5.11 | 5.04 Alumina | 6.84 | 8.09 | 6.22 Sulfuric acid | .02 | .02 | .02 Volatile matter | 3.14 | 3.14 | 3.14 | ------ | ------ | ----- | 100.02 | 100.69 | 99.29 | | | Amount of soluble matter | 24.00 | 27.02 | 22.27 Amount of soluble bases | 13.50 | 14.70 | 12.83 -------------------------+--------+--------+-------

It will be noted that the strongest acid produced the smallest amount of decomposition of the soil silicates, e. g. the silica soluble in carbonate of soda solution being 3% less than in the case of the acid of medium strength; a result possibly due to some difficultly-soluble compound formed on the surface of the soil grains. The weakest acid had a stronger solvent power; but the maximum effect was produced by the acid of 1.115 density. This being also the most readily obtainable, by simple steam distillation of acid of any other strength, the writer adopted it as best suited to the purposes of soil analysis.

To ascertain the time required for the desired action, viz., the solution of the plant-food ingredients to the extent likely to be of any avail to growing plants, digestions of the same soil were made in the same manner for periods of 1, 3, 4, 5 and 10 days, with the acid of 1.115 density. The results were as follows:

ANALYSIS AFTER DIFFERENT TIMES OF DIGESTION. =========================+================================== | No. of Days’ Digestion. Ingredients. +------+------+------+------+------ | 1 | 3 | 4 | 5 | 10 -------------------------+------+------+------+------+------ Insoluble Residue | 76.97| 72.66| 71.86| 70.53| 71.79 Soluble Silica | 8.60| 11.18| 11.64| 12.30| 10.96 Potash | .35| .44| .57| .63| .62 Soda | .06| .06| .03| .09| .28 Lime | .26| .29| .28| .27| .27 Magnesia | .42| .44| .47| .45| .44 Br. Ox. Manganese | .04| .06| .06| .06| .06 Ferric Oxid | 4.77| 5.01| 5.43| 5.11| 4.85 Alumina | 5.15| 7.38| 7.07| 7.88| 7.16 Phosphoric acid | .21| .21| .21| .21| .21 Sulfuric acid | .02| .02| .02| .02| .02 Volatile matter | 3.14| 3.14| 3.14| 3.14| 3.14 +------+------+------+------+------ Total | 99.63|100.68|100.55|100.69| 99.80 | Amount of soluble matter | 19.67| 24.88| 25.57| 27.02| 24.87 Amount of soluble bases | 11.05| 13.68| 13.91| 14.49| 13.68 -------------------------+------+------+------+------+------

While these results pointed clearly to the five-day period as being sufficiently effective so far as the plant-food ingredients are concerned, it was not easy to understand why a ten-day digestion should be less incisive than a five-day one. Instead of repeating the ten-day experiment, it was thought preferable to re-treat the residue from the five-day digestion for five days more. The result was that only more silica and alumina went into solution—in other words, additional clay was alone being decomposed. This being of no interest in the matter of plant nutrition, the five-day period was definitely adopted by the writer for his work; and it, together with the acid of 1.115 density, is the basis of all the results given in this volume, except where otherwise stated. There appeared to him to be no good reason for the acceptance of the arbitrary method of soil-extraction suggested by Kedzie and since adopted by the Association of Official Agricultural Chemists; the more as to do so would throw out of comparison all the previous work done by Owen, Peter, and himself and his pupils, which had already been definitely correlated with the natural conditions and with cultural experience.[117]

[117] While regretting to thus “secede” from the fellowship of his colleagues, the writer cannot but regret equally their voluntary decision to do over again, or lightly reject, all that had been done before in correlating soil-composition and plant-growth. He still thinks that it is idle to expect any unification, national or international, of methods of soil analysis based upon purely arbitrary prescriptions, unless previously shown to be definitely correlated with natural and cultural conditions; as is measurably the case with Dyer’s method.

Virgin Soils with High Plant-food Percentages are Always Productive.—In strong contrast to the contradictory evidence deduced from the analysis, by any method, of cultivated soils when compared with cultural experience, it seems to be generally true that virgin soils showing high percentages of plant-food as ascertained by extraction with strong acids (such as hydrochloric, nitric, etc.), invariably prove highly productive: provided only that extreme physical characters do not interfere with normal plant growth, as is sometimes the case with heavy clays, or very coarse sandy lands.—To this rule no exception has thus far been found. The composition of some representative soils falling within this category is given in the annexed table, which at the same time conveys some idea of the proportion of acid-soluble ingredients usually found in the best class of natural soils.

TABLE EXEMPLIFYING HIGH PLANT-FOOD PERCENTAGE IN SOILS.

(A) = Buckshot soil. Yazoo Bottom. (B) = Black Prairie. Rankin County. (C) = Loamy Sediment. Houma, Terrebonne parish. (D) = Rio Grande Bottom. Sandy Sediment. ============================+===================+==========+========= | Mississippi |Louisiana.| Texas. +---------+---------+----------+--------- | | | | | (A) | (B) | (C) | (D) | (Heavy Clay). | (Loam). | ----------------------------+---------+---------+----------+--------- Number of Sample | 390 | 188 | 240 | 37 ----------------------------+---------+---------+----------+--------- Chemical Analysis of | | | | Fine Earth. | | | | Insoluble matter | 51.06 | 69.95 | 35.48 | 36.04 | 71.77| 74.40| 56.24 | 53.30 Soluble silica | 20.70 | 4.46 | 20.76 | 17.26 ----------------------------+---------+---------+----------+--------- Potash (K₂O) | 1.10 | .90 | 1.03 | 1.31 Soda (Na₂O) | .33 | .24 | .13 | .22 Lime (CaO) | 1.35 | 1.04 | .72 | 14.43 Magnesia (MgO) | 1.67 | .91 | .88 | 1.53 Br. ox. of Manganese (Mn₃O₄)| .12 | .12 | .014 | .07 Peroxid of Iron (Fe₂O₃) | 5.82 | 4.77 | 7.10 | 4.09 Alumina (Al₂O₃) | 10.54 | 7.25 | 15.45 | 9.11 Phosphoric acid (P₂O₅) | .30 | .47 | .15 | .20 Sulfuric acid (SO₃) | .02 | .16 | .25 | .04 Carbonic acid (CO₂) | | | | 9.91 Water and organic matter | 7.37 | 10.74 | 18.52 | ? ----------------------------+---------+---------+----------+--------- Total |100.38 |101.01 |100.48 |100.22 ----------------------------+---------+---------+----------+---------

(E) = San Diego Co. Colorado Bottom. Silt Sediment.[118] (F) = Riverside Co. Palm Valley. Micaceous Sandy Soil. (G) = Tulare Co. Experiment Station. Plains Loam. (H) = Solano Co. Putah Valley. Dark Loam. (I) = San Luis Obispo Co. Arroyo Grande Dark Loam. ============================+========================================= | California. +---------+------+------+--------+-------- | | | | | | (E) | (F) | (G) | (H) | (I) | | | | | ----------------------------+---------+------+------+--------+-------- Number of Sample | 506 | 1092 | 1159 | 110 | 2061 ----------------------------+---------+------+------+--------+-------- Chemical Analysis of | | | | | Fine Earth. | | | | | Insoluble matter | 58.57 | 71.45| 72.98|67.33 |53.43 | 63.90| | | 71.00| 72.43 Soluble silica | 5.33 | 5.50| 6.60| 3.67 |19.00 ----------------------------+---------+------+------+--------+-------- Potash (K₂O) | 1.18 | 1.42| 1.20| .93 | .67 Soda (Na₂O) | .16 | .18| .52| .12 | .18 Lime (CaO) | 8.67 | 2.20| 1.86| .77 | 2.11 Magnesia (MgO) | 2.97 | 2.09| 1.81| 2.29 | 2.26 Br. ox. of Manganese (Mn₃O₄)| .03 | .05| .08| .11 | .06 Peroxid of Iron (Fe₂O₃) | 4.14 | 6.68| 6.86| 8.01 | 5.23 Alumina (Al₂O₃) | 8.40 | 5.78| 5.66| 9.16 | 7.40 Phosphoric acid (P₂O₅) | .13 | .35| .10| .11 | .71 Sulfuric acid (SO₃) | .15 | .01| .03| .12 | .22 Carbonic acid (CO₂) | 7.82 | .18| | | 1.82 Water and organic matter | 3.34 | 4.29| 2.54| 7.12 | 6.63 ----------------------------+---------+------+------+--------+-------- Total |100.89 |100.18|100.24|99.74 | 99.72 | | | | | Humus | | | | | 3.06 Nitrogen in humus | | | | | 22.00 Nitrogen in soil | | | | | .67 Hygroscopic moisture. | | | | | absorbed at 15°C | | | | | 10.70 ----------------------------+---------+------+------+--------+-------- Available phos. acid .14 Available potash .14

[118] The Rio Grande and Colorado bottom soils contain amounts of lime carbonate largely in excess of requirements, 2 to 3% of that compound being all that is needed to insure all the advantageous effects of lime in any soil (see this chapter, page 367).

Discussion of Table.—It will be noted in this table that while the total of the matters soluble in acids (inclusive of silica) ranges from a little below 50 to over 77 per cent, the total of directly important mineral plant-food ingredients (potash, lime, magnesia and phosphoric acid), constitute in moderately calcareous soils only from about 2.5 to somewhat over four per cent of the whole. Yet if all these were in available form, the supply would be abundant for many hundreds and even thousands of crop years. For, one-tenth of one per cent in the case of the clayey soils of the preceding table would amount to about 3500 pounds per acre-foot, and to 4000 in the case of the sandy ones. Hence the amount of phosphoric acid in e. g., the Mississippi delta soil from Houma would suffice for the production of about 440 crops of wheat grain (at 20 bushels per acre) if only one foot depth were drawn upon; but as the roots of grain easily penetrate to twice and half and three times that depth even in the humid region, the number might be tripled. As a matter of fact, however, that soil has produced full crops for from forty to fifty years only; yet this is considered an exceptionally long duration of profitable production without fertilization.

The first and last soils in the above list represent probably the highest types of productiveness known. The Yazoo bottom soil has produced up to one thousand pounds of cotton lint per acre when fresh, and is still producing from four to five hundred pounds after thirty years’ culture. The Arroyo Grande soil of California with its extraordinary percentages of phosphoric acid and nitrogen, as well as exceptionally high proportion of available phosphoric acid and potash, has made such a record of productiveness, and high quality of the seeds produced, that it has for a number of years been excluded from competition for prizes offered by seed-producers elsewhere, in order to give other sections a chance. Both these soils are rather heavy clays, but readily tillable in consequence of their abundant lime-content. The remarkably high content of acid-soluble silica, indicating the presence of much easily available zeolitic matter, is doubtless connected with the exceptional productiveness.

Experience, then, proves that lands showing such high plant-food percentages will yield profitable harvests for a long time without fertilization, or with only such partial returns as are afforded by the offal of crops. Also that when fertilization comes to be required, instead of supplying all the ingredients usually constituting fertilizers, only one or two of these will as a rule be actually needed, and even these in smaller amounts than in “poor” lands; thus materially reducing the expense of fertilization. The high production and durability of such lands therefore amply justify their higher pecuniary valuation; for which there would be no rational permanent ground if they required fertilization to the same extent as poor lands. In other words, if the entire amount of soil-ingredients removed by crops had had to be currently replaced equally in all cases (as is implied in the hypothesis, advanced by some, that the chemical composition of soils is of no practical consequence), the high prices which from time immemorial have been paid for black prairie and rich alluvial lands as against meagre uplands and barrens, would have been so much money wasted.

The explanation of these advantages evidently lies largely in the larger amounts of soil ingredients annually rendered available in rich soils by the fallowing effect of the atmospheric agencies, because of the generous totals present. The actual amounts of soil ingredients thus rendered accessible to plants, other things being equal, are evidently more or less directly proportional to the totals of acid-soluble plant-food ingredients present. And if this is true in cultivated lands, the inevitable conclusion is that the same must be true of virgin lands; whose productive capacity and duration can therefore be forecast by such analyses. It will be observed that the above data, which could be indefinitely increased by corroborative analyses, seem to establish the fact that about one per cent of acid-soluble potash, one of lime, the same, or less, of magnesia, and .15% of phosphoric acid, are thus shown to be “high” percentages of these ingredients in virgin soils.

It is not easy to see how the above conclusions can be successfully controverted; they are, moreover, thoroughly in accordance with cultural experience. Difficulties of interpretation arise mainly in the case of medium soils, which show neither very high nor very low percentages of plant-food; and which raise the question of what amount or percentage constitutes “adequacy” of each of the several substances.

Low Percentages.—On the other hand, whenever in virgin soils acid-analysis shows the presence of but a very small proportion of one or several of the essential ingredients, we have a valuable indication as to the one of these that will first be required to be added when production slackens.

What are “Adequate” Percentages of Potash, Lime, Phosphoric Acid and Nitrogen?—It is evident that a very critical discussion of cultural experience can alone answer this question; and at first sight such experience often appears very contradictory when compared with the results of analysis.

One of the chief causes of such apparent discrepancies is readily intelligible when we consider the differences in root-development of the same plant in different soils. In “light” or sandy lands the roots may penetrate to several times the depth attained by them in heavy clay soils. Having thus within their reach a soil-mass several times larger, and aerated to a much greater depth, it is but reasonable to expect that in deep, sandy lands plants would do equally well with correspondingly smaller percentages of plant-food than would suffice in clay soils, in which the root-range is very much more restricted. The well-known fact that the production of heavy clay lands may be increased by their intermixture with mere sand, adding nothing to their store of plant-food, emphasizes this expectation and elevates it into a maxim. On this ground alone, therefore, it is evident that the mere consideration of plant-food percentages found, can be a true measure of productiveness only in the case of virgin soils with high percentages.

Soil Dilution Experiments.—The extent to which dilution with mere “lightening” materials can be carried without impairing production, can of course be determined for concrete cases only; but the following experiment made at the California Station is a case in point:

One kilogram of the heavy but highly productive black clay soil of the experimental grounds of the University of California was used in each of five experimental cultures, each made in duplicate, in cylindrical vessels of zinc-covered (“galvanized”) sheet iron, all proportioned alike in height and diameter, but containing respectively one, two, four, five and six volumes of total soil. In the smallest was placed one kilogram of the undiluted, original soil, in the others successively the same amount of the soil thoroughly mixed with one, three, four, and five volumes of a dune sand fully extracted with chlorhydric acid, and washed with distilled water. The water capacity of each of the mixtures was determined and the earth in the pots kept at the point of half-saturation generally admitted to be the optimum (best condition) for plant growth. Each pot was sown with ten seeds of white mustard, subsequently reduced to five plants selected for their vigor.

The (“galvanized”) vegetation pots were made as nearly as possible of similar proportions in depth and width for each dilution, so as to give opportunity for the proportional development of the root systems. The photographs show the latter as nearly as practicable in their natural form, restored after washing off the adherent soil. It was of course extremely difficult to preserve intact the extreme circumferential rootlets and hairs; yet the general development is correctly shown.

The following table shows the percentage composition of the original as well as the diluted soils, while the photographs show the development of the plants in their successive stages, so far as these could be observed; the continued attacks of mildew and plant lice preventing full maturity being attained.

COMPOSITION OF BLACK ADOBE AND SAND DILUTIONS. ============================+========+============================== |Original| Dilutions. Chemical analysis of | soil. | fine earth. | 1:0 | 1:1 1:3 1:4 1:5 ----------------------------+--------+-------+-------+-------+------ Insoluble matter | 54.50 | 77.25 | 88.62 | 90.00 | 92.42 Soluble silica | 19.60 | 9.50 | 4.75 | 3.80 | 3.17 Potash (K₂O) | .73 | .36 | .18 | .15 | .12 Soda (Na₂O) | .20 | .10 | .05 | .04 | .03 Lime (CaO) | 1.15 | .57 | .29 | .23 | .19 Magnesia (MgO) | 1.08 | .54 | .27 | .22 | .18 Br. ox. of Manganese (Mn₃O₄)| .04 | .02 | .01 | .01 | .01 Peroxid of Iron (Fe₂O₃) | 8.43 | 4.22 | 2.11 | 1.68 | 1.40 Alumina (Al₂O₃) | 7.92 | 3.96 | 1.98 | 1.58 | 1.32 Phosphoric acid (P₂O₅) | .19 | .10 | .05 | .04 | .03 Sulfuric acid (SO₃) | .04 | .02 | .01 | .01 | .01 Carbonic acid (CO₂) | | | | | Water and organic matter | 6.54 | 3.27 | 1.64 | 1.31 | 1.09 Loss in analysis | 1.18 | .09 | .04 | .03 | .03 +--------+-------+-------+-------+------ Total | 100.00 |100.00 |100.00 |100.00 |100.00 | | | | | Humus | 1.21 | .60 | .30 | .24 | .20 “ Ash | .94 | .47 | .23 | .19 | .16 “ Nitrogen, p. cent | | | | | in Humus | 18.58 | 18.58 | 18.50 | 18.58 | 18.58 “ “ p. cent. | | | | | in soil | .203| .10 | .05 | .04 | .034 ----------------------------+--------+-------+-------+-------+------

The restricted volume of soil occupied by the roots in the undiluted adobe soil, together with the very abundant development of root-hairs, is very striking. A marked change in these respects is manifest in the first dilution, and increasingly so as dilution increases; the paucity of root-hairs is very marked in the last (greatest) dilution, in which, as the photograph of the plants shows, the development was decidedly behind that in the pot containing dilution 1:4. The latter in fact showed the best development not only in this case, but in two other series of tests conducted at the same and subsequent times; and strangely enough, also in the pulverulent, “sandy loam” soil of the southern California substation tract. In the latter series, which for lack of space cannot be figured here, the main difference was that in the undiluted soil the roots filled the entire soil mass, instead of remaining near the surface, as in the pure adobe. It is possible that the latter was too wet when given the full half of its water-capacity, although, as the figures show, the water was slowly introduced from below by means of glass tubes, ending within a shield to prevent puddling.

Limitation of Root Action.—These results, representing five soils of different percentage-composition and physical character, but identical chemical composition and ratios between the several ingredients, and similarly acted upon by the atmospheric agencies in the past, illustrate strikingly the impossibility of judging correctly of a soil’s productiveness from percentages of chemical ingredients alone. It is clear that the physical characters of the land as well as its depth, must be essentially taken into account. But there is obviously a certain limit beyond which greater perviousness and root-penetration cannot make up for deficiency in the absolute amounts of plant-food within possible reach of the plant; for in the case of excessive dilution these are rendered partially inaccessible within the time-limits of a season’s growth.

It is hardly necessary to say that these experiments require repetition with the aid of the experience acquired in these first trials, not only in the laboratory but also in the field. It will be especially interesting to compare with the results obtained in these strongly calcareous soils, the effects of dilution in such soils as those of Florida, mentioned below; the probability being that where lime is naturally deficient, the effects of dilution will be much more pronounced in diminishing production, because of the absence of the previous favorable action of lime upon the availability of the soil-ingredients.

Lowest Limit of Plant-food Percentages and Productiveness found in Virgin Soils.—The subjoined table shows some of the very low plant-food percentages found in natural soils, all being of a sandy character:

============================+==================================== | MISSISSIPPI SOILS. +----------+--------+--------+------- |Homochitto| Shell | Pine | Pine | Bottom. |Hammock.| Woods. |Flats. ----------------------------+----------+--------+--------+------- Number of Sample. | 68 | 83 | 206 | 214 ----------------------------+----------+--------+--------+------- CHEMICAL ANALYSIS OF | | | | FINE EARTH. | | | | ----------------------------+----------+--------+--------+------ Insoluble matter | | | | | 92.16 | 96.08 | 93.23 | 95.59 Soluble silica | | | | ----------------------------+----------+--------+--------+------ Potash (K₂O) | .15 | .05 | .26 | .06 Soda (Na₂O) | .04 | .06 | .07 | .05 Lime (CaO) | .12 | .10 | .12 | .02 Magnesia (MgO) | .21 | .12 | .18 | .07 Br. ox. of Manganese (Mn₃O₄)| .28 | .05 | .15 | .05 Peroxid of Iron (Fe₂O₃) | 1.18 | .52 | 1.25 | .46 Alumina (Al₂O₃) | 3.22 | .46 | 2.36 | .85 Phosphoric acid (P₂O₅) | .08 | .10 | .03 | .02 Sulfuric acid (SO₃) | .05 | Trace | .02 | Trace Carbonic acid (CO₂) | | | | Water and organic matter | 2.70 | 3.02 | 2.33 | 2.28 +----------+--------+--------+-------- Total | 100.19 | 100.56 | 100.00 | 99.45 ----------------------------+----------+--------+--------+--------

============================+==================== | FLORIDA SOILS. +-------------------- | Pine Lands. |-------+------------ | First | Second |Class. | Class. ----------------------------+-------+------------ Number of Sample. | 6 | 7 ----------------------------+-------+------------ CHEMICAL ANALYSIS OF | | FINE EARTH. | | ----------------------------+-------+------------ Insoluble matter | 94.46 | 95.63 | | 96.51 Soluble silica | 1.67 | .88 ----------------------------+-------+------------ Potash (K₂O) | .19 | .12 Soda (Na₂O) | .04 | .06 Lime (CaO) | .07 | .06 Magnesia (MgO) | .04 | .04 Br. ox. of Manganese (Mn₃O₄)| .06 | .05 Peroxid of Iron (Fe₂O₃) | .32 | .22 Alumina (Al₂O₃) | .92 | .47 Phosphoric acid (P₂O₅) | .11 | .09 Sulfuric acid (SO₃) | .09 | .06 Carbonic acid (CO₂) | | Water and organic matter | 1.88 | 1.81 +-------+------------ Total | 99.85 | 99.49 ----------------------------+-------+------------

The average of plant-food percentages in all these soils is quite low, and at first sight there seems to be little choice between them. Yet two of them—Nos. 68 and 88, from Mississippi—are not only quite productive at the outset, but also fairly durable. This becomes measurably intelligible when it is known that both are of great depth, and so well drained that roots can descend for many feet; while the composition of the soil-material is almost identical for three or four feet. On the other hand, both Nos. 206 and 214 are quite shallow, being underlaid by sand almost devoid of plant-food at about two feet. In addition, both have extremely low percentages of phosphoric acid; while the rest show near .10% of that ingredient, an amount which, as will be seen hereafter, is considerably above the recognized limit of deficiency. The two Florida soils however bear only pine; they are underlaid by almost clean sand at two or three feet, and are therefore quickly exhausted. It will also be noted that their lime-percentage is only about half of that of the two first-named Mississippi soils, both of which bear a strong growth of deciduous timber trees, grape vines, and other vegetation indicating the presence of lime carbonate.

It is noteworthy, also, that the popular classification of the two Florida soils corresponds exactly with the differences in the percentages of plant-food; those in the “second-class” soil being uniformly lower than those in the one designated as first-class. This indicates, again, that as between soils of similar character and origin, the production and durability are sensibly proportional to the plant-food percentages when the latter fall below a certain limit; a point more fully illustrated farther on.

In the light of the above experiment and tables, it becomes pertinent to consider what are the lowest percentage limits of each of the more important plant-food ingredients compatible with profitable production.

LIMITS OF ADEQUACY OF THE SEVERAL PLANT-FOODS IN VIRGIN SOILS.

It is obvious that the lower limits of adequacy of the critical plant-food ingredients are best ascertained in the case of virgin soils containing very small amounts of some one ingredient, while fairly or fully supplied with the rest. In such cases, which are not at all infrequent, the use of the deficient ingredient as a fertilizer should produce a very marked effect so soon as the first flush of production (always noted in fresh soil) is over. This first productiveness may, even in poor lands, range from one to three years, when there is a sudden decline.

Lime a Dominant Factor.—When we investigate the cases of such lands, it soon becomes apparent that besides the low percentage of any one ingredient, the proportions of others present require consideration. Among these, lime in the form of carbonate stands foremost. Its presence exerts a dominant and beneficial influence in many respects, as is readily apparent from the prompt change in vegetation whenever it is introduced into soils deficient in it. In discussing the results of soil analysis, its consideration is of first importance in forecasting correctly the adequacy or inadequacy of other soil ingredients (see chapter 20, page 379). For in general, we find that lower percentages of potash, phosphoric acid and nitrogen are adequate, when a large proportion of lime carbonate is present.—This has already been referred to in connection with the table of soils of low percentages, given above. In the interpretation of results obtained by analysis this point must always be kept in view; and in the numerical statements made below, it must be understood that they refer to virgin soils sufficiently supplied with lime to assure a constant excess of lime carbonate, maintaining the conditions of nitrification and insuring the absence of acidity. (See chapter 9, page 146).

Potash.—In respect to potash, the writer was led by his early investigations in the State of Mississippi to conclude that less than one-fourth of one per cent (.25) of potash constituted a deficiency likely to call for early fertilization with potash salts; while as much as .45% of the same seemed to cause the land to respond but feebly to such fertilization. He has not found it necessary to revise materially that early conclusion, whether from his own work or from that of others. Within the last decade, Prof. Liebscher of Göttingen[119] has arrived at this identical figure from analyses made of soils upon which he had conducted a seven-year series of fertilizer tests; he having found that potash fertilization produced no sensible, or at least no paying results on land giving that figure, and otherwise well provided with plant-food. The different (lower) figures given by Schloesing, Risler and other French chemists in discussing the soils of France are doubtless due to the weak acid and short period of digestion employed in the analysis; an unfortunate discrepancy of methods which precludes any direct comparison of results.

[119] Untersuchungen über die Bestimmung des Düngerbedürfnisses der Ackerböden und Kulturpflanzen, von G. Liebscher; Journal für Landwirtschaft 43 (1895), Nos. 1 & 2, pp. 48-216.

These figures apply both to the arid and the humid regions in the temperate zones. In the tropics we find very much lower percentages quoted as adequate; thus in the laterite soils of India and Samoa, according to Wohltmann, in the soils of Jamaica according to Fawcett, and in those of Madagascar according to Müntz and Rousseaux.[120] There, potash-percentages over .10% seem to be high, and in Madagascar some lands in fair production range as low as .01%. The soil-extractions have however in these cases been made with a weaker acid than above specified, so that some increase of the figures (perhaps 33 to 50%) have to be allowed for. But even then there can be no question that a far less amount of potash, as determined by acid-extraction, is found sufficient for crop production in the tropics; doubtless because of the very intense decomposing (“fallowing”) effect of the continuous heat and moisture, tending also to a rapid decomposition of organic matter and a proportionally rapid formation of carbonic and nitric acids. Such soils are of course constantly kept in a leached condition, as a result of the heavy and continuous rainfall.

[120] La Valeur Agricole des Terres de Madagascar. Ann. de la Science Agronomique, 2’me série, tome 1, 1901.

Phosphoric Acid.—As regards the lower limit of adequacy of phosphoric acid, there is a remarkable agreement in the investigations made everywhere. It was placed at .05% by the writer as long ago as 1860, as the result of investigations made in the State of Mississippi; and the same figure has since been arrived at independently by agricultural chemists in France, Russia, Germany and England. The cause of this remarkable agreement is undoubtedly the readiness with which the phosphates that come under consideration at all for the nutrition of plants, are dissolved by almost any acid treatment likely to be used in soil analysis. Almost the same agreement exists in regard to the “adequacy” of .1% of P₂O₅; while all soils showing percentages between .1 and .05% are considered weak on this side, and liable to need phosphate fertilization soon. One-fourth of one per cent is an unusually high percentage in most countries; .30% and over is exceptional in non-ferruginous soils. But as stated on a previous page, a high percentage of lime carbonate may offset a smaller percentage of phosphoric acid, apparently by bringing about greater availability; and a similar effect seems to result from the presence of a large supply of humus.

On the other hand, very large percentages of finely divided ferric hydrate may, especially in the absence of lime carbonate, render even large supplies of phosphoric acid inert and useless, by the formation of the totally insoluble ferric phosphate. Aluminic hydrate probably acts in a similar manner. The following table gives examples in point, as regards ferric hydrate.

HAWAIIAN SOILS SHOWING HIGH CONTENTS OF FERRIC OXID. (Rept. Cal. Exp. Sta. 1894-5, page 27.) =============================+===============+======================= | Oahu. | Hawaii. -----------------------------+-------+-------+-------+-------+------- NUMBER OF SAMPLE. |No. 21.|No. 22.|No. 24.|No. 26.|No. 27. -----------------------------+-------+-------+-------+-------+------- Coarse Materials. 0.55ᵐᵐ | 2.00 | 2.50 | 4.00 | 3.00 | 5.00 Fine Earth | 98.00 | 97.50 | 96.00 | 97.00 | 95.00 | | | | | CHEMICAL ANALYSIS OF | | | | | FINE EARTH. | | | | | Insoluble matter | 15.84 | 14.49 | 26.99 | 28.66 | 21.07 Soluble Silica | 14.07 | 30.37 | 10.26 | 7.35 | 2.68 Potash (K₂O) | .45 | .26 | .40 | .61 | .44 Soda (Na₂O) | .14 | .08 | .26 | .17 | .25 Lime (CaO) | .26 | 1.04 | .52 | .68 | .28 Magnesia (MgO) | .65 | .80 | .96 | 1.04 | .60 Br. ox. of Manganese (Mn₃O₄) | .05 | .03 | .21 | .20 | .07 Peroxid of Iron (Fe₂O₃) | 39.05 | 19.68 | 19.10 | 18.23 | 30.10 Alumina (Al₂O₃) | 14.61 | 18.29 | 21.41 | 20.18 | 14.38 Phosphoric acid (P₂O₅) | .19 | .32 | .64 | .70 | .97 Sulfuric acid (SO₃) | .03 | .09 | .32 | .21 | .29 Carbonic acid (CO₂) | | | | | Water and organic matter | 14.18 | 14.59 | 18.60 | 21.65 | 28.60 +-------+-------+-------+-------+------- Total | 99.52 |100.04 | 99.67 | 99.61 | 99.73 | | | | | Humus | 3.35 | 3.24 | 4.84 | 5.43 | 9.95 “ Ash | 3.12 | 2.22 | 2.76 | 3.56 | 6.70 “ Nitrogen, p.c. in Humus | 3.30 | 9.800| 2.800| 3.100| 1.71 “ “ , p.c. in soil | .112| .314| .134| .168| .17 Phosph. acid in humus ash | .110| .166| .580| .500| Soluble in 2% Citric acid | .004| .020| .035| .037| .025 in Nitric acid, 1.20 sp. g.| .190| .320| .640| .700| .970 in Chlorhydric acid | | | | | (1.115 sp.g.) | .430| .350| 1.600| 1.280| Hygroscopic moisture 15°C. | 18.50 | 21.25 | 23.07 | 23.14| 23.81 -----------------------------+-------+-------+-------+-------+-------

Unavailability of Ferric Phosphate.—It will be noted that in the soils from Oahu with an overwhelming amount of ferric oxid (mostly in the form of hydrate or rust) the citric acid has taken up only an insignificant amount of phosphoric acid; nitric acid took up 40 to 50 times as much, and chlorhydric doubled even this. In the much less ferruginous Hawaiian soils, though containing more alumina, the citric acid extracted nearly ten times as much; proving that it is chiefly ferric oxid, and not the alumina as has been supposed, that causes the insolubility of phosphoric acid in soils and doubtless also in fertilizers. The very unusually high content of phosphoric acid in the Hawaiian soils, exceeding all others on record, so far as known to the writer, emphasize the effects of ferric hydrate upon soluble phosphates; while the fact that these very soils are greatly benefited by the use of phosphate fertilizers, proves that the Dyer (citric acid) method for the determination of available phosphoric acid which in soils Nos. 21 to 26 yielded results largely in excess of the established limit in European soils, cannot be successfully applied to these highly ferruginous soils. It should also be noted that the amounts of phosphoric acid found in the humus extracted by the Grandeau method is in the first two Hawaiian soils over ten times the amount extracted by citric acid, but that while they rise and fall together, no definite quantitative ratio exists between the two.

It is obvious that in such soils, fertilization with water-soluble phosphates would be likely to result in the quick partial withdrawal of the same from useful action, and that any excess not promptly taken up by the crop, is likely to become inert and useless. It will evidently be desirable to use the phosphates in the form of bone meal or basic slag (Thomas Phosphate), which because of their difficult solubility will be acted upon but very slowly, if at all, by the ferric and aluminic hydrates.

Nitrogen.—In determining the nitrogen-content of the soil, a great variety of methods has been followed. Some include all that can be obtained by the combustion of the organic matters of soil and from the nitrates present in the same; while others, the writer among the number, believe that the mainly important source of nitrogen to the plant being the nitrification of the humus-nitrogen, the determination of the humus by the method of Grandeau, and of the nitrogen contained in it, should be the standard; the unhumified vegetable matter being of no definitely ascertainable value, and the nitrates varying from day to day and being liable to be lost by leaching at any time; therefore forming no permanent feature of the soil. Considering the variety of methods, the unanimity with which about one-tenth of one per cent (.10) has been assumed as the ordinarily adequate percentage is remarkable. In view of the extremely variable amount of nitrogen in the humus (ranging from 1.7 to nearly 22%), the amount of the latter cannot, of course, afford even an approximation to the nitrogen-content; except that as in the humid region, the nitrogen-percentage is not known to exceed about 5 or 5.5%, an approximate estimate can be made on that basis. In the arid region, according to location, the nitrogen-percentage may be from three to six times greater for a similar amount of humus. (See chap. 8. p. 135). In the writer’s experience, a nitrogen-percentage of .1% in the arid region is a very satisfactory figure, indicating that the need of nitrogen-fertilization is not likely to arise for a number of years.

Nitrification of the Organic Matter of the Soil.—In order to test the question whether or not the nitrogen of the unhumified debris existing in surface soils is directly nitrifiable, the writer selected a soil which in its natural condition sustains intense nitrification, so that at some points it contains as much as 1200 pounds of sodic nitrate per acre. The composition of this soil, representing the land of the “ten-acre tract” of the southern California substation, is as follows:

SOIL FROM “TEN-ACRE TRACT,” SOUTHERN CALIFORNIA SUB-STATION, NO. 1284.

Coarse Materials > 0.55ᵐᵐ 1.00 Fine Earth 99.00 ------ 100.00 CHEMICAL ANALYSIS OF FINE EARTH.

Insoluble matter 62.62} 70.92 Soluble silica 8.30} Potash (K₂O) .95 Soda (Na₂O) .50 Lime (CaO) 5.07 Magnesia (MgO) .84 Br. ox. of Manganese (Mn₂O₄) .06 Peroxid of Iron (Fe₂O₃) 6.43 Alumina (Al₂O₃) 3.88 Phosphoric acid (P₂O₅) .21 Sulfuric acid (SO₃) .06 Carbonic acid (CO₂) 3.66 Water and organic matter 6.02 ----- Total 99.70

Water-soluble matter, per cent. .137 Sodic nitrate, per cent. .020

Humus 1.99 “ Ash 1.13 “ Nitrogen, per cent. in Humus 10.30 “ “ , per cent. in soil .203 Total Nitrogen in soil .330 “ “ in unhumified matter .127 Available Potash {citric} .03 Available Phosphoric acid {method} Hygroscopic Moisture absorbed at 15° C. 5.81

It will be noticed that this is a rather strongly calcareous soil, (nearly 9% of calcic carbonate), slightly impregnated with alkali, of which about one-ninth is saltpeter. One portion of this soil was thoroughly leached with distilled water until not a trace of nitrates could be detected in the leachings. Another portion was treated for the removal of humus according to the Grandeau method (see chapter 8, page 132); the extracted soil showed under the microscope an abundance of vegetable debris, some slightly browned as from incipient humification.

The calcic and magnesic carbonates withdrawn in the humus-extraction were then restored to the soil in the form of finely divided precipitates and thoroughly mixed in, first in the dry and then in the wet condition; the extracted soil being repeatedly wetted with turbid water from the leached soil, in order to replace and reinfect it with the nitrifying bacteria. Both soils were then spread out in flat glass dishes and placed in a wooden box containing also a similar flat dish with distilled water, upon which played the draught from the inlet pipe opening into the outer air, with outlet-holes in the cover at the opposite end; thus keeping the air within fairly moist. In addition, the soils themselves were moistened with distilled water every three days and restored to a loose condition by stirring. The whole was placed so as to maintain, during the greater part of the 24 hours, a temperature of from 30 to 35 degrees C. At intervals the samples of both soils were leached and color-titrated for their nitrate content by the picric-acid test. The results, calculated as sodic nitrate, during two years were as follows:

It will be noted that in the course of four months, nitrification had not sensibly set in the extracted soil; while in the leached natural soil the nitrate-content had reached to three-fifths the amount originally present, and in the course of a year the nitrate-content of the latter was more than double that of the original (unleached) soil; while that in the extracted soil had only reached one-seventh of the same. At the end of two years we find a still farther increase of nitric nitrogen in both, the ratio between the two remaining about the same (1:14). At the same time the ratio of increase attained at first had materially diminished in the water-leached soil, probably on account of the accumulation of the niter itself.

It thus appears that although the nitrogen of the unhumified organic matter constituted about 40% of the total in the original soil, it would during the entire year have contributed only to an insignificant extent to the available nitrate-supply; while the fully humified “matière noire” contributed fourteen times as much. During the ordinary growing-season of four or five months the unhumified organic matter would have yielded practically nothing to the crop.

Functions of the unhumified Vegetable Matter.—The chief utility of the unhumified matter in the soil consists of course in its gradual conversion into true humus, in the course of which it evolves carbonic gas to act on the soil minerals; while at the same time it helps to render the soil more porous and thus facilitates the action of the aerobic bacteria, for which it serves as food. Hence the addition of vegetable matter to soils not already too “light” is always advantageous, so long as it does not introduce injurious, non-humifiable ingredients, like turpentine in the sawdust of resinous pines. But it is always advisable to first use such matter as litter for stock, in order to better prepare it for the processes of humification, under the influence of ammoniacal fermentation, such as occurs in the decay of green plants or animal matter. A portion of the ash ingredients also is quickly utilized by solution in the soil-water.

Matière Noire the Only Guide.—According to these results it is clear that in order to gain any tangible indications with respect to crop-bearing, it is the nitrogen in the humus proper, the matière noire only, that should serve as the basis; and that as a current source of nitrogen to the plant, the unhumified matter is hardly entitled to more consideration than the “insoluble silicates.” For, the favorable conditions for nitrification under which the above experiment was conducted, will very rarely be even approached under field conditions.

What are the Adequate Nitrogen Percentages in the Humus?—The nitrification of the matière noire being, apparently, the main source of plant-nutrition with that element under ordinary conditions, the question naturally arises as to what may be considered an adequate nitrogen-content of that substance, so as to permit a full supply of nitrates to the crop.

The data extant on this subject are rather scanty, and thus far have all been obtained at the California Experiment Station.[121] But they seem to be very cogent in proving that the growth of crops removed from the soil causes a rapid depletion of the nitrogen in the humus-substance, and that so soon as the nitrogen-percentage in the same falls below a certain point, the soil becomes “nitrogen-hungry;” so that the application of nitrogenous fertilizers is needed and is very effective. The data in the table below, as well as the figure of a culture experiment (No. 52 below), illustrate this point.

ADEQUACY AND INADEQUACY OF NITROGEN CONTENTS OF HUMUS. =========+=======+==================+=========+=========+=========== Collection|Kind of| Locality. |Per cent.|Per cent.| Per cent. Number. | Soil. | |Humus in |Nitrogen | Nitrogen | | | Soil. |in Humus.|in Soil. | | | | | [122] ---------+-------+------------------+---------+---------+----------- 6 | Black |Near Stockton, San| | | | Adobe.| Joaquin Co., Cal.| 1.05 | 18.66 | .196 ---------+-------+------------------+---------+---------+----------- 1679 | “ |Virgin Soil, | | | | | University | | | | | Grounds, Berkeley| 1.20 | 18.58 | .203 ---------+-------+------------------+---------+---------+----------- 1842 | “ |Ramie plot, Univ. | | | | | Grounds, 10 years| | | | | cultivated | 1.80 | 4.17 | .075 ---------+-------+------------------+---------+---------+----------- 1841 | “ |Grass plot, Univ. | | | | | Grounds, 10 years| | | | | cultivated | 1.65 | 3.40 | .056 ---------+-------+------------------+---------+---------+----------- 29 | Dark |Sugar-cane land, | | | | loam. | Maui, H. T. | 10.90 | 3.15 | .347 ---------+-------+------------------+---------+---------+----------- 27 | Dark |Guava-land hills, | | | | loam. | near Hilo, Hawaii| 9.95 | 1.71 | .170 | | Island | | | ---------+-------+------------------+---------+---------+-----------

[121] The Supply of Soil Nitrogen, Rep. Cal. Expt. Station, 1892-93, page 68; ibid., 1894-95, page 28; The Recognition of Nitrogen Hungriness in Soils, in Bull. 47, Div. of Chemistry, U.S. Department of Agriculture, 1895; Landw. Presse, No. 53, July 1885. See also for detailed data chapter 8, page 135.

[122] Calculated upon the true humus substance (matière noire), not by determining total (incl. unhumified) nitrogen in the soil.

Nos. 6 and 1679 show the usual humus-and nitrogen-percentages in the “black adobe” or “prairie” soils of California. Nos. 1842 and 1841 represent the same soil as 1679, upon which, however, ramie and ray grass had respectively been growing, without fertilization, for about ten years; showing that while the humus-content of the soil has increased, the nitrogen-content of the humus has decreased in the case of ramie by 72.78%, in that of the grass by 76.78%; reducing the land to figures commonly found in the humid region. In the case of the ramie, the partial return through the leaves has resulted in a higher humus-content, together with higher nitrogen-percentage, than in the case of the grass, which in the several cuttings annually made, caused a greater depletion in nitrogen and a smaller accession of humus. The grass was very weak in its growth and partially dying out.

No. 29, the sugar-cane land from Maui, was still in fair production, but beginning to weaken as against its first production. No. 27, the guava land from Hawaii, originally bore a luxuriant cover of wild guava, but after bearing one fair crop of seed-cane and one of ratoons, the cane planted on it “spindled up” and died so soon as the seed-cane planted was exhausted. Both the island soils, originally derived from the weathering of the black basaltic lavas of the region, were well supplied with mineral plant-food (see above, page 356), and the humus-content in both was exceptionally high; and neither was in an acid condition. The difference in their nitrogen-content, both in the totals and in the humus itself, suggested that notwithstanding the relatively high total of nitrogen in No. 27, it might be nitrogen-hungry, in view of the low percentage of the nitrogen in the humus.

Confirmatory Experiment.—A pot-culture with wheat, the results of which are shown in the figure below, fully confirm this suspicion. One kilogram of soil was used in each of two pots, one being fertilized with half a gram of Chile saltpeter. The experiment could not be carried to full completion on account of the overwhelming invasion of mildew; but the figures speak for themselves. Moreover, a field trial made on the island with saltpeter, in pursuance of the writer’s recommendation, resulted in a luxuriant growth of the cane.

Data for Nitrogen-adequacy.—It appears from the facts shown above, that for the growth of grasses a nitrogen-percentage in the humus of 1.7 is wholly inadequate, no matter how much humus may be present. A percentage of 3.15 in the Maui soil, No. 29, containing nearly 11% of humus, gave only a fair crop of sugar-cane; on the Berkeley grass plot, with 3.40% and only 1.65 of total humus, the ray grass was barely maintaining life. The ramie, with 4.17% of nitrogen in the soil-humus, was still doing fairly well.

It is doubtless impossible to give one and the same absolute figure for nitrogen-deficiency for all plants and soils. Where the conditions of nitrification are favorable, as in the presence of much of the earth carbonates, a smaller percentage may suffice for the same plants that elsewhere suffer; and it is highly probable that different minima will be found for plants of different relationship and root-habits. But there is every reason to believe that in the nitrogen-percentage of soil-humus, considered in connection with other chemical and physical conditions and soil derivations, we have a means of ascertaining the needs of plants with respect to nitrogen-fertilization, if proper study be given to the subject. Broadly speaking, it appears to be necessary to keep the nitrogen-percentage of soil-humus near 4% to insure satisfactory production.

It having been suggested that the frequent and disastrous crop failures on the noted tchernozem or black-earth soils of Russia might be due in part at least to nitrogen-depletion of the humus, the writer obtained through the courtesy of Prof. P. Kossovitch of St. Petersburg soil samples from the center of the Black-earth region, both cultivated and uncultivated. These samples are in appearance exactly like some of the dark alluvial soils of Louisiana and California, and approach them very nearly in the essentials of composition, as will be seen from the table below:

ANALYSES OF BLACK SOILS, =============================+=================+==================== | Tchernozem | Alluvial | (Russia.) | Black clay lands. +------+----------+---------+---------- | | |Louisiana|California | | | No. 240.| No. 1167. |Virgin|Cultivated+---------+---------- | | |Back-land|Black-land | | | Houma. | Tulare. -----------------------------+------+----------+---------+---------- CHEMICAL ANALYSIS OF | | | | FINE EARTH. | | | | (No coarse material in soils.)| | | | | | | | Insoluble matter | 48.38| 55.09 | 35.48 | 62.43 Soluble silica | 13.21| 12.28 | 20.76 | 16.99 Potash (K₂O) | .72| .52 | 1.03 | 1.09 Soda (Na₂O) | .20| .13 | .13 | .77 Lime (CaO) | 1.51| 1.31 | .72 | 1.46 Magnesia (MgO) | .73| .75 | .88 | 1.44 Br. ox. of Manganese (Mn₃O₄) | .05| .03 | .01 | .06 Peroxid of Iron (Fe₂O₃) | 7.12| 4.80 | 7.10 | 4.98 Alumina (Al₂O₃) | 5.22| 4.73 | 15.45 | 6.87 Phosphoric acid (P₂O₅) | .14| .13 | .15 | .12 Sulfuric acid (SO₂) | .07| .08 | .25 | .02 Carbonic acid (CO₂) | | | | Water and organic matter | 22.78| 19.94 | 18.52 | Total |100.13| 99.79 | 100.48 | 100.59 | | | | Humus | 5.11| 5.54 | 5.07 | 1.33 “ Ash | 1.80| 1.40 | .91 | .36 “ Nitrogen, per cent. | | | | in Humus | 4.63| 4.22 | | “ “ per cent. | | | | in soil | .27| .24 | | Available Potash | | | | (citric acid method) | .014| .010 | | Available Phosph. acid | .011| .008 | .08 | .01 (citric acid method) | | | | Hygroscopic Moisture | | 12.07 | 18.82 | 5.38 absorbed at | | 17°C | 13°C | 15°C -----------------------------+------+----------+---------+----------

It will be seen that the Russian soil is of high fertility according to the standards given above, and that the nitrogen-content of the abundant humus is amply within the limits of adequacy suggested by the experience in California and Hawaii. The humus-content of the arid California soils is characteristically low as compared with the Russian tchernozem as well as with the Houma backland of humid Louisiana; but its nitrogen-content is doubtless at least three times that of the latter, as is that of the humus of similar lands in which it has been determined.

INFLUENCE OF LIME UPON SOIL FERTILITY.

Assuming as substantially correct the numerical data given above in respect to the three leading ingredients of plant-food—phosphoric acid, potash and nitrogen,—the dominant role of lime in soil fertility, already mentioned, requires some farther illustration and discussion.

“A Lime Country is a Rich Country.”—The instant change of vegetation when we pass from a non-calcareous region to one having calcareous soils, has already been alluded to. (See this chapter, p. 354). But it is not necessary to be a botanist to see the change in the prosperity of the farming population as one enters a lime district. The single log-cabin with, probably, a wooden barrel terminating the mud-plastered chimney, is replaced, first by double log-houses, then by frame, and farther on by brick buildings, with the other unmistakable evidences of prosperity. Thus this is seen in passing from the mountain region of Kentucky into the “blue-grass” country, which is throughout underlaid by calcareous formations; and thus, likewise, in crossing the strike of the formations of Alabama, Mississippi and Louisiana, or any other region where underlying calcareous formations have contributed to the formation of the soils, as compared with some adjacent district where this is not the case. The calcareous loess areas bordering on the Mississippi river and some of its chief tributaries, are conspicuous cases in point, as are also the prairies of Illinois and Indiana.

Effects of High Lime-content in Soils.—The table below illustrates the fact that in the presence of high lime-percentages, relatively low percentages of phosphoric acid and potash may nevertheless prove adequate; while the same, or even higher amounts, in the absence of satisfactory lime-percentages prove insufficient for good production.[123]

[123] This statement appears contradictory of the observations of Schloesing upon the solubility of phosphoric acid in presence of lime carbonate (Am. Sci. Agron., tome 1, 1899), but the natural conditions seem to justify fully the above conclusion.

SOILS SHOWING LOW PHOSPHORIC ACID PERCENTAGE. ============================+====================================== | HIGH LIME. +-----------+---------+---------------- |Mississippi|Louisiana| California +-----------+---------+------+--------- | Kemper | Vernon | Yuba | Amador | County | County |County| County ----------------------------+-----------+---------+------+--------- Number of Sample. | 139 | 171 | 499 | 1113 ----------------------------+-----------+---------+------+--------- Chemical Analysis of | | | | Slate Fine Earth. | | | | Soil ----------------------------+-----------+---------+------+--------- Insoluble matter | | 53.19 | 78.79| 49.96 | 67.08 | 74.29| | 64.92 Soluble silica | | 21.10 | 3.80| 14.96 ----------------------------+-----------+---------+------+--------- Potash (K₂O) | .70 | .33 | .25| 1.48 Soda (Na₂O) | .14 | .06 | .04| .43 Lime (CaO) | 1.37 | 1.40 | 1.02| .60 Magnesia (MgO) | 1.00 | .74 | .40| 2.21 Br. ox. of Manganese (Mn₃O₄)| .25 | .15 | .02| .05 Peroxid of Iron (Fe₂O₃) | 6.75 | 4.52 | 5.81| 11.52 Alumina (Al₂O₃) | 13.07 | 11.36 | 6.28| 12.31 Phosphoric acid (P₂O₅) | .03 | .05 | .04| .05 Sulfuric acid (SO₃) | .08 | .12 | .02| .02 Carbonic acid (CO₂) | | | | Water and organic matter | 9.45 | 7.27 | 3.64| 6.63 +-----------+---------+------+--------- Total | 99.91 |100.29 |100.19|100.22 +-----------+---------+------+--------- Hygroscopic Moisture | 11.45 | 18.11 | 4.80 | 5.74 absorbed at °C | 8.0 | 25.5 | 15.0 | 15.0 ----------------------------+-----------+---------+------+---------

============================+======================================= | LOW LIME. +-----------------+--------------------- | Mississippi | California +---------+-------+-----------+--------- |Chickasaw|Carroll| Shasta |Humboldt | County |County | County | County ----------------------------+---------+-------+-----------+--------- Number of Sample. | 164 | 48 | 559 | 207 ----------------------------+---------+-------+-----------+--------- Chemical Analysis of | | | | Fine Earth. | | | | ----------------------------+---------+-------+-----------+--------- Insoluble matter |93.62 | | 76.27 | 65.35 | 94.98| 89.39 | 80.38| 72.24 Soluble silica | 1.36 | | 4.10 | 6.90 ----------------------------+---------+-------+-----------+--------- Potash (K₂O) | .09 | .19 | .50 | 1.13 Soda (Na₂O) | .07 | .08 | .04 | .28 Lime (CaO) | .07 | .08 | .10 | .11 Magnesia (MgO) | .13 | .07 | .40 | 3.33 Br. ox. of Manganese (Mn₃O₄)| .02 | .12 | .01 | .12 Peroxid of Iron (Fe₂O₃) | 1.09 | 1.21 | 6.67 | 6.99 Alumina (Al₂O₃) | 1.47 | 4.37 | 8.48 | 10.24 Phosphoric acid (P₂O₅) | .03 | .05 | .04 | .17 Sulfuric acid (SO₃) | .01 | .05 | .01 | .02 Carbonic acid (CO₂) | | | | Water and organic matter | 2.00 | 4.09 | 3.97 | 5.63 +---------+-------+-----------+--------- Total |99.94 | 99.70 |100.62 | 100.24 +---------+-------+-----------+--------- Hygroscopic Moisture | 1.80 | 4.66 | 5.05 | 7.87 absorbed at °C |11.0 | 11.0 | 17.0 | 13.0 ----------------------------+---------+-------+-----------+---------

Nos. 139 and 171 are heavy black prairie soils of high productive capacity, whose production had, at the time of sampling, lasted almost undiminished for over twenty years. Nearly the same is true of the two California soils, Nos. 499 and 1113; which, however, are ferruginous loams of only moderate clay-content. In all, the percentage of phosphoric acid shown by the analysis is at or below the recognized limit of deficiency, while the lime-content of all is as high as is required for the welfare of any soil, however constituted. The potash-percentage also is low in all except the “red foothill soil,” No. 1113.

Passing to the soils of low lime-content, we find the two Mississippi soils, poor in both potash, lime and phosphoric acid, so low in production as to be wholly unprofitable in cultivation without previous fertilization; No. 559, from California, produced two fair crops of barley and then no more. No. 207, is the soil of Eel river bottom, California; profusely productive at first, by virtue of its high content of both potash and phosphoric acid; but “giving out” under a few years’ culture of clover or alfalfa (which draw heavily upon lime), and quickly restored to productiveness under the influence of dressings of quicklime. In this case the soil had become acid, a condition which always militates against the success of culture plants, and more especially against those of the leguminous relationship.

What are Adequate Lime Percentages?—We have in the presence or absence of the natural vegetation peculiar to calcareous soils (“calciphile”) an excellent index of the presence or absence of such amounts of lime carbonate as fulfil the conditions of its beneficial effects. Lists of such plants for the United States are given farther on; they agree almost throughout with such plants as are everywhere recognized by American farmers as indicating productive soils.

All soils bearing such vegetation show with red litmus paper, when wetted, a neutral reaction at first, which after the lapse of twenty or thirty minutes turns to a blue alkaline one; such as is given under the same conditions by the carbonates of lime and magnesia.

But the reverse is not necessarily true; for we occasionally find soils containing considerable amounts of lime carbonate that yet fail to bear lime vegetation. This is the case of extremely heavy clay soils, as exemplified in the table below in the case of the last three soils; while the first, No. 220, exemplifies a case where although potash is exceptionally high, only scrubby oak growth is produced in presence of an amount of lime that in sandy lands would show profuse lime growth.

TABLE ILLUSTRATING THE NEED OF HIGH LIME-PERCENTAGES IN HEAVY CLAY SOILS. ============================+===========================+=========== | Mississippi. |California. +---------+-------+---------+----------- |Flatwoods| Hog- | Ridge | Yellow | Pontotoc| wallow| Prairie,| ridge, | Co. | Jasper|Smith Co.|Alameda Co. | | Co. | | ----------------------------+---------+-------+---------+----------- No. Sample. | 230 | 242 | 203 | 4 ----------------------------+---------+-------+---------+----------- CHEMICAL ANALYSIS OF | | | | FINE EARTH. | | | | ----------------------------+---------+-------+---------+----------- Insoluble matter | 77.85 | 76.76 | 51.75 | 86.00 Soluble silica | | | | ----------------------------+---------+-------+---------+----------- Potash (K₂O) | .75 | .53 | .53 | .19 Soda (Na₂O) | .11 | .19 | .22 | .15 Lime (CaO) | .18 | .42 | .48 | .48 Magnesia (MgO) | .83 | .67 | 1.01 | .45 Br. ox. of Manganese (Mn₃O₄)| .17 | .56 | .10 | .04 Peroxid of Iron (Fe₂O₃) | 5.90 | 4.12 | 23.79 | 4.01 Alumina (Al₂O₃) | 10.30 | 10.06 | 10.85 | 5.53 Phosphoric acid (P₂O₅) | .05 | .06 | .15 | .06 Sulfuric acid (SO₃) | .03 | .06 | .02 | .02 Carbonic acid (CO₂) | | | | Water and organic matter | 3.69 | 5.73 | 11.39 | 4.05 +---------+-------+---------+----------- Total | 99.86 | 99.17 | 100.29 | 100.99 +---------+-------+---------+----------- | | | | Hygroscopic Moisture | 9.3 | 6.8 | 19.7 | absorbed at °C | 22.0 |air-dry| 17.0 | ----------------------------+---------+--------+---------+-----------

All of the soils in this table are heavy clays, very difficult to till; in all, the lime-percentage falls below .5%; and none bear any lime vegetation, the Mississippi soils having a stunted growth of black jack and post oaks, such as is universally known to indicate soils too poor for profitable cultivation. The California soil bears stunted live-oak (Q. agrifolia); but not being as heavy as its brethren from Mississippi, though unthrifty, is more readily improved.

Comparison with the two first sandy soils in the table on p. 352 shows, that with plant-food percentages equal to, or even much below those here shown, not only was vigorous lime growth present, but crop-production was good and even high.

We are thus led to the conclusion that the greater the clay percentage in a soil, the more lime carbonate it must contain in order to possess the advantages of a calcareous soil; and that while in sandy lands lime growth may follow the presence of only .10% of lime, in heavy clay soils not less than about .6% should be present to bring about the same result. This is apparent to the eye in that the dark-tinted humus characteristic of truly calcareous lands, does not appear in clay soils until the lime-percentages rise to nearly 1%; while in sandy lands a much smaller amount (say .2%) will produce this effect.

European Standards.—It is of interest to consider, in connection with preceding discussions, the estimates given by Maercker of Halle, of the practical value of soils corresponding to chemical composition as ascertained by analysis with strong acids, substantially in accordance with the methods adopted by the writer.

PRACTICAL RATING OF SOILS BY PLANT-FOOD PERCENTAGES ACCORDING TO PROF. MAERCKER, HALLE STATION, GERMANY. =====================+==========+==========+==================== | | | Lime. Grade of Soil. | Potash. |Phosphoric+----------+--------- | | Acid. | Clay | Sandy | | | Soil. | Soil. ---------------------+----------+----------+----------+--------- Poor |Below 0.05|Below 0.05|Below .10|Below .05 Medium | 0.05-0.15| .05 - .10| .10- .25| .10-.15 Normal | 0.15-0.25| .10 - .15| .25- .50| .15-.20 Good | 0.25-0.40| .15 - .25| .50-1.00| .20-.30 Rich |Above 0.40| Above .25|Above 1.00|Above .30 =====================+==========+==========+==========+========= Av’age for California| 0.70 | 0.08 | 1.08 “ “ Arid Reg. | .73 | .12 | 1.36 “ “ Humid Reg.| .22 | .11 | .11 ---------------------+----------+----------+-------------------- | | Grade of Soil. | Total | Humus | Nitrogen.| Nitrogen. | | ---------------------+----------+---------- Poor | Below .05| Medium | .05-.10| Normal | .10-.15| Good | .15-.25| Rich | Above .25| =====================+==========+ Av’age for California| | .102 “ “ Arid Reg. | .11 | (?) “ “ Humid Reg.| .12 | .166 ---------------------+----------+----------

It will be observed that according to Maercker’s valuation, the average California soil is “rich” in potash and lime, but only “medium” as regards its contents of phosphoric acid and nitrogen. In this respect, and almost throughout, Maercker’s ratings are in remarkable agreement with those made by the writer as far back as 1860.[124] It also appears that Maercker’s figures for “normal” soils correspond to those of the American humid regions; the “arid” figures for potash and lime being “abnormally” high.

[124] See discussions of analyses of Mississippi soils in the Report on the Agriculture and Geology of Mississippi, 1860; same in Rep. On Cotton Production, Tenth Census, 1880, Vol. 5; also Appendix to the Report on the Experiment Stations of the University of California, 1890, p. 163.

Unfortunately neither Maercker’s method of preparing the soil extract, nor his ratings as given in the table, are accepted by all soil chemists even in Germany. As will be seen by reference to Wohltmann’s work on the soils of Samoa and Kamerun (chap. 21, p. 404), his methods and numerical estimates differ widely from those given by Maercker, and also from those adopted by the Prussian soil surveys. Reference to the analyses of the soils of Madagascar by Müntz and Rousseaux, given in the same chapter, page 406, shows still another different method, although as it happens their numerical estimates do not differ very widely from those of Wohltmann. In both cases, a special, more incisive extraction is made for the determination of potash. Why the same more energetic action is not used for the other ingredients also, is not stated, and is obscure. Fortunately, in all cases the action is at least sufficiently strong to secure the dissolution of all the lime existing in the form of carbonate, and of all, or nearly all, the phosphoric acid not securely locked up as ferric phosphate; the latter being inert, is of no special interest (see Analyses of Hawaiian Soils, this chapter, page 256).

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XX. SOILS OF THE ARID AND HUMID[125] REGIONS

Composition of Good Medium Soils.—In the preceding tables examples have been given of rather extreme types of soils, both rich and poor throughout, and also of such as are deficient in one or several of the important ingredients. In the table below are given the analyses of some of the good average farming lands; uplands of several states, both of the humid and arid regions. In the former, the representative timber trees of such lands are the black, red, white and (less characteristically) the post, black-jack, Spanish, overcup and locally some other oaks; grading higher in proportion to the presence of more or less hickory, and lower as the latter is replaced by pine. In the states south of Ohio, the “oak and hickory uplands” are what the farmer usually looks for, outside of the valleys or bottoms.

[125] In the discussion in this chapter the “humid region” referred to is always that of the temperate zones, unless expressly otherwise stated. The most humid region of all—the tropics—is treated under a special head.

Criteria of Lands of the Two Regions.—In the country west of the Rocky Mountains, the timber, while locally very characteristic, cannot be as broadly used as a criterion, partly on account of its scarcity, partly because the dominant factor in the growth of trees is moisture, which is measurably independent of chemical soil-composition. The latter, moreover, on account of climatic conditions, already alluded to (chapter 16), does not vary as materially in the arid as the humid region, on account of the almost universal presence of larger proportions of lime carbonate; the variations of which in the humid region govern largely the vegetative changes. For we there find the timber growth of the lowlands ascending into the uplands so soon as the latter becomes decidedly calcareous; as is abundantly exemplified in the loess or “bluff” formations bordering the Mississippi, Ohio, and Missouri rivers, where the black walnut, tulip tree, ash, honey-locust, together with the lowland oaks, hickories and cane usually characterizing the stream bottoms, grow abundantly and with luxuriant development on the adjoining steep hill country as well (see below, chapters 24, 25).

UPLAND SOILS OF HUMID REGION. ==================================================================== OAK UPLANDS WITH HICKORY AND WALNUT. -----------------------+----------+---------------+--------+-------- State. |Tennessee.| Mississippi. |Alabama.|Georgia. | | | | -----------------------+----------+--------+------+--------+-------- County. |Rutherford|Pontotoc|Benton|Cherokee| Polk -----------------------+----------+--------+------+--------+-------- Number of Sample. | 7 | 226 | 216 | 110 | 502 -----------------------+----------+--------+------+--------+-------- ANALYSIS OF FINE EARTH.| | | | | | | | | | Insoluble matter |75.35 |83.27 | |78.73 |72.32 | 82.66 | 88.83|83.35 | 84.77| 76.55 Soluble silica | 7.31 | 5.56 | | 6.04 | 4.23 Potash (K₂O) | .26 | .37 | .55| .26 | .73 Soda (Na₂O) | .26 | .22 | .08| .12 | .17 Lime (CaO) | .34 | .28 | .25| .33 | .29 Magnesia (MgO) | .30 | .23 | .48| .40 | .26 -----------------------+----------+--------+------+--------+-------- Bro. ox. of Manganese | | | | | (Mn₃O₄) | .04 | .28 | .76| .22 | .18 -----------------------+----------+--------+------+--------+-------- Peroxid of Iron (Fe₂O₃)| 5.18 | 2.39 | 4.80| 3.71 | 6.29 Alumina (Al₂O₃) | 5.57 | 4.51 | 6.28| 5.08 | 7.10 Phosphoric acid (P₂O₅) | .08 | .08 | .07| .09 | .26 Sulfuric acid (SO₃) | .08 | .02 | .06| .10 | .11 Carbonic acid (CO₂) | | | | | -----------------------+----------+--------+------+--------+-------- Water and organic | | | | | matter | 5.50 | 3.11 | 4.20| 5.15 | 6.60 -----------------------+----------+--------+------+--------+-------- Total |99.77 |100.32 |100.88|100.23 |99.54 -----------------------+----------+--------+------+--------+-------- Humus | | | | | “ Ash | | | | | | | | | | Hygroscopic Moisture | 7.29 | 4.08 | 6.84| 4.50 | 8.71 absorbed at °C | 22°C | | | | 18°C -----------------------+----------+--------+------+--------+-------- SHORT-LEAVED PINE. -----------------------+--------+---------+-----------------+--------- State. | North | South | Mississippi |Louisiana |Carolina|Carolina | | -----------------------+--------+---------+--------+--------+--------- County. |Cabarrus|Spartan- | Sumner |Franklin|Morehouse | | burgh | | | -----------------------+--------+---------+--------+--------+--------- Number of Sample. | 9 | 5 | 142 | 71 | 232 -----------------------+--------+---------+--------+--------+--------- ANALYSIS OF FINE EARTH.| | | | | | | | | | Insoluble matter |78.79 | 43.74 |90.23 |88.75 |81.70 | 86.19| 49.61| 92.55| 90.56| 87.45 Soluble silica | 7.40 | 5.87 | 2.32 | 1.81 | 5.75 Potash (K₂O) | .13 | .21 | .24 | .14 | .44 Soda (Na₂O) | .01 | .09 | .09 | .09 | .27 Lime (CaO) | .34 | .03 | .09 | .07 | .10 Magnesia (MgO) | .31 | .21 | .20 | .19 | .24 -----------------------+--------+---------+--------+--------+-------- Bro. ox. of Manganese | | | | | (Mn₃O₄) | .05 | .01 | .07 | .08 | .39 -----------------------+--------+---------+--------+--------+-------- Peroxid of Iron (Fe₂O₃)| 4.99 | 11.70 | 1.84 | 2.41 | 3.55 Alumina (Al₂O₃) | 4.02 | 26.54 | 1.86 | 2.20 | 4.87 Phosphoric acid (P₂O₅) | .14 | .13 | .09 | .08 | .10 Sulfuric acid (SO₃) | .08 | .01 | .01 | .01 | .08 Carbonic acid (CO₂) | | | | | -----------------------+--------+---------+--------+--------+-------- Water and organic | | | | | matter | 3.88 | 11.60 | 2.83 | 4.31 | 2.54 -----------------------+--------+---------+--------+--------+-------- Total | 100.14 |100.22 | 99.87 | 100.14 |100.03 -----------------------+--------+---------+--------+--------+-------- Humus | | | | | “ Ash | | | | | | | | | | Hygroscopic Moisture | 3.65 | 11.21 | 3.57 | 4.4 | 5.47 absorbed at °C | 21.8°C | 21.8°C | 21.8°C | | -----------------------+--------+---------+--------+--------+--------

UPLAND SOILS OF ARID REGION.

======================+============================================ | CALIFORNIA. +----------+---------+------------+---------- |Placer Co.|San Diego| Ventura Co.|Riverside | Auburn. | Co. | | Co. | |National | |Arlington. | | City. | | ----------------------+----------+---------+------------+---------- Number of Sample. | 51 | 47 | 182 | 1406 ----------------------+----------+---------+------------+---------- ANALYSIS OF FINE EARTH.| | | | ----------------------+----------+---------+------------+---------- Insoluble matter | | | 74.91 | 76.41 | 69.52 | 86.21 | 82.84 | 84.61 Soluble silica | | | 7.93 | 8.20 ----------------------+----------+---------+------------+---------- Potash (K₂O) | .38 | .48 | .62 | .87 Soda (Na₂O) | .07 | .14 | .16 | .29 Lime (CaO) | .96 | .36 | .95 | 1.57 Magnesia (MgO) | 1.09 | .54 | .96 | 1.33 ----------------------+----------+---------+------------+---------- Br. ox. of Manganese | | | | (Mn₃O₄) | .39 | .10 | .04 | .04 ----------------------+----------+---------+------------+---------- Peroxid of Iron(Fe₂O₃)| 12.42 | 3.69 | 5.07 | 4.20 Alumina (Al₂O₃) | 10.97 | 5.12 | 5.94 | 5.30 Phosphoric acid (P₂O₅)| .16 | .23 | .13 | .14 Sulfuric acid (SO₃) | .01 | .03 | .04 | .01 Carbonic acid (CO₂) | | | | ----------------------+----------+---------+------------+---------- Water and organic | 5.14 | 2.60 | 2.67 | 1.60 matter | | | | ----------------------+----------+---------+------------+---------- Total | 101.10 | 99.50 | 99.41 | 100.05 ----------------------+----------+---------+------------+---------- Humus | 1.14 | .56 | 1.06 | .20 “ Ash | 1.12 | 1.04 | 1.00 | .64 Hygroscopic Moisture | | 2.30 | 6.59 | 1.77 absorbed at °C | | 15 | 15 | 15 ----------------------+----------+---------+------------+---------- ======================+=======================++================== | WASHINGTON. || MONTANA. +---------+-------------++---------+-------- | Bunch | Bunch grass || Judith | Near | grass |Selah Valley,|| Gap. | Bozeman |Ritzville| Rolling || | Allen’s | Ridge. | Upland. || | Ranch. ----------------------+---------+-------------++---------+-------- Number of Sample. | 46 | 37 || 371 | 387 ----------------------+---------+-------------++---------+-------- ANALYSIS OF FINE EARTH.| | || | ----------------------+---------+-------------++---------+-------- Insoluble matter |76.71 | 77.18 || 74.17 |67.28 | 82.00 | 81.69 || 78.80| 73.83 Soluble silica | 5.28 | 4.59 || 4.61 | 6.54 ----------------------+---------+-------------++---------+-------- Potash (K₂O) | .72 | .62 || 1.07 |1.20 Soda (Na₂O) | .09 | .24 || .16 | .21 Lime (CaO) | 1.04 | 1.32 || .71 | 2.92 Magnesia (MgO) | .94 | .92 || 1.16 | 1.44 ----------------------+---------+-------------++---------+-------- Br. ox. of Manganese | | || | (Mn₃O₄) | .05 | .05 || .97 | .62 ----------------------+---------+-------------++---------+-------- Peroxid of Iron(Fe₂O₃)| 5.14 | 5.62 || 4.20 | 4.63 Alumina (Al₂O₃) | 5.74 | 5.24 || 7.08 | 8.09 Phosphoric acid (P₂O₅)| .16 | .13 || .12 | .18 Sulfuric acid (SO₃) | .01 | .05 || .02 | .01 Carbonic acid (CO₂) | | || 1.76 | ----------------------+---------+-------------++---------+-------- Water and organic | | || | matter | 4.58 | 3.52 || 6.54 | 5.37 ----------------------+---------+-------------++---------+-------- Total |100.48 | 99.37 || 99.94 | 99.69 ----------------------+---------+-------------++---------+-------- Humus | .90 | .48 || | “ Ash | .42 | .32 || | Hygroscopic Moisture | 5.60 | 4.84 || 9.77 |10.37 absorbed at °C | 15 | 15 || 15 | 15 ----------------------+---------+-------------++---------+--------

Soils of the Humid Region.—Taking a view, first, of the table showing the soils of the humid region, it appears that the change of vegetation from walnut and hickory to the short-leaved pine bears no visible relation to the increase or decrease of potash or phosphoric acid, but is plainly governed mainly by the amount of lime present. Where the short-leaved pine prevails the soil is almost always either neutral or shows the alkaline reaction in the course of half an hour; but where the long-leaved pine predominates the soil has almost always an acid reaction. The latter is also usually found in bottoms in which the loblolly pine (P. taeda) prevails, and where, although the soil may show a fair proportion of lime in the analysis, it does not exist in the form of carbonate.

The examples here given are from lands not derived from, or underlaid by, limestone formations. Where the latter exist the percentage of lime is usually materially increased; as it is also in the lowlands or bottoms when compared with adjacent uplands (see above, chapter 10, p. 162; chapter 18, p. 331); as well as in the delta lands of rivers.

Soils of the Arid Region.—Even a cursory comparison of the soils of the arid regions of the Pacific slope with those of the humid, as given in the above tables, shows some striking points of difference. The most obvious is the uniformly high percentage of lime, and usually also of magnesia, in the arid soils, and that quite independently of underlying formations, calcareous or otherwise. This occurs despite the fact that while limestone formations are very prevalent east of the Rocky Mountains, they are quite scarce west of the same. The red (Laramie) sandstones of Wyoming, the slates of the foothills of the Sierra Nevada, the clay shales, granites and eruptives of the Coast Ranges of California, Oregon and Washington, and the varied black rocks of the great lava sheet of the Pacific Northwest, all alike produce soils of high lime content as compared with Eastern soils not derived from calcareous formations. This fact has already been referred to, but is more fully illustrated in the table below.

Aside from the lime-content, however, it will be noted in the preceding table that the potash-content of the arid soils is on the average considerably higher than in those of the humid region. In fact it is hard to find west of the Rocky Mountains (except where high elevation causes a humid climate) any soils as poor in potash as are many of the commonly cultivated lands of the Eastern United States.

Other ingredients do not show such marked differences from the purely chemical standpoint: yet, as will be shown below, the forms in which silica and alumina occur are also not inconsiderably modified.

GENERAL COMPARISON OF SOILS FROM THE ARID AND HUMID REGIONS OF THE UNITED STATES.[126]—In order to verify the conclusions just mentioned upon the broadest basis possible, the following table has been compiled from all available sources; partly published, partly in manuscript only, having remained in the writer’s hands since the cessation of the Northern Transcontinental Survey, prosecuted from 1880 to 1883, under the auspices of the Northern Pacific Railroad, in Washington and Montana. The published data are derived partly from the records of State surveys, partly from the soil work connected with the Tenth Census; partly also from those of Experiment Stations. In most cases it has of course been necessary to restrict the comparison to such analyses as have been made by substantially identical methods, for reasons already given; but in the cases of some states from which numerous analyses made by the Kedzie method, adopted by the Association of Official Chemists, were available, the average has been given but the name of the state starred, to indicate that the percentages, excepting phosphoric acid, are lower than they would be if made by the method adopted by the writer, particularly as regards potash. The adoption of the one-millimeter mesh for the fine-earth sieve instead of the half-millimeter size also creates an unfortunate and ineliminable discrepancy.

[126] Abstracted and revised from Bulletin No. 3, U. S. Weather Bureau, 1893.

In order to exhibit clearly the influence of climate as distinct from other local conditions, it was also necessary to eliminate, in both the arid and humid regions, the soils directly derived from, or connected with calcareous formations; such as the prairies of the Southwestern States, the Bluegrass region of Kentucky, etc. This rule having been applied impartially to the soils of both climatic regions, it can hardly be questioned that the conclusions flowing from a discussion of the results of the comparison are entitled to as much weight as are those of any comparison based on large numbers of observations made, not with reference to the special point under consideration, but with a practical object of which the governing conditions were more or less uncertain, and required to be ascertained by a process of elimination.

The table gives, first, the averages for each ingredient for each of the states represented, the number of analyses from which the averages are derived being given in each case. These averages are given separately for the states of the humid and the arid regions respectively; and at the base of each group the grand average is shown in two forms. The first gives the figures as derived from the aggregate number of soil analyses in each great group, being 696 for the humid, 178 for the transition region and 573 for the arid, divided into the totals resulting from the summation of each ingredient for the whole 696, 178 and 573, respectively.

The second form is that in which the soils of each state are considered as representative of the general character of such state, as the result of intentional selection; such as actually occurred in the cases of those included in the census work of 1880. The figures given here are therefore the result of a summation of the state averages as such, and of their division by the number of states represented.

It will be noted that while these two modes of presentation do change the figures a little, yet in either form the same general result is outlined with striking accuracy. It is also notable that notwithstanding the less complete extraction of soil-ingredients in the starred (★)states, the general ratios between arid and humid soils remain substantially the same. For Western Oregon, local calcareous formations compel omission of three lime figures from the averages.

AVERAGE COMPARISON OF SOILS IN THE HUMID AND ARID REGIONS OF THE UNITED STATES. (A) = Number analyzed. (B) = Insoluble Residue. (C) = Soluble Silica. (D) = Sum of Insoluble Residue and Soluble Silica. (E) = Potash (F) = Soda. (G) = Lime. (H) = Magnesia. ===========================+===+=====+=====+=====+===+====+====+==== | | | | | | | | |(A)| (B) | (C) | (D) |(E)| (F)| (G)| (H) | | | | | | | | ---------------------------+---+-----+-----+-----+---+----+----+---- | | | | | | | | HUMID REGION. | | | | | | | | | | | | | | | | Rhode Island ★ | 7|82.41| 1.69|84.10|.15| .09| .43| .27 North Carolina | 20|81.63| 3.50|85.13|.15| .06| .09| .08 South Carolina | 30|85.54| 3.39|88.93|.12| .07| .07| .12 Georgia | 40|86.07| 2.89|88.96|.15| .07| .08| .10 Florida | 10|85.33| 1.33|86.66|.07| .03| .09| .03 Alabama | 50|81.58| 4.89|86.47|.23| .07| .17| .21 Mississippi | 97|85.87| 4.39|90.26|.28| .11| .15| .31 Louisiana | 35|81.12| 3.54|84.66|.19| .09| .16| .23 Arkansas | 38| | |88.54|.17| .06| .08| .43 Kentucky |185| | |86.72|.20| .10| .08| .19 Ohio ★ |140| | |87.00|.26| .35| .28| .44 | | | | | | | | Oregon (W. of Cascades) | 44|64.82| 5.38|70.20|.23| .19| .83| .73 Average for Humid Region |696|84.17| 4.04|88.21|.21| .14| .13| .29 “ by States | |81.59| 3.45|85.04|.18| .11| .11| .26 | | | | | | | | TRANSITION REGION. | | | | | | | | | | | | | | | | Minnesota. ★--Semi-humid |144|76.60| 8.74|85.34|.30| .23| .65| .43 North Dakota. ★--Semi-arid| 34|68.44| 7.28|75.72|.42| .73| .91| .64 Average for region |178|75.04| 8.46|83.50|.33| .32| .70| .47 | | | | | | | | ARID REGION. | | | | | | | | | | | | | | | | Montana | 59|70.98| 4.17|75.15|.87| .27|1.03|1.36 Idaho | 17|75.34| 5.22|80.56|.56| .26| .85|1.11 Wyoming ★ | 23|76.86| 2.25|79.11|.64| .41|1.91|1.31 Colorado ★ | 16|77.70| 7.10|84.80|.44| .44|1.43| .81 Utah ★ | 38| | |81.04|.98| .53|1.77| .73 Arizona | 20|64.58|13.78|78.36|.82| .43|2.37|1.89 Nevada ★ | 22|71.77| 5.95|77.72|.54| .93|2.04| .96 California |262|66.28| 9.79|76.07|.61| .29|1.25|1.50 ---------------------------+---+-----+-----+-----+---+----+----+---- Oregon East of | 7|72.10| 9.68|81.78|.54| .26|1.23| .73 Washington Cascades |109|71.60| 6.09|77.69|.65| .36|1.25| .96 ---------------------------+---+-----+-----+-----+---+----+----+---- Averages for Arid Region |573|69.16| 6.71|75.87|.67| .35|1.43|1.27 “ by States | |71.91| 7.11|79.02|.67| .42|1.61|1.14 ---------------------------+---+-----+-----+-----+---+----+----+----

(I) = Br. oxide Manganese. (J) = Peroxid of Iron. (K) = Alumina. (L) = Phosphoric Acid. (M) = Sulfuric Acid. (N) = Water and Organic Matter. (O) = Hygroscopic Moisture. (P) = Humus. (Q) = Nitrogen in Humus. (R) = Nitrogen in Soil. ===========================+===+====+====+===+===+=====+====+====+=====+=== | | | | | | | | | | |(I)| (J)| (K)|(L)|(M)| (N) | (O)|(P )| (Q) |(R) | | | | | | | | | | ---------------------------+---+----+----+---+---+-----+----+----+-----+--- | | | | | | | | | | HUMID REGION. | | | | | | | | | | | | | | | | | | | | Rhode Island ★ |.04|3.59|3.66|.09|.10| 7.43|5.23|2.59| | North Carolina |.07|4.72|5.71|.12|.06| 3.98|4.18| | | South Carolina |.05|2.47|4.59|.11|.06| 3.39|4.22| .42| | Georgia |.09|2.75|4.02|.11|.10| 3.62|3.54| | | Florida |.08| .60|1.17|.08|.05| 1.89|1.72| | | Alabama |.12|3.81|2.70|.13|.05| 4.04|7.07| | | Mississippi |.14|2.64|4.07|.09|.03| 3.33|5.41| | | Louisiana |.02|3.12|4.24|.10|.05| 4.26|6.66| | | Arkansas |.21|3.10|3.51|.15|.05| 3.70|2.47| | | Kentucky |.20|6.01|3.52|.11|.04| 3.69|1.85| | | Ohio ★ | |3.11|3.36|.11|.04| 5.04| | | | Oregon (W. of Cascades) |.09| 11.64 |.23|.16| 3.20| |1.55| | Average for Humid Region |.13|3.88|3.66|.12|.05| 4.40| |1.22| | “ by States |.10|3.26|3.68|.12|.06| 3.96| |1.52| | | | | | | | | | | | TRANSITION REGION. | | | | | | | | | | | | | | | | | | | | Minnesota. ★--Semi-humid | |2.83|4.43|.22|.01| 7.30| |2.91| 6.53|.19 North Dakota. ★--Semi-arid| |3.62|5.17|.19|.05|13.85| |4.67| 7.28|.34 Average for region | |2.08|4.57|.21|.02| 8.55| |3.24| 6.67|.22 | | | | | | | | | | ARID REGION. | | | | | | | | | | | | | | | | | | | | Montana |.37|4.28|6.81|.22|.06| |7.14| | | Idaho |.02|3.85|6.38|.16| | 5.01|2.00|1.68| 5.95|.10 Wyoming ★ | |3.05|6.61|.18|.11| 5.48| | | | Colorado ★ | |3.82|4.98|.23|.03| 3.57|2.31| | |.03 Utah ★ |.03|3.08|5.50|.22| | 6.27|2.37| | | Arizona |.06|4.92|6.43|.13|.06| 3.57| |1.65| | Nevada ★ |.32|5.67|5.00|.32|.14| 5.93| | | |.12 California |.06|6.61|8.44|.10|.06| 4.74|6.09|0.91|15.23|.14 Oregon } East of |.08| 10.75 |.11|tr.| 4.40| | .67| | Washington } Cascades | |5.37|6.31|.21|.03| 5.77|5.14| | | Averages for Arid Region |.11|5.48|7.21|.16|.06| 5.15|5.46|1.13|12.50|.13 “ by States |.13|4.74|6.27|.19|.07| 4.71|5.34|1.03|10.59|.10 ---------------------------+---+----+----+---+---+-----+----+----+-----+---

New Mexico.—Few analyses of New Mexico soils have been made, but the average results of six partial determinations made by Goss, and one full analysis made by Hare according to the method of the writer, and given below, show substantial accord with the averages of the above table. The averages of Goss’ determinations are: Potash .780, Phosphoric acid .221, Nitrogen .108 per cent.

CHEMICAL ANALYSIS OF RIO GRANDE SILT (by Prof. R. F. Hare.)

Deposited on land by irrigation.

Insoluble matter 63.70 Potash (K₂O) 1.06 Soda (Na₂O) .22 Lime (CaO) 4.97 Magnesia (MgO) 2.43 Br. ox. of Manganese (Mn₃O₄) .14 Peroxid of Iron (Fe₂O₃) 5.80 Alumina (Al₂O₃) 6.86 Phosphoric acid (P₃O₅) .16 Sulfuric acid (SO₃) .13 Carbonic acid (CO₂) 7.45 Water and organic matter 9.98

Humus 1.17 “ Nitrogen 11.11 “ “ per cent. in soil .13

Hygroscopic Moisture absorbed at °C 2.63

DISCUSSION OF THE TABLE.

Lime.—Considering in this table, first, lime, a glance at the columns for the two regions shows a surprising and evidently intrinsic and material difference, approximating in the average by totals to the proportion of 1 to 11; in the average by states, 1 to 14½. This difference is so great that no accidental errors in the selection or analysis of the soils can to any material degree weaken the overwhelming proof of the correctness of the inference drawn upon theoretical grounds, viz., that the soils of the arid regions must be richer in lime than those of the humid countries. For the differences in derivation would, in view of the wide prevalence of limestone formations in the humid regions concerned, produce exactly the reverse condition of things from that which is actually found to exist; and if further proof were needed it can readily be found in the detailed discussion of the analyses of the soils of the arid areas forming the contrast. This shows that for instance, in Washington highly calcareous soils are directly derived from the black basaltic rocks; while similarly, calcareous lands are found in California to be the outcome of the decomposition of granites, diorites, lavas, clay-shales and sandstones.

It is not easy to overrate the importance of this feature of the soils of the arid region, as it is intimately connected with other theoretically and practically important facts, in part already mentioned.

Summary of Effects of Lime Carbonate in Soils.—It is best to summarize, briefly, at this point, the advantages (and possible disadvantages), resulting from the presence of a proper amount of lime carbonate in soils, so far as these are at present understood.

Physically, even a small amount of lime carbonate, by its solubility in the carbonated soil-water, will act most beneficially in causing the flocculation of clay and in the subsequent conservation of the flocculent or tilth condition, by acting as a light cement holding the soil-crumbs together when the capillary water has evaporated; thus favoring the penetration of both water and air, and of the roots themselves. It should be added that according to the experience of the writer, amounts of lime carbonate in excess of 2% do not add to the favorable effects, except as would so much sand.

As to chemical effects, among the most important are:—

1. The maintenance of the neutrality of the soil, by the neutralization of acids formed by the decay of organic matter, or otherwise.

2. The maintenance, in connection with the proper degrees of moisture and warmth, of the conditions of abundant bacterial life (see above, chapter 9, p. 146); more especially those of nitrification, thus supplying the readily assimilable form of nitrogen. Also in favoring the development and activity of the root bacteria of legumes, and of the other nitrogen-gathering bacteria, such as Azotobacter (ibid. p. 156).

3. The rendering available, directly or indirectly, of relatively small percentages of plant-food, notably phosphoric acid and potash; as shown in the preceding pages.

4. The prompt conversion of vegetable matter into black, neutral humus, and (as shown in the case of the soils of the arid region) the concentration of the nitrogen in the same; while accelerating the oxidation of the carbon and hydrogen, as shown by S. W. Johnson and others.

6. It counteracts the deleterious influence of an excess of magnesia in the soil, as first shown by Loew,[127] and verified by his pupils in Japan.

7. In alkali soils, according to Cameron and May, it counteracts the injurious action of the soluble salts upon the growth of plants, not only in the form of carbonate, but also in those of sulfate and chlorid.[128]

[127] Bull. No. 1, Div. Veget. Physiol, and Plant Pathol. U. S. Dept. Agr.; et al.

[128] Loeb, Publications of the Spreckel’s Physiological Laboratory of the University of California, has shown a similar protective influence of the lime salts in sea-water, against the other salts, in the case of the lower marine organisms.

8. As a matter of experience, both in the case of grapes and orchard as well as wild fruits, an adequate but not excessive supply of lime in the soil will produce sweeter fruit than when lime is in small supply.

9. An excess of carbonate of lime in soils (from eight to twenty per cent and more), constituting “marliness,” tends to seriously disturb the nutrition and general functions of many plants (calcifuge), and to produce a suppression or diminution of the formation of chlorophyll and starch; as in the case of grape vines, citrus fruits and others, which nevertheless flourish best in lands moderately calcareous.

Among the points thus enumerated the third and fourth require some comment. Without pretending to define exactly how lime acts in rendering other ingredients more available to plant assimilation, attention may be called to the fact that lime carbonate may be considered as acting similarly to, albeit more mildly than, caustic lime, in the displacement of other bases from their compounds. It doubtless acts thus in liberating potash from its zeolitic compounds. As to phosphoric acid, the connection of the effect of lime carbonate with the remarkable availability of that substance when present in the form of tetra-basic salt, in the case of phosphate slag, is at least possible.

As to the action of lime carbonate in forming humus,[129] no one who has observed the characteristic dark black tint of our calcareous “prairie soils” can question the fact; which moreover is perfectly explicable upon the analogy already alluded to, with caustic lime, which, together with caustic alkalies (potash and soda), is known to act powerfully in the conversion of vegetable matter into humus. That instead of liberating the nitrogen in the form of ammonia, as do the caustic hydrates, the milder carbonate should only cause the formation of humic amides, is quite intelligible. That such is really the case, has been conclusively proved by the investigations of the writer made conjointly with M. E. Jaffa (Rep. Sta. Cal. Agr. Expt. 1892-4); the general result being that while in the humid region the average nitrogen-content of soil-humus is less than 5%, in the upland soils of the arid region (where all soils are calcareous) that percentage rises as high as 22.0%, with a general average of between 15 and 16%. That such highly nitrogenous material can be more readily attacked by the nitrifying bacteria than when a large excess of other oxidable matter is present, is at least a legitimate presumption, especially in view of the very active nitrification known to take place in the arid regions everywhere. So long as a large excess of carbohydrates is present, the oxidation of these will naturally take precedence over that of the relatively inert nitrogen. The accumulation of the latter in the humus-substance of the arid region, where oxidation of the organic matter of the soil is very active, points strongly to this view of the case.

[129] “Black Soils;” Agric. Science, January, 1892.

Magnesia.—While the differences in respect to the proportions of lime are the most prominent and decided, yet the related substance, magnesia, shows also a very marked and constant difference as between the soils of the humid and arid regions. It will be observed that the general average for magnesia in the soils of the Atlantic Slope is about double that of lime; Florida and Rhode Island being the only states in which the average is lower for magnesia than for lime. In the arid region, on the contrary, magnesia on the general average is nearly the same as lime; in the average by states, somewhat less; thus bringing the ratio for the two regions for magnesia up to one to six or seven. This also is so decisive a showing that no accident could bring it about. We must conclude that climatic influences have dealt with magnesia similarly as with lime; which from the standpoint of the chemist is just what might be expected, since magnesia carbonate behaves very much like that of lime toward carbonated waters.

That magnesia is a very important plant-food ingredient is apparent from its invariable and rather abundant presence in the seeds of plants, where it takes precedence of lime. Its functions in plant nutrition have been specially investigated by O. Loew,[130] particularly with respect to its relations to lime. As already stated in connection with the soil-forming properties of magnesian minerals (see chapter 2), soils containing large proportions of magnesia generally are found to be unthrifty, the lands so constituted being frequently designated as “barrens.” Loew finds that certain proportions of lime to magnesia must be preserved if production is to be satisfactory, the proportion varying with different plants, some of which (e. g. oats) will do well when the proportion of lime to magnesia is as 1:1, while others require, that that ratio should be as 2 or 3 is to 1, to secure the best results. In general it is best that lime should exceed magnesia in amount.

[130] Bull. No. 18, Div. Vegetable Physiology and Plant Pathology; Bull. No. 1, Bureau of Plant Industry, U. S. Dept. of Agr.; Bull. College of Agriculture, Tokyo, Vol. 4, No. 5.

Loew explains the injurious action of magnesium salts thus: The calcium nucleo-proteids of the organic structures are transformed in presence of soluble salts of magnesium into magnesium compounds, while the calcium of the former enters into combination with the acid of the magnesium salt. By this transformation the capacity for imbibition will change, which must result in a fatal disturbance of functions. The presence of soluble lime salts will prevent that interchange. Thus certain algæ perished in a solution containing 1 per 1000 of magnesium nitrate, but remained alive when .3 per 1000 of calcium nitrate was added.

Magnesia seems to be specially concerned in the transfer of phosphoric acid through the plant tissues, in the form of dimagnesic-hydric phosphate, which is rather soluble in the acid juices of plants. It is probable that, apart from the relations just referred to, such excess of lime as is known to produce chlorosis in plants interferes with the transfer of the magnesic phosphate. Some plants, as already stated, dispose of an excess of lime by depositing it in the form of oxalate, while others (such as the stone crops) excrete it on the surface of leaves and stems in the form of carbonate. But others seem to possess this power to a limited extent only.

In the case of soils containing much magnesia the proper proportion between it and lime may easily be disturbed by the greater ease with which lime carbonate is carried away by carbonated water into the subsoil, thus leaving the magnesia in undesirable excess in the surface soil. Hence the great advantage of having in a soil, from the outset, an ample proportion of lime. From this point of view alone, then, the analytical determination of lime and magnesia in soils is of high practical value.

Aso, Furuta and Katayama (Bull. Coll. Agr. Tokyo, Vol. 4 No. 5; Ibid. Vol. 6), have by direct experiment determined the most advantageous ratio of lime to magnesia in several crop plants. They find for rice and oats 1:1, for cabbage 2:1, for buckwheat 3:1; there being apparently a connection between the extent of leaf-surface and lime requirement, since leaves contain predominantly lime, while in the fruit, magnesia predominates.

Manganese.—A decided difference in the manganese content of the arid as against the humid soils appears in the table, the ratio being about 11:13 in favor of the humid soils. Manganese has not been regarded as being of special importance to plant growth in general, although, as already stated, some plants contain a relatively large proportion of manganese in their ashes; thus, e. g., the leaves of the long-leaved pine of the cotton states.[131] But no definite data showing the importance of this element to crops were available until Loew and his co-workers at Tokyo[132] established its stimulating action in a number of cases, in which crop production was materially increased by the use of protoxid salts of manganese. Aso[133] applied manganous chlorid to an experimental plot of thirty square meters, at the rate of twenty-five kilos of Mn₃O₄ per acre, and thus obtained a yield of rice one-third greater than on the control plot, at a cost of about $2.00, while the value of the increase of the product was nearly $68.00. More experimental evidence on this subject is required to establish the general value of the large-scale use of the salts of manganese; which are obtained in large quantities as a comparatively valueless by-product of the bleaching industries.

[131] Rep. Agr. and Geology of Mississippi, 1860, p. 360.

[132] Bull. Agr. Coll. Tokyo, Vol. V., Nos. 2 and 4.

[133] Ibid. Vol. 6.

The “Insoluble Residue.”

Remembering, in discussing the facts shown by the table, that the fundamental difference between the regime of the humid and arid regions is the presence in the latter of an almost continuous leaching process, in which the carbonated water of the soil is the solvent; remembering, also, that the least soluble portion of rocks and soils is quartz or silica (sand, as usually understood), it would be predicable that this ingredient should in the humid region be found to be more abundant in soils than in the arid. This portion is represented by the “insoluble residue” of the table.

Inspection shows that both in the averages of the single states, and in both of the general averages, this difference between the soils of the humid and the arid regions of the United States is strongly pronounced; the ratio being substantially as 69% in the arid region to 84% in the humid.

We must then conclude that the leaching process must have influenced materially other soil ingredients than lime, which have remained behind in such amounts as to depress the percentage of insoluble residue in the soils. It remains to be shown what are the substances so retained.

Insoluble and Soluble Silica and Alumina.

The ingredient most nearly correlated with the insoluble residue is the free silica which remains behind with it when the acid with which the soil has been treated is evaporated to dryness. The silica is separated from the practically insoluble, undecomposed minerals by boiling with a strong solution of sodic carbonate. The amount of this “soluble silica” is obviously the measure of the extent to which the soil-silicates have been decomposed in the treatment with acid.

The most prominent of these is usually supposed to be clay—the hydrous silicate of alumina that in its purest condition forms kaolinite or porcelain earth. Any alumina found in the usual course of soil analysis is generally referred to this mineral, which contains silica and alumina nearly in the proportion of 46% to 40%.

In very many cases, however, the reference of these two ingredients to clay is manifestly unjustified. This is clearly so when (as not unfrequently happens) the amount of alumina found exceeds that which would form clay with the ascertained percentage of soluble silica; it is almost as certainly so when, in addition to the alumina, other bases (notably potash, lime and magnesia), are found in proportions which preclude their being in combination with any other acidic compounds present. The only possible inference in such cases is that these bases, together with at least a portion of the alumina, are present in the form of hydrated, and therefore easily decomposable silicates or zeolites.

The subjoined analysis by R. H. Loughridge, of a clay obtained in the usual process of mechanical soil analysis (by precipitating with common salt the turbid water remaining after 24 hours subsidence in a column of 200 millimeters) from a very generalized soil of northern Mississippi, shows one of the many cases in which the numerical ratios of the several ingredients are incompatible with the assumption that silica and alumina are present in combination as clay (kaolinite) only:

ANALYSIS OF COLLOIDAL CLAY.

Insoluble matter 15.96 Soluble silica 33.10 Potash (K₂O) 1.47 Soda (Na₂O) 1.70 Lime (CaO) .09 Magnesia (MgO) 1.33 Br. ox. of Manganese (Mn₃O₄) .30 Peroxid of iron (Fe₂O₃) 18.76 Alumina (Al₂O₃) 18.19 Phosphoric acid (P₂O₅) .18 Sulfuric acid (SO₃) .06 Carbonic acid (CO₂) .00 Water and organic matter 9.00 ------ Total 100.14

If in this case we assign all alumina to silica, as required for the composition of kaolinite or pure clay, there yet remains a trifle over twelve (12.17) per cent of silica to be allotted to the other bases present. Deducting from this the ascertained amount of silica soluble in sodic carbonate, pre-existing in the raw material (.38 per cent), we come to 11.79 per cent as the amount of silica which must have been in combinations other than kaolinite, viz., hydrous silicates, or soil zeolites, formed either with the bases other than alumina shown in the analysis or, more probably, containing some of the alumina itself in essential combination.

We are thus enabled to obtain from the determination of the soluble silica an estimate of the extent to which these soil zeolites, that form so important a portion of the soil in being the repositories of the reserve of more or less available mineral plant-food, are present in the soils of the several regions. A glance at the table shows that the general average of soluble silica is very much greater in the soils of the arid regions than in those of the humid, approximating one to two in favor of the arid division.[134]

[134] Looking at the details of the several states, we find that on the arid side Washington has a relatively low figure for soluble silica, which in the average, however, is overborne by the high figures for California and Montana. The explanation of this fact probably lies in the derivation of the majority of the Washington soils examined, from lake deposits brought down gradually from the humid region at the heads of the Columbia drainage, where sandy beds are very prevalent; while the country rock—the basaltic eruptives—are very basic, and moreover slow to disintegrate. In California and Montana the rocks are infinitely varied, and the general outcome of their weathering is plainly a predominance of complex hydrous silicates in the soils, as compared with humid regions.

Differences in the Sands of the Arid and Humid Regions.—In chapter 5 mention has been made of the fact that while in the humid regions, “sand” as a rule means quartz grains, mostly with a clean surface and very frequently rounded and polished, in the arid regions even the coarse sand grains consist of, or are covered with, a great variety of minerals in a partially decomposed condition. This is owing to the absence of the abundant rainfall which in humid climates continually washes down the finely divided, half-decomposed mineral matter into the subsoil; while in arid climates the light rains cannot produce any such washing effect and hence the sand grains remain incrusted with the products of either their own decomposition, or of that of neighboring particles; it being therefore not concentrated in the finer portion only, viz., the clay and finest silts. This fundamental difference, which is illustrated in the analytical table below, at once explains why in the arid regions generally, sandy soils are found so highly productive that, owing to their easy cultivation they are preferred to the clayey lands, in which tillage and irrigation are more difficult. It is a well-known fact that on the “sands of the desert” when either irrigated, or wetted by rain, vegetation at once springs up with remarkable luxuriance, even on sand drifts; and this productiveness appears to be quite as lasting as that of “strong” clay soils of the humid regions.

This difference is curiously illustrated on the southern edge of the “black adobe” or prairie soil area which surrounds Stockton, Cal. Here we find on the opposite sides of a small stream (French Camp slough) the two extremes, of heavy clay and the sandy soils which for many years made Stanislaus county the “banner” county for wheat. The grain product of both banks ranked alike in quantity and quality in average years; but in extreme seasons sometimes one, sometimes the other failed, according to the weather conditions which favored one or the other soil. No one would think of sowing wheat on so sandy a soil in the humid States.

Table Illustrating Difference in Sands of the Humid and Arid Regions.

===============================+========+=======+=====+========= |Per cent|Potash.|Lime.|Magnesia. Clay. |in Soil.| | | -------------------------------+--------+-------+-----+--------- Mississippi[135] | 21.64 | .32 | .03 | .29 California 1281 Chino[136] | 7.60 | .16 | .14 | .17 “ Jackson[137] | 16.43 | .13 | .12 | .08 Silt .06-.016 mm. diam. | | | | Mississippi | 35.10 | .41 | .15 | .36 California (Chino) | 18.53 | .24 | .53 | .29 “ Jackson | 34.90 | .10 | .04 | .08 Silt .016-.025 mm. diam. | | | | Mississippi | 13.67 | .12 | .09 | .10 California, Chino | 5.49 | .05 | .11 | .02 “ Jackson | 9.96 | .08 | .04 | .10 Silt .025-.036 mm. diam. | | | | Mississippi | | | | California, Chino | 3.92 | | | “ Jackson | 7.68 | .06 | .02 | Silt .036-.047 mm. diam. | | | | Mississippi | | | | California, Chino | 6.40 | .05 | .18 | “ Jackson | 8.21 | .04 | .01 | Coarse Silt .047-.072 mm. diam.| | | | California, Chino | 7.92 | .06 | .23 | “ Jackson | 5.91 | .01 | .01 | Fine sand .072-.12 mm. diam. | | | | California, Chino | 11.87 | .06 | .26 | “ Jackson | 4.03 | .01 | .01 | Sand .12-.50 mm diam. | | | | California, Chino | 36.11 | .11 | .69 | “ Jackson | 10.10 | | | -------------------------------+--------+-------+-----+---------

===============================+==========+=======+================= |Phosphoric|Soluble|Alumina.| Summ- Clay. | Acid. |Silica.| | ation. -------------------------------+----------+-------+--------+-------- Mississippi[138] | .04 | 7.17 | 3.97 | 11.82 California 1281 Chino[139] | .04 | 1.70 | 1.35 | 3.56 “ Jackson[140] | .05 | 2.83 | 2.13 | 5.34 -------------------------------+----------+-------+--------+-------- Silt .06-.016 mm. diam. | | | | Mississippi | .07 | 2.87 | 1.36 | 5.22 California (Chino) | .06 | 4.96 | 1.76 | 7.84 “ Jackson | .02 | 2.50 | 2.44 | 5.18 -------------------------------+----------+-------+--------+-------- Silt .016-.025 mm. diam. | | | | Mississippi | .02 | .32 | .17 | .82 California, Chino | .01 | .80 | .51 | 1.50 “ Jackson | .007 | 1.01 | 1.01 | 2.25 -------------------------------+----------+-------+--------+-------- Silt .025-.036 mm. diam. | | | | Mississippi | | | | .36 California, Chino | | | | lost “ Jackson | .006 | 0.82 | .74 | 1.70 -------------------------------+----------+-------+--------+-------- Silt .036-.047 mm. diam. | | | | Mississippi | | | .55 | trace California, Chino | .01 | .80 | .64 | 1.66 “ Jackson | .001 | .43 | | 1.12 -------------------------------+----------+-------+--------+-------- Coarse Silt .047-.072 mm. diam.| | | | California, Chino | .02 | .89 | .59 | 1.79 “ Jackson | .003 | .42 | .30 | .77 -------------------------------+----------+-------+--------+-------- Fine sand .072-.12 mm. diam. | | | | California, Chino | .03 | .98 | | 1.43 “ Jackson | .003 | .28 | .09 | .40 -------------------------------+----------+-------+--------+-------- Sand .12-.50 mm diam. | | | | California, Chino | .04 | 2.43 | 1.59 | 4.98 “ Jackson | | | |Not detd -------------------------------+----------+-------+--------+--------

[135] Analyses by R. H. Loughridge.

[136] Analyses by L. M. Tolman.

[137] Analyses by E. H. Lea.

[138] Analyses by R. H. Loughridge.

[139] Analyses by L. M. Tolman.

[140] Analyses by E. H. Lea.

It thus appears that while in the Mississippi soil, solubility of plant-food practically ceased at grain-diameter of .036 mm, in the arid California soils, as large an amount was found in the sand-grain sizes between .12 and .50 millimeters as in the fine silt .016 to .025 mm. in Mississippi.

Hydrous Silicates are More Abundant in Arid than Humid Soils.—This predominance of hydrous silicates in the soils of the arid regions should not be a matter of surprise when we consider the agencies which are brought to bear upon these soils with so much greater intensity than can be the case where the solutions resulting from the weathering process are continually removed as fast as formed, by the continuous leaching effect of atmospheric waters. In the soils of regions where summer rains are insignificant or wanting, these solutions not only remain, but are concentrated by evaporation to a point that, in the nature of the case, can never be reached in humid climates. Prominent among these soluble ingredients are the silicates and carbonates of the two alkalies, potash and soda. The former, when filtered through a soil containing the carbonates of lime and magnesia, will soon be transformed into complex silicates, in which potash takes precedence of soda, and which, existing in a very finely divided (at the outset in a gelatinous) condition, serve as an ever-ready reservoir to catch and store the lingering alkalies as they are set free from the rocks, whether in the form of soluble silicates or carbonates. The latter have another important effect: in the concentrated form at least, they, themselves, are effective in decomposing silicate minerals refractory to milder agencies, such as calcic carbonate solution; and thus the more decomposed state in which we find the soil minerals of the arid regions is intelligible on that ground alone.

It must not be forgotten that lime carbonate, though less effective than the corresponding alkali solutions, nevertheless is also known to produce, by long-continued action, chemical effects similar to those that are more quickly and energetically brought about by the action of caustic lime. In fact, the agricultural effects of “liming” are only in degree different from those produced by marling with finely pulverized carbonate; and in nature the same relation is strikingly exemplified in the peculiarly black humus that is characteristic of calcareous lands, but which can be much more quickly formed under the influence of caustic lime on peaty soils.

In the analysis of silicates we employ caustic lime for the setting-free of the alkalies and the formation of easily decomposable silicates, by igniting the mixture; but the carbonate will slowly produce a similar change, both in the laboratory and in the soils in which it is constantly present. This is strikingly seen when we contrast the analyses of calcareous clay soils of the humid region with the corresponding non-calcareous ones of the same. In the former the proportions of dissolved silica and alumina are almost invariably much greater than in the latter, so far as such comparisons are practicable without assured absolute identity of materials. That is, calcareous clays or clay soils are so sure to yield to the analyst large precipitates of alumina, that experience teaches him to employ smaller amounts for analysis than he would of non-calcareous materials, in order to avoid unmanageably large bulks of aluminic hydrate. It is but rarely that even the heaviest non-calcareous soils yield to the acid usually used in soil analysis more than 10 per cent of alumina; while heavy calcareous clay (prairie) soils commonly yield between 13 and 20 per cent.[141] It would be interesting to verify this relation by artificial digestions of one and the same clays with calcic carbonate at high temperatures, as it must always be extremely difficult to insure absolute identity of all other conditions in natural materials.

In most of these cases, what is true of alumina is also true of the soluble silica. But since the latter is constantly liable to be dissolved out by solutions of carbonated alkalies, it is not surprising that this relation is not always shown.

[141] Report of the Tenth Census, Vols. 5 & 6; see especially the analyses of soils from Mississippi and Alabama. Also the Reports of the California Experiment Station.

Aluminic Hydrate.—In numerous cases, the amount of alumina dissolved in analysis is greatly in excess of the soluble silica, so as to force the conclusion that a portion of the latter must be present in a different form from that of clay (kaolinite); the only choice being between that of complex hydrous silicates (none of which, however, could contain as large a percentage of alumina as clay itself) and aluminic hydrate. The latter is alone capable of explaining the presence of more alumina than silica in easily soluble form;[142] and the visible occurrence of “gibbsite” and “bauxite” in modern formaations renders this a perfectly simple and acceptable explanation. Since these minerals are known to be incapable of crystallization, we are moreover led to the presumption that it will as a rule be found in the finest portions of the soil, viz., in the “clay” of mechanical analysis.

[142] Excepting the relatively rare minerals of the Allophane, Kollyrite, and Miloshite group.

Some illustrations of these conditions are given below, for soils from Mississippi and California. The soluble silica being all assigned to kaolinite, the rest of the alumina must be assumed to be present as hydrate, since no other compound could fulfil the stoichiometrical requirements.[143] The table therefore shows the differences between the amounts of alumina found by analysis, and those assignable to kaolinite, calculated to the mineral bauxite—the most abundant, as well as the one containing the medium proportion of water, among the three naturally occurring aluminic hydrates.

TABLE SHOWING EXCESS OF ALUMINA OVER SILICA IN SOILS; CALCULATED AS BAUXITE.

(A) = Total soluble in HCl. (B) = Corresponding to Bauxite. (C) = Other Soluble Matters. Miss. = Mississippi. Cal. = California. -------+---------------+--------+------+-----+----+-----+-----+----- Number.| Name of Soil. |County. |State.| (A) |SiO₂|Al₂O₃| (B) | (C) -------+---------------+--------+------+-----+----+-----+-----+----- 195 |Prairie |Alcorn | Miss.|28.57| 3.6| 14.4|14.12| 2.92 346 |Dark Loam |Chicasaw| “ |10.32| 6.6| 11.2| 6.91| .86 288 |Flatwoods Clay |Pontotoc| “ |26.94| 5.0| 11.3| 8.75| 3.48 676 |Red Volcanic |Lake | Cal.|41.00| 5.9| 22.6|21.90| 2.00 332 |Mojave Desert |Kern | “ |24.82| 5.0| 9.2| 6.10| 5.13 191 |Red Foothill |Merced | “ |23.32| 4.5| 8.8| 6.20| 3.05 -------+---------------+--------+------+-----+----+-----+-----+----- 705 |Red Chaparral |Shasta | “ |28.75| 5.5| 14.4|12.10| 1.12 706 | “ “ | | | | | | | | Subsoil | “ | “ |28.40| 4.7| 17.4|16.70| 1.32 -------+---------------+--------+------+-----+----+-----+-----+----- 573 |Tulare Plains |Tulare | “ |29.27| 3.4| 8.7| 7.20|11.16 701 |Dry Bog | “ | “ |27.29| 4.3| 12.4|10.90| 5.04 1004 |“Slickens“ Sed.|Butte | “ |30.80| 8.0| 14.2| 9.20| 1.95 656 | “ “ |Yuba | “ |22.23| 3.0| 10.4| 9.80| 2.19 517 |Brownish Loam |Butte | “ |29.80| 4.8| 12.0| 9.80| 4.42 561 |Black Loam | “ | “ |30.21| 3.2| 13.0|12.80| 4.67 -------+---------------+--------+------+-----+----+-----+-----+----- 563 |Sacramento | | | | | | | | Alluvium | “ | “ |23.46| 2.7| 10.4|10.90| 4.58 -------+---------------+--------+------+-----+----+-----+-----+----- 863 |Red Foothill |Nevada | “ |56.80|11.0| 36.4|33.60| 1.22 861 | “ “ | “ | “ |45.46|11.5| 22.0|14.10| 3.97 -------+---------------+--------+------+-----+----+-----+-----+-----

It is apparent from this table that if, as is probable, the aluminic hydrate accumulates in the “clay” of the analysis, it will in some cases form a very considerable percentage of the same, and detract to that extent from its plastic, adhesive and other properties. But it must be remembered that the assumption upon which this table is calculated, leaves out of consideration the zeolitic portion, which as the 6th column shows, is frequently quite large as measured by the bases found, to which no other form of combination can be assigned. Since some of the alumina undoubtedly takes part in the formation of such zeolites, the silica must to that extent be withdrawn from the estimate made for kaolinite. While it is impossible to make any definite numerical allowance for this fact, it clearly will tend in many cases to increase materially the amount of alumina that must be assigned to the hydrate condition. It will be noted that in most cases given, the alumina per cent is rather large.

[143] Since any complex zeolite would contain less alumina than kaolinite, this assumption more than covers the possible zeolitic alumina.

The relatively large number of such cases shown in the table for California soils is not a matter of accident; for even a cursory glance at the columns of analyses of California (and Washington and Montana) soils, shows that the cases in which the alumina exceeds the silica in amount are rather predominant, while the reverse is the case in the humid region.[144] But it must not be inferred that the reverse relation is not also frequently observed even in the arid region; it occurs in fact in close proximity to the localities where some of the most striking instances of excess of alumina over soluble silica have been found.

[144] See for comparison the data given in vols. 5 and 6 of the report of the Tenth Census of the United States.

Thus Nos. 861 and 863 from the neighborhood of Grass Valley, which show this excess most strikingly, occur within 15 miles of localities which show almost the reversal of the numbers given for the two former, and at a level of about a thousand feet lower. It would seem, on the whole, that the excess of alumina occurs most frequently in connection with soils formed from eruptive rocks; in the case referred to, from volcanic ash. It will require more detailed study to detect the causes of these marked differences.

Retention of Soluble Silica in Alkali Soils.—It is somewhat surprising that, contrary to the expectation one would naturally entertain, the alkali lands, so frequently rich in the carbonates of the alkalies that would dissolve free silica, on the contrary, show most frequently an excess of soluble silica over alumina. This is probably to be explained from the very liberal opportunities afforded in the alkali soils for the formation of complex zeolitic masses by the retention in soil of the soluble alkali salts, and the abundance of lime always present in them. As already stated, we usually find in alkali soils a very large proportion of both alkaline and earthy bases in acid-soluble silicate combinations. But much farther research is needed to explain fully the marked discrepancies observed in this respect between soils not only occurring in closely contiguous localities, but also showing marked similarities in their general composition.

Ferric Hydrate.—There is no obvious reason, from the chemical standpoint, why iron, that is, ferric hydrate or iron rust, should be more abundant in the soils of the arid regions, as the averages given in the table suggest; moreover, the fact does not impress itself upon the eye, since the orange or reddish tints are by far more common in the humid than in the arid regions of the United States at least. The California average is considerably influenced by the very highly ferruginous soils from the foothills of the Sierra Nevada, and by the black (magnetite) sand so commonly present; that of Oregon by the black, highly ferruginous country rock (basalts), from which they are partly derived. The average for Montana is not higher than that of three states of the humid region, and less than that of Kentucky. We might imagine a cause for depletion of iron in the soils of the humid areas in the frequency with which humid moisture and high temperature will during the summers concur toward the bringing about of a reducing process in the soil, which by getting the iron into proto-carbonate solution would make it liable to be leached into the subsoil, as is frequently the case; yet the resulting “black gravel” or bog ore, in its various forms, is of not infrequent occurrence in the arid regions also. A constant quantitative difference due to climatic conditions does not appear to be shown by the data thus far at command, but the finer distribution of the ferric hydrate in the humid temperate as well tropical regions is obvious to the observer, from the frequent redness of humid and tropical soils.

Manganese.—An unexpected and apparently well-defined contrary relation appears to be shown as regards the related metal manganese; the average percentage of which is in all cases less in the arid than in the humid region. The cause of this relation is altogether obscure; it is too frequent to be accidental.

Phosphoric Acid.—As regards that highly important soil ingredient, phosphoric acid, the indication in the table that there is no characteristic difference in the average contents in soils of the arid and humid regions, respectively, is doubtless correct. This substance is so tenaciously retained by all soils that there is no obvious reason why there should be any material influence exerted upon its quantity by leaching, or by any of the differences in the process of weathering that are known to exist between the two climatic regions. Moreover, it is apparent that the average for the arid region is made up out of very widely divergent figures; that of California exceptionally low (lower than any of those for the states of the humid regions), while those for Washington and Montana are exceptionally high. The latter is due to country rocks (“basalts”) showing abundance of microscopic crystals of apatite, which in some cases raise the contents of the soils in phosphoric acid to nearly twice the average given for the states.

The forecast that for most California soils, fertilization with phosphates is of exceptional importance, has already been abundantly confirmed by cultural experience. Few definite data are as yet available from other arid states, where fertilization is thus far sporadic and unsystematic. But it is predictable that in view of the presence of an excess of lime carbonate in the arid soils, and the unfavorable effect of this compound on the rapid solubility of tri-calcic phosphate demonstrated by Schloesing, Jr.,[145] by Böttcher and Kellner[146] and Nagaoka,[147] fertilization with readily available phosphate fertilizers will be found necessary among the first, all over the arid region, especially in view of the scarcity of humus in arid soils. [145] Ann. Sci. Agronomique, tome 1, 1899.

[146] Landw. Presse, 1900, No. 52; ibid. 1901, Nos. 23 and 24.

[147] Bull. Univ. Tokyo, Vol. 6, No. 3. Production was diminished to less than one half when lime was used with bone meal, and actual assimilation of phosphoric acid to one fifth.

A curious instance of the effects of continued warm maceration in rock decomposition is afforded by the highly ferruginous soils derived from the black basaltic lavas of the Hawaii Islands. These lavas, like the basalt sheet of the Pacific Northwest, contain a large amount of crystallized phosphate minerals, notably apatite and vivianite. A correspondingly large proportion of phosphoric acid is found in the soils derived from these rocks, up to nearly two per cent.[148] But almost the entirety of this substance is present in the form of an insoluble, basic iron compound, difficultly soluble even in acids, and rendering it wholly unavailable to vegetation. So that actually the most pressing need of most of these soils is phosphate fertilization. The same is probably true of some of the highly ferruginous soils of California and of the Cotton States.

[148] See table, chapter 19, p. 256.

Sulfuric Acid.—From the absence of the leaching process in the soils of the arid region, we should expect that sulfates would be more abundant in them than in the soils of the humid. This is certainly true in the case of the alkali soils, which are characteristic of the regions of deficient rainfall. See below, chapter 22.

Hence the showing made in the general table, indicating that sulfates are equally abundant in the soils of the humid than in those of the arid regions, is surprising in view of the efflorescences of alkali sulfates so frequently observed in the latter. This is obviously due to the fact that the majority of such alkali soils has, on account of their local nature and usually heavy lime content, been excluded from the comparison; which otherwise would have made a very different showing.

Potash and Soda.—The compounds of the alkali metals potassium and sodium, being on the whole much more soluble in water, even without the concurrence of carbonic acid, than those of calcium and magnesium, the leaching process that creates such pronounced differences in the case of the two earths must affect the alkali compounds very materially. Comparison of the soils of the two regions in this respect shows, indeed, very great differences in the average contents of potash and soda. For potash the ratio is .216 to .670 per cent on the general average, and .187 to .670 per cent, in the average by states; for soda, .140 per cent to .350 per cent on the general average, and .110 per cent to .420 per cent in the average by states. For both, therefore, the general average ratio is as one to between three and four for the humid as against the arid region.

It is curious that an approximation to the ratio of one to two, or somewhat less, is maintained in the average proportion of soda to potash in both regions; but this does not by any means hold good in detail, very high potash-percentages being often accompanied by figures for soda very much below the above ratio. This is the result of an important difference in the chemical behavior of the two alkalies, which has already been alluded to in connection with the discussion of the zeolites. (See chapter 3, p. 38).

The process of “kaolinization,” being that by which clays are formed out of feldspathic minerals and rocks such as granite, syenite, trachyte, etc., results in the simultaneous formation of solutions of carbonates and silicates of potash and soda. These coming in contact with the corresponding compounds of lime and magnesia, also common products of rock decomposition, are partly taken up by the latter, forming complex, insoluble, hydrous silicates (zeolites). In these, however, potash whenever present takes precedence of soda; so that when a solution of a potash compound is brought in contact with a zeolite containing much soda, the latter is partially or wholly displaced and, being soluble, tends to be washed away by the rainfall into the country drainage. Hence potash, fortunately for agriculture, is tenaciously held by soils, while soda accumulates only where the rainfall or drainage is insufficient to effect proper leaching, and in that case manifests itself in the formation of what is popularly known as “alkali soils;” namely those in which a notable amount of soluble salts exists, and is kept in circulation by the alternation of rainfall and evaporation, the latter causing the salts to accumulate at the surface and to manifest themselves in the form of saline crusts or efflorescenses. Alkali lands are a characteristic feature of all regions of scanty rainfall, and are found more or less on all the continents. The substances composing the alkali salts are retained not only in their soluble form, but by their continued presence influence profoundly, in several ways, the processes of soil formation. A more detailed discussion of this important subject is given in chapters 22 and 23.

Arid Soils are Rich in Potash.—One of the most important practical conclusions flowing from the comparison of the potash contents of the humid and arid soils respectively is that while in the former, potash is usually among the first substances to be supplied by fertilization when production languishes, in the arid regions it will as rule come last in order among the three ingredients commonly so furnished. Aside from the water-soluble potash salts always forming part of the salts of the alkali lands proper, which in many cases will alone hold out for many years under the demands of cultivation,[149] they rarely contain much less than one per cent of acid-soluble potash; occasionally rising as high as 1.8 per cent. That in such lands potash-fertilization is uncalled-for and ineffective, hardly requires discussion; while on the other hand, phosphates are commonly required for full production after ten or fifteen years of cultivation without returns. Nitrogen usually comes next in order, but sometimes is the first need.

[149] In the light alkali lands of the southern California Experiment Substation at Chino, the average content of water-soluble potash in ten acres amounts to the equivalent of 1,200 pounds of potash sulphate per acre. Outside of this the acid-soluble potash of the soil is .95%., equal to 38,000 pounds per acre-foot.

The constant indiscriminate purchase and use of all three ingredients, so urgently recommended by fertilizer manufacturers because of their success in the humid Eastern States, is therefore very poor economy for the farmers of the arid region. Excepting cases of very intense culture, e.g. of vegetables or berries, the use of potash salts is but rarely remunerative, and therefore uncalled-for, in arid soils for a number of years.

Humus.—The figures shown in the table for the average humus-percentages in the soils of the two regions do not adequately represent the very important differences actually existing; partly because of the inadequate number of determinations made by the same method (Grandeau’s), partly because of the differences in the composition, and especially in the nitrogen-content of this substance, which render direct comparison delusive. A detailed discussion of the marked differences existing between the humus of arid and humid soils in this respect has already been given (chapter 8, p. 135); showing that the high nitrogen-percentage in the arid humus probably compensates largely the lower humus-percentage, while rendering nitrification more rapid, because the oxygen is not consumed by overwhelming amounts of carbon and hydrogen; which, as is already known, take precedence of nitrogen in the oxidation of humus substances. Nitrates are almost always more abundant in the soils of the arid region than in those of the humid, sometimes to the extent of influencing injuriously the quality of certain crops, such as tobacco and sugar beets. Nevertheless, nitrogen is ordinarily, in the arid region, the substance requiring replacement next to phosphoric acid. And when considered in connection with the small humus-content, so liable to burning-out, this places green-manuring with leguminous plants among the first and most vital improvements to be employed there.

The Transition (semi-humid or semi-arid) Region.—The sloping plains country lying between the Rocky Mountains and the Mississippi, quite arid at the foot of the mountains, but with rainfall increasing more or less regularly to eastward, form a transition-belt between the arid and humid region of which but a small portion has been systematically studied in respect to its soil formations. The analyses made of soils of the two adjacent states of Minnesota and North Dakota, have been placed in the general table (p. 377) to show how far in their general relations their soils correspond to the generalizations deduced from the comparison of the decidedly arid and humid soil areas chiefly represented in the table. Although it has not been possible, for lack of detailed data, to eliminate the soils originating from calcareous formations, it will be seen that those of semi-arid Dakota differ from those of more humid Minnesota, almost throughout, as would be anticipated from the studies of the extremes, given in this chapter.

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XXI. SOILS OF ARID AND HUMID REGIONS (Continued)

SOILS OF THE TROPICS.

Within the ordinary limits of atmospheric temperatures, and in the presence of adequate moisture, chemical processes active in soil-formation are intensified by high and retarded by low temperatures, all other conditions being equal. We can usually artificially imitate, and produce in a short time by the application of relatively high temperatures, most of the chemical changes that naturally occur in soil-formation. While it is true that the changes of temperature are nearly as great in the tropical as in the temperate climates, these changes all occur at a higher level and within the limits favoring bacterial and fungous action.

This being true we should expect that the soils of tropical regions should, broadly speaking, be more highly decomposed than those of the temperate and frigid zones, and that the intensified processes continue currently. This fact has not been as fully verified as might be desirable, by the direct comparative chemical examination of corresponding soils from the several regions, owing to the want of uniformity in methods and the fewness of such investigations in tropical countries. Yet the incomparable luxuriance of the natural as well as artificial vegetation in the tropics, and the long duration of productiveness that favors so greatly the proverbial easy-going ways and slothfulness of the population of tropical countries, offers at least presumptive evidence of the practical correctness of this induction.

In other words, the fallowing action, which in temperate regions takes place with comparative slowness, necessitating the early use of fertilizers on an extensive scale, is much more rapid and effective in the hot climates of the equatorial rainy belt; thus rendering currently available so large a proportion of the soil’s intrinsic stores of plant-food, that the need of artificial fertilization is there largely restricted to those soils of which the parent rocks were exceptionally deficient in the mineral ingredients of special importance to plants, that ordinarily form the essential material of fertilizers. Quartzose, magnesian, and other soils resulting from the decomposition of “simple” rocks will, of necessity, be poor in plant-food everywhere.

Humus in Tropical soils.—Another inference from the climatic conditions of the tropics is that the properly tropical soils are likely to be rich in humus, as a result of the luxuriant vegetation which in the decay of its remnants must leave abundant humic residues. This seems to be generally verified wherever the interval between rainy seasons is not too long; for otherwise, under the great and constant heat of the tropics a rapid burning-out of the humus, such as is known to occur in the arid regions, must also take place. A good example illustrating the inter-tropical regime as regards humus is given in the table in chap. 8, p. 137, showing the humus-content of some Hawaiian soils. Both are of the same order as in the soils of the temperate humid region, though the nitrogen-content evidently can, consistently with productiveness, range lower than has thus far been observed in temperate climates. This again forms a striking contrast with the soils of the arid regions.

It is greatly to be regretted that not even approximate determinations of the organic matter, much less of the humus-substance proper, have been made by any of those who have analyzed tropical soils; excepting those made of Hawaiian soils at the California Experiment Station.

The “loss by ignition” is of course always very largely water, mostly referable to ferric hydrate and clay substance, the latter presumably essentially in the form of kaolinite. When, therefore, ferric oxid and alumina have been determined, we may approximate to the amount of total organic matter by making allowance for ferric hydrate at the rate of about 14% of the ferric oxid, for kaolinite at that of 34.92% of the alumina found. Deducting these amounts of water from the total “loss by ignition,” we may obtain at least an approximate idea of the organic matter, and the probable availability of the nitrogen determined by the analysis. See chapter 19, p. 357.

While the continuous heat and moisture of the tropics concur toward rapid rock decomposition, it must be remembered that the copious rainfall is equally conducive to an intense leaching effect. Striking examples of this action occur in the Hawaiian Islands, in the highly ferruginous soils resulting from the decomposition of the black (pyroxenic and hornblendic) lavas that are so characteristic of the volcanic effusions of that region. The soils formed from these rocks are sometimes so rich in ferric hydrate (iron rust) that they might well serve as iron ores elsewhere. But these soils are very unretentive, and though very productive at first they are soon exhausted, the abundant rains having sometimes deprived them of almost every vestige of lime, and of most of the potash contained in the original rock. At the same time the abundant phosphoric acid of the original rock has been reduced to almost total unavailability by combination with ferric oxid, just as in the case of the bog ore of the temperate climates; so that phosphate fertilization is urgently needed in these lands, though showing high percentage of phosphoric acid. (Chap. 19, p. 356.)

Soils highly colored by ferric hydrate occur rather frequently in the tropics, and have received the general name of “laterite” soils. Curiously enough, the intense reddish tint mostly shown in these soils, and which is emphasized in the “terra roxa” of the Brazilians, and the general “red” aspect of Madagascar, and of the Malabar and Bengal coasts, is by no means always accompanied by markedly high percentages of iron oxid; but the latter is very finely diffused, so as to be very effective in coloration. The plant-food percentages of tropical soils are generally quite low, so that in the temperate humid regions such lands would be adjudged to be rather poor. Yet they mostly prove quite productive and lasting, even without fertilization.

This is doubtless to be explained by the continuous and rapid rock and soil-decomposition which goes on under tropical climatic conditions, already alluded to; so as to supply enough available plant-food for the demands of each season’s vegetation, analogously to the proverbial “nimble penny.” This is supplemented also by the rapid decay and leaching-out of the ash ingredients of the rapidly decaying and dying vegetation. Nitrification must likewise, of course, be very active under the continual heat and moisture, and the humus formed under these circumstances is likely to be quite poor in nitrogen. On this latter point, however, definite data are almost wholly wanting.

Investigations of Tropical Soils.—The most extended chemical investigations of properly tropical soils have been made by Wohltmann in his investigations of the soils of India, German Southwest and Southeast Africa, and Samoa;[150] and by Müntz and Rousseaux of soils collected under Government auspices in Madagascar. Leather, Bamber and Mann have also analyzed a large number of soils of India. But we find in many of these cases a failure to specify distinctly the local climatic conditions, and even the depth to which the samples have been taken; so that the investigator is obliged to examine laboriously the local climates, and especially the amount and distribution of rainfall, before being enabled to discuss intelligently the data given. Even Wohltmann, in his discussion of North African and Saharan soils, classes these distinctly arid types among the tropical ones.

[150] Samoa Erkundung, by F. Wohltmann, Kolonial-Wirthsch. Komitee, Berlin, 1904.

Again, the dry seasons intervening between the tropical rains, varying in length and from locality to locality, obscure somewhat the relations of the soils to the climatic conditions. Under the lee of mountains, even of slight altitude, we find xerophytic (arid-land) vegetation, as has been noted by many observers in Brazil, even near the Amazon; in Hawaii, in Jamaica, and in Madagascar. Unless, therefore, a close discrimination is exercised by field observers, many contradictory results will appear in analyses of soils of inter-tropical countries. This is naturally the case in India, where the topographic surface conformation and seasonal climatic conditions are so complicated and contrasted. On the whole, the results obtained in Samoa, Kamerun and Madagascar seem, of those available, to be the most characteristic of true tropical conditions. In comparing these with the soils of low plant-food percentages in the temperate humid region (see chapter 19, p. 352), it must be remembered that those mentioned as being productive are so by virtue of great depth and relatively high proportions of lime; while in the tropics, the intense leaching process prevents lime from reaching any high absolute or relative percentages, save where limestone formations prevail. Moreover, the mode of preparation of the soil extracts for analysis by Wohltmann, and by Müntz and Rousseaux, differ so widely from that forming the basis of discussion of soil-composition in this volume, that it becomes necessary to make separate allowances in each case; since some of the ingredients, phosphoric acid, lime and magnesia, are fully dissolved by the weaker treatments, while others,—e. g., potash—are not, and are therefore not directly comparable with the data obtained in the writer’s work. The analyses made in India by Leather and others have apparently been made substantially in accordance with the author’s methods and may be considered directly comparable.

SOILS OF SAMOA AND KAMERUN.

Wohltmann has investigated the soils of Samoa, notably those of the main island of Upolu, under the auspices of the German “Kolonial-Wirthschaftliche Komitee” in 1903, and gives the results of his observations and analyses in a report published at Berlin in 1904. The analyses are quite numerous, but unfortunately are made by a special method which renders them only partly comparable with those of any other analyst.

Wohltmann’s method is this: “450 grams of fine earth (below 2 millimeters diameter) is treated for 48 hours with 1½ liters of cold chlorhydric acid of 1.15 density. Another portion, designed for a fuller determination of potash, is treated for one hour with the same acid, boiling hot. Potash was determined in both soil extracts; the hot extract gave from one-third to twice the amount obtained in the cold extraction.”[151]

Wohltmann justifies this method by the statement that it has yielded him results more nearly in accord with experience than any other tried, both with tropical and European soils.

Under these conditions only a few of the determinations in Wohltmann’s analyses are directly comparable with those upon which the discussions in this volume have been based. The figures for nitrogen and phosphoric acid may be assumed to be fully comparable; that of lime will in general represent fully only that which is present in the forms of carbonate, sulfate and humate, and a part of that existing in zeolitic or hydrous silicate form. Of the two potash determinations only the one made in hot extraction will be even remotely comparable, being probably at least 30% lower than would have been obtained by the writer’s method.

[151] Wohltmann states that the hot extraction sometimes yielded as much as five times more than the cold; but no such case appears in his reports on Samoa and Kamerun.

Even thus, however, Wohltmann’s results are highly instructive. He gives the following summary of his mode of interpreting such analyses:

==================+============+=======+============= | Very rich. | Good. | Inadequate. ------------------+------------+-------+------------- Potash | .2 | .1 | .05 Lime and Magnesia | 1.0 | .4 | .07 Phosphoric acid | .2 | .1 | .06 Nitrogen | .2 | .1 | .05 ------------------+------------+-------+-------------

It will be observed that the figures of this table differ materially only in the matter of potash from those given in chapter 19, p. 354; for the latter substance they would have to be multiplied by from 2 to 4, according to the lime-content and other conditions.

With this understanding a number of Wohltmann’s analyses of soils from Samoa and Kamerun are given below, the potash determinations made with hot acid being placed in parentheses after the other.

Soils of Samoan Islands.—A discussion of these analyses shows, from the writer’s point of view, a very low content of potash and lime, with the peculiarity that both are somewhat higher at the depth of a meter than in the surface ten-inches. This is probably to be accounted for from the very high content of organic matter (humus), which is apparent from the high “loss by ignition,” a very large proportion of which must be credited to the burning of the organic matter. That this humus reaches to the lowest depths examined, is clear from the nitrogen-content given for these samples. Wohltmann, whose estimate of these soils agrees in most respects with the writer’s, attributed to them a very satisfactory nitrogen-content. This would be true of the total; but as he has not determined either the true humus or its nitrogen-content, it remains uncertain whether or not a sufficiency is in an available form, and whether their case may not be like that of the Hawaiian soil mentioned above (chapter 19, p. 362), in which despite 10% of humus and .17% of nitrogen, the land was found to be nitrogen-hungry. Again, as regards the phosphoric acid, which Wohltmann considers satisfactory to high, it is questionable to what extent it is rendered unavailable by the very high content of ferric hydrate. We are thus left in some uncertainty as to the real manurial requirements of the Samoan soils, which doubtless represent very closely also those of Tutuila, the chief American island of the group.

ANALYSES OF TROPICAL SOILS BY F. WOHLTMANN.

Extraction with cold chlorhydric acid, sp. g. 1.15, for 48 hours.

(A) = Tuanaimato. Virgin Forest Soil, not given. (B) = Le Utu Sao Vaa Cultivated Soil. (C) = 0—25 cm. (10 ins.) (D) = 75—100 cm. (30—40 ins.) ======================+=========================================== | SAMOAN ISLANDS. | | UPOLU. | SAVAII. | | | (A) | (B) | Cleared Land. | | | | | Depth. | | (C) | (D) | (C) | (D) ----------------------+----------+-------+-------+--------+--------- Potash[152] | .05 | .048 | .022 | .063 | .036 | (.07) |(.077) |(.043) | (.102) | (.043) ----------------------+----------+-------+-------+--------+--------- Lime | .07 | .113 | .033 | .042 | .023 Magnesia | .37 | .285 | .144 | .067 | .074 Ferric Oxid |21.53 |17.600 |19.653 |15.333 |18.733 Alumina |12.40 | 9.621 |11.217 |18.941 |16.413 ----------------------+----------+-------+-------+--------+--------- Silica | .99 | 2.043 | 2.853 | .366 | 1.250 Titanic Acid | | | | | ----------------------+----------+-------+-------+--------+------- Phosphoric Acid | .30 | .179 | .213 |29.096 | .187 Org. Matter and Water |18.49[153]|17.288 |12.770 |29.146 |16.332 Nitrogen in Soil | .30 | .447 | .186 | .697 | .128 | | | | | Hygr. Moisture | 6.80 |15.062 |13.242 |15.288 |12.862 ----------------------+----------+-------+-------+--------+-------

======================+================================= | KAMERUN. | | ISONGO. | MUNDAME II. | | | | Depth. | (C) | (D) | (C) | (D) ----------------------+-------+-------+-------+--------- Potash[154] | .097 | .104 | .076 | .110 |(.101) |(.163) |(.123) | (.168) ----------------------+-------+-------+-------+--------- Lime | .193 | .154 | .150 | .125 Magnesia | .283 | 1.415 | .198 | .099 Ferric Oxid | 7.305 | 7.497 |13.920 |11.707 Alumina |14.298 |14.504 | 5.223 | 5.531 ----------------------+-------+-------+-------+--------- Silica | .047 | .108 | .227 | .120 Titanic Acid | | | | ----------------------+-------+-------+-------+--------- Phosphoric Acid | .064 | .224 | .131 | .205 Org. Matter and Water |23.335 |12.299 |10.154 | 9.549 Nitrogen in Soil | .187 | .079 | .164 | .103 | | | | Hygr. Moisture |20.065 |15.329 |14.811 |16.498 ----------------------+-------+-------+-------+----------

[152] The numbers in brackets are determinations made after boiling with acid for one hour.

[153] Soil air-dry.

[154] The numbers in brackets are determinations made after boiling with acid for one hour.

It is probable that for crops requiring so much potash as do the banana and cacao trees, potash is the first need when they cease to produce well on these soils.

Soils of Kamerun.—In the soils of Kamerun, also analyzed by Wohltmann, and of which two are placed alongside of those of Samoa, it is at once seen that the materials from which they have been formed are richer in both potash and lime than the parent rocks of the Samoan, and not quite so rich in iron. They are also very rich in organic matter, evidently down to the depth of a meter, as are those of Samoa. It is probably due to the high humus-content that these soils, washed as they have been by the second-highest rainfall in the world (about 35 feet annually) have not been as thoroughly leached as have been those of the Brahmaputra valley. The annual rainfall of Samoa is only from nine to eleven feet on the lower levels, but ranges as high as 18 feet at higher elevations.

It is noticeable that in most of these true tropical soils the content of magnesia is considerably above that of lime; a fact readily intelligible from the more ready solubility of lime in carbonated water. It is hardly doubtful that this disproportion will in many cases explain a lack of thriftiness, which could be effectually remedied by a simple application of lime or marl, without resorting to the more costly fertilizers.

THE SOILS OF MADAGASCAR.

The soils of the island of Madagascar have been analyzed to the number of about 500 by Müntz and Rousseaux, under the auspices of the French government.[155] So large a number of analyses should give a very full understanding of the agricultural capacity and adaptation of so comparatively limited an area; unfortunately, we are here again confronted by more or less imperfect data accompanying the samples collected by government agents, and by the use of an analytical method different from those of all other nations, and hence incommensurable except, as in the case of Wohltmann’s method, in regard to certain ingredients.

[155] Annales de la Science Agronomique, tome 1er, 1901, fasicules 1, 2, 3.

The French chemists use nitric instead of chlorhydric acid; cold for phosphoric acid and lime, boiling-hot for five hours for potash; considering the remainder as of no practical importance. Since nitric acid is in general much less incisive than chlorhydric in its solvent power, comparison with the analyses made by other nations becomes difficult. As in the case of Wohltmann, magnesia, lime, and phosphoric acid may be considered to be quite thoroughly extracted by the treatment; while extraction of possibly available potash is doubtless very incomplete. On the whole, however, the estimates of soil-fertility based on percentages is very nearly the same as those assigned by Wohltmann in the table given above. Like Wohltmann, they emphasize the axiom that the same percentage-gauge of fertility cannot be applied in the tropics as in the temperate zones.

General Character of the Island.—The island of Madagascar, lying between the 11th and 25th degrees of south latitude, is quite mountainous in its central and eastern portion, where the coast falls off pretty steeply into the sea, leaving only a narrow coast belt of properly agricultural land in the lower valleys and at the mouth of the torrential streams. The mountains rise at one point to the height of nearly 10,000 feet. The western portion of the Island is much less broken, has much plateau land with low intersecting ranges and streams of moderate fall, with considerable alluvial lands near the coast. The rocks are almost throughout gneisses and mica-schists, which, as heretofore stated (chapter 4, p. 51), form mostly poor soils. There are a few areas of eruptive rocks and tertiary calcareous deposits, and on these the lands are much more thrifty. The rocks and red soils of the central mass, however, extend seaward almost everywhere.

The rainfall is high on the east side, where the moisture of the southeast trade winds is first condensed, the precipitation reaching ten to twelve feet (120 to 144 inches) annually. The western portion is relatively dry, but rains fall more or less throughout the year; while in the eastern and central mountainous part there is a distinct subdivision into a wet and a dry season. Here, while the rivers are largely torrential, many large fertile valleys have been created by the heavy denudation of the mountain slopes. This is especially the case in the Imerina province (in which the capital, Tananarivo, is situated), and here the valley soils are deep, and rich in humus. The western portion is but thinly forested. The soils of most of the island are “red” with ferric hydrate, resembling the laterite soils elsewhere; yet the iron percentages are not usually very heavy, ranging mostly from 4 to 6, more rarely to 10% and more, of ferric oxid. Most of the red soils are clayey, crack open in summer and become very hard in drying.

Of the 476 soils analyzed by Müntz and Rousseaux, 156 are from the province of Imerina, 56 from the adjacent province of Betsileo, therefore 212 from the central, mountainous part of the island. The remainder are scattered around the coasts; the most productive being apparently those of the northern end, Diego Suarez, which is mostly underlaid by the eruptive rocks forming the mountain mass of Mount Amber, from which numerous fertile valleys radiate. The valleys of the west coast also, in the provinces of Bara, Tulear and Betsiriry, have some very productive soils.

The subjoined table, giving fourteen analyses selected as representative from the mass of material presented by Müntz and Rousseaux, gives a fair general idea of the character of the soils of the great island. It is at once apparent that lime and potash are extremely deficient in the soils of the mountain slopes of central and southern Madagascar, these substances having, as elsewhere in the humid region, been leached down into the valleys; and the materials being mostly quite clayey, these valley soils have not, as in the case of the sandy alluvium of the Brahmaputra, themselves been again leached of their mineral ingredients. Practically these valleys seem to form the only profitably cultivable area of the central portion; while along the larger river courses, such as the Mangoky, Ikopa, Mahajamba and others, good alluvial “bottoms” and deltas form available lands. It seems to the writer that, in view of their own expressed opinion that tropical soils are not to be gauged on the same percentage-basis of soil-ingredients as those of temperate regions, Müntz and Rousseaux rather underestimate the productive value of many of these lands; regarding which the field notes report good production, and the crops of which are certainly not the first that they have borne in the course of Malagassy history. It is as though their anxiety to forestall overestimates of agricultural prospects by intending settlers, had led them to somewhat overshoot the mark.

ANALYSES OF MADAGASCAR SOILS BY MUNTZ AND ROUSSEAUX. ======================+=================================+=========== | IMERINA. | BETSILEO ----------------------+---------------------------------+----------- | CENTRAL MOUNTAIN REGION. ----------------------+----------+------------+---------+----------- Number of Soil. | 4--4 | 115 | 71--4 | 272 ----------------------+----------+------------+---------+----------- Locality | Ambohitr-| Ankazobe | Ambohib-|Fandrandava | omby. | North. | zaka. | Valley. ----------------------+----------+------------+---------+----------- | Red Soil.| Ochreous | | | | Soil. | | ----------------------+----------+------------+---------+----------- Potash (K₂O) | .020 | .006 | .071 | .017 Lime (CaO.) | .350 | .060 | traces | trace Magnesia (MgO.) | .022 | | | Ferric Oxid (Fe₂O₃) | 9.333 | | | Phosphoric Acid (P₂O₅)| .050 | .032 | .061 | .267 Nitrogen (N) | .096 | .027 | .030 | .020 ----------------------+----------+------------+---------+----------- Remarks | Small |Deficient in|No great | Only | cultural | plant-food |cultural | moderately |resource. |ingredients.|resource.| fertile. | |May maintain| | | |vegetation | | | |on account | | | |of humidity.| | ----------------------+----------+------------+---------+-----------

======================+==========+=======================+======== | TULEAR. | MAINTIRANO. |MAJUNGA. ----------------------+----------+-----------------------+-------- | West Coast. ----------------------+----------+------------+----------+-------- Number of Soil. | 253 | 261 | No. 267 | 343 ----------------------+----------+------------+----------+-------- Locality |R. bank of| Plateau of |Village of| | Sakondry |Antsoamena. |Anhozorabe| | north of | | South of | |Tongobory | |Mandroso. | | Valley. | | | ----------------------+----------+------------+----------+-------- | | Sandy | | Hill | | soil. | |summit. ----------------------+----------+------------+----------+-------- Potash (K₂O) | .086 | .015 | .014 | .012 Lime (CaO.) | 4.180 | trace | trace | trace Magnesia (MgO.) | | | | Ferric Oxid (Fe₂O₃) | | | | Phosphoric Acid (P₂O₅)| .083 | .043 | .011 | .017 Nitrogen (N) | .075 | .051 | .039 | .043 ----------------------+----------+------------+----------+-------- Remarks | Potatoes,| Manioc, |Remarkable|Grasses, | manioc, | maize, | crops, | tall |represents| beans, | manioc, | herbs. | soil of | peanuts, | rice. | | whole |coco-trees, | | | region. | mangoes. | | ----------------------+----------+------------+----------+--------

======================+========================+=================== | ANDEVORANTE. | AMBATONDRAZAKA. ----------------------+------------------------+------------------- | EAST COAST. ----------------------+------------+-----------+----------+-------- Number of Soil. | 107 | 370 | 81 | 105 ----------------------+------------+-----------+----------+-------- Locality | Dist. of | Ambaniman-| Imeriman-|Sabotsy. | Vatomandry.| hovinana. | droso. | |Ampitamafana| | | ----------------------+------------+-----------+----------+-------- | Plain | | | | valley, | Valley. | |Alt 1000 | sandy | | | M. | soil. | | | ----------------------+------------+-----------+----------+-------- | | | | Potash (K₂O) | .069 | .012 | .490 | .005 Lime (CaO.) | .070 | trace | 1.200 | .070 Magnesia (MgO.) | | | | Ferric Oxid (Fe₂O₃) | | | | Phosphoric Acid (P₂O₅)| .047 | .161 | 1.566 | .369 Nitrogen (N) | .016 | .254 | .249 | .164 ----------------------+------------+-----------+----------+-------- Remarks | Coffee, | Very | Amply |Fertile | vanilla, | fertile | rich in |soil fit | rice, &c., | soil. |plant food|for all | rich red | |suited for|cultures. | soil. Rep. | | intensive| | whole | | cultures.| | valley. | | | ----------------------+------------+-----------+----------+--------

Be that as it may, the influence of the tropical climate and rainfall upon the composition of these soils is certainly very marked. While gneiss is not credited with producing first-class soils, its usual content of orthoclase feldspar should at least insure a respectable average content of potash; but this, it will be seen, is mostly not the case; and that of lime seems even worse, aside from the case where, as in some regions near the coast (especially in the west and south), calcareous formations, probably of tertiary age, have contributed to soil-formation. At some points there seem to exist phosphate deposits, well known elsewhere to occur in such rocks, which impart to the soils exceptionally high percentages of phosphoric acid, even exceeding one per cent. The phosphates of course remain practically untouched by the leaching processes, and appear to be somewhat widely diffused; so that the soils of Madagascar may be said to be, on the whole, well supplied with this important plant-food.

In the central province of Imerina the valleys and lower slopes show a fair content of both lime and potash; but in the province of Betsileo, adjoining it on the south, nearly every one of the soils analyzed is reported as containing only “traces” of lime, together with very small amounts of potash in most cases. The ultimate analyses of ignited red earths, of which an average is here given, are of interest in this connection.

ULTIMATE ANALYSIS OF IMERINA RED SOILS, IGNITED; AVERAGE OF THREE.

Silica 55.2 Potash .3 Lime trace Magnesia 1.1 Ferric oxid 10.6

It is quite obvious that only leaching-down and concentration of the feeble resources of such material in the valleys can produce soils worthy of permanent cultivation.

One point, however, is strikingly illustrated in several of the analyses given in the subjoined table. We find in the original quite a number of cases in which the field notes report considerable fertility, while the chemists’ judgment is very unfavorable. Thus we find recorded for the soil No. 267, taken near the village of Anjozorabe, in the Maintirano region, “luxuriant vegetation and remarkable crops,” with such minute proportions of potash, lime and phosphoric acid that the authors are compelled to say that the land offers “no cultural resources.” The same occurs in the cases of soils Nos. 370, 261, and several others having either “good crops” or “abundant natural vegetation.” Unless we assume that in these cases the samples were not properly taken, we are obliged to conclude that under the local climatic conditions, such minute amounts of plant-food are developed with sufficient rapidity to supply good growth. This would be quite parallel to the case of the tea soils of Assam, whose production lasted 30 years before showing exhaustion, on plant-food percentages only slightly greater than those here noted, and determined by a much more incisive method.

It is thus quite obvious that a different standard of interpretation must be applied to tropical soils as compared with either the temperate humid, or the arid regions; and that uniform methods of analysis are needed to evolve a definite rule from the present uncertainties.

THE SOILS OF INDIA.

The soils of India have been investigated to some extent by the geological survey of India; by Voelcker, who went there on a special mission to investigate agricultural conditions; and since, more especially by Leather, Bamber and Mann; and by Moreland. Leather’s account is the most complete on the general subject and can best serve as the basis for a review of the entire peninsula.[156]

[156] On the Composition of Indian Soils. Agr. Ledger, 1898, No. 2.

According to Dr. Leather, “the four main types of soils to be dealt with, and which certainly occupy by far the larger of the Indian cultivated area,” are: The Indo-Gangetic alluvium, covering the chief cultivable areas of the Indo-Gangetic plain; the black cotton soils or regur, occupying the main body of the plateau of the Central provinces (the Deccan) from the Vindhya range south; the red soils lying on the metamorphic rocks of Madras; and the “laterite” soils which are met with in many parts of India. To these should be added the alluvial soils of the Brahmaputra valley, in Assam. It is hardly to be expected that so large an area as that of India, with its diversified topography, and a climate ranging from about four inches of rainfall in the northern Panjab to the world’s maximum in Assam, and southward to typical tropical conditions, could be even thus briefly characterized. The observers have rarely given for the several soils analyzed, special local and climatic data, which cannot always be obtained from the official publications; so that it is not easy to discuss them from the points of view of aridity and humidity.

The Indo-Gangetic Plain.—The general rain-map of India shows the Panjab and Rajputana to be arid throughout; thence eastward the rainfall increases to 25 and 30 inches on the Ganges; notwithstanding which, alkali (reh) is abundant about Aligarh, Meerut and Agra. Thence toward Calcutta there is a steady increase of rainfall until, at the head of the Bay of Bengal, 70 inches is reached.

If under these conditions the Indo-Gangetic plain admits of any generalizations as regards soil composition, it must be attributed in the main to its predominantly alluvial character. It should therefore be relatively rich in lime, magnesia and potash. So far as the first is concerned, Leather remarks that the only rocky particles larger than sand to be found in all this large belt of land is the nodular limestone called kankar, formed by the deposition of calcium carbonate within the soil, at the depth of a few feet. It occurs very generally in India, and as stated above (chapters 9 and 19), this occurrence of calcareous hardpan, of varying hardness, is almost universal in the arid regions. The analysis given in the table, selected as representative from those given by Leather, show that the general forecast is realized in them, as soils of an arid region.

ANALYSES OF SOILS OF INDIA.

NORTHERN INDIA. ============================+================================= | Indo-Gangetic Alluvium. +----------------+---------------- | Panjab. | Lower Ganges. | | +--------+-------+-------+-------- | Sotar |Changa | Ison, | Sibpur, | Valley.|Manga. |Ganges,|Calcutta | Clay |Loamy | Doab. | Clay | Loam. |soils. | Sandy | soils. | [157] | | Loams.| ----------------------------+--------+-------+-------+------- Insoluble matter | 81.57 | 81.54 | 88.08 | 73.58 Soluble silica | | | | ----------------------------+--------+-------+-------+-------- Potash (K₂O) | .74 | .54 | .64 | Soda (Na₂O) | .08 | .25 | .09 | 1.82 ----------------------------+--------+-------+-------+-------- Lime (CaO) | 1.44 | .98 | .47 | 1.01 Magnesia (MgO) | 1.97 | 1.72 | .32 | 1.64 Br. ox. of Manganese (Mn₃O₄)| | .11 | | 7.19 Peroxid of iron (Fe₂O₃) | 4.32 | 5.11 | 3.10 | 7.58 Alumina (Al₂O₃) | 5.85 | 4.36 | 4.38 | 9.89 Phosphoric acid (P₂O₅) | .23 | .14 | .08 | .07 Sulfuric acid (SO₃) | ? | .02 | .05 | .00 Carbonic acid (CO₂) | 1.13 | .45 | .37 | .28 Water and org. matter | 2.67 | 4.78 | 2.42 | 5.93 +--------+-------+-------+-------- Total | 100.00 |100.00 |100.00 |100.00 | | | | Nitrogen | .02 | .082| .027| .051 ----------------------------+--------+-------+-------+-------- | Brahmaputra Alluvium. +--------------------------------------- | Assam.[158] +---------+---------+----------+-------- | Fezpur |Lakhimpur|Golaghat. |Sipsagar | bank. | New |Nigriling.| Teela | Old |Alluvium.| New | land. |Alluvium.| |Alluvium. | New | | | |Alluvium ----------------------------+---------+---------+----------+-------- Insoluble matter | 85.18 | 84.60 | 85.88 | 91.52 Soluble silica | | | | ----------------------------+---------+---------+----------+-------- Potash (K₂O) | .35 | .24 | .26 | .14 Soda (Na₂O) | .30 | .12 | .23 | .16 ----------------------------+---------+---------+----------+-------- Lime (CaO) | .04 | .11 | .03 | .06 Magnesia (MgO) | .46 | .33 | .36 | .20 Br. ox. of Manganese (Mn₃O₄)| | | | Peroxid of iron (Fe₂O₃) | 2.08 | 2.78 | 2.74 | 1.52 Alumina (Al₂O₃) | 5.03 | 5.63 | 5.10 | 3.30 Phosphoric acid (P₂O₅) | .05 | .06 | .06 | .06 Sulfuric acid (SO₃) | .02 | .02 | .02 | .02 Carbonic acid (CO₂) | | | | Water and org. matter | 5.59 | 6.11 | 5.32 | .18 +---------+---------+----------+-------- Total | | | | | | | | Nitrogen | .14 | .20 | .18 | .08 ----------------------------+---------+---------+----------+-------- | Laterite Soils. +------------------------------- | Bengal. +----------+---------+---------- |Lohardaga.| Chota |Hazaribagh | | Nagpur. |District. | |Singhbhum| | |District.| ----------------------------+----------+---------+---------- Insoluble matter | 29.67 | 59.06 | 80.46 Soluble silica | | | ----------------------------+----------+---------+---------- Potash (K₂O) | .10 | .27 | .38 Soda (Na₂O) | .04 | | .32 ----------------------------+----------+---------+---------- Lime (CaO) | .38 | .28 | 1.72 Magnesia (MgO) | .21 | .33 | .38 Br. ox. of Manganese (Mn₃O₄)| .07 | .48 | .50 Peroxid of iron (Fe₂O₃) | 48.71 | 26.64 | 6.12 Alumina (Al₂O₃) | 8.81 | 7.27 | 7.19 Phosphoric acid (P₂O₅) | .64 | .08 | Trace. Sulfuric acid (SO₃) | | | Trace. Carbonic acid (CO₂) | .06 | .16 | .12 Water and org. matter | 11.31 | 5.43 | 2.81 +----------+---------+---------- Total | 100.00 | 100.00 | | | | Nitrogen | .010 | .024 | .03 ----------------------------+----------+---------+----------

[157] Analysis by Voelcker.

[158] Analyses by Mann.

SOUTHERN INDIA. ============================================================= Madras Presidency. ----------------------------+--------+----------------------- |Laterite| Upland. | Soil. | Red Soils. +--------+---------+------------- |Sidapet.| Madura. |Trichinopoli. | |Dindigal.| Parambalur. | | | | | | ----------------------------+--------+---------+------------- Insoluble matter | 76.86 | 90.47 | 86.74 Soluble silica | | | ----------------------------+--------+---------+------------- Potash (K₂O) | .09 | .24 | .05 Soda (Na₂O) | .17 | .12 | .15 ----------------------------+--------+---------+------------- Lime (CaO) | Trace.| .56 | .48 Magnesia (MgO) | .77 | .70 | .70 Br. ox. of Manganese (Mn₃O₄)| .19 | .08 | .10 Peroxid of iron (Fe₂O₃) | 10.09 | 3.51 | 5.70 Alumina (Al₂O₃) | 8.84 | 2.92 | 5.68 Phosphoric acid (P₂O₅) | Trace.| .09 | .05 Sulfuric acid (SO₃) | | | Carbonic acid (CO₂) | .12 | .30 | .11 Water and org. matter | 2.87 | 1.01 | .24 +--------+---------+------------- Total | 100.00 | 100.00 | 100.00 | | | Nitrogen | .015| .006 | .021 ----------------------------+--------+----+----+-------------

==================================================================== Madras Presidency. ----------------------------+-------------+------------------------- | Alluvium. | Regur. | Averages. | +------+------+------+----------+------- |Loamy |Sandy |Regur.| Trichin- |Kistina |Soils.|Soils.| Avg. | opoli. | | | | 18 |Parambalur|Narsa- | | |Soils.| Taluk. | opet. ----------------------------+------+------+------+----------+------- Insoluble matter |71.79 |93.09 | 68.41| 65.16 | 68.29 Soluble silica | | | | | ----------------------------+------+------+------+----------+------- Potash (K₂O) | .22 | .04 | .41| .14 | 1.14 Soda (Na₂O) | .17 | .07 | .31| .01 | 1.30 ----------------------------+------+------+------+----------+------- Lime (CaO) | .54 | .13 | 2.90| 2.18 | 3.43 Magnesia (MgO) | 1.29 | .33 | 2.27| 2.47 | 1.94 Br. ox. of Manganese (Mn₃O₄)| .13 | .04 | .17| .25 | .09 Peroxid of iron (Fe₂O₃) | 9.59 | 2.46 | 7.13| 9.27 | 6.96 Alumina (Al₂O₃) | 9.98 | 1.74 | 10.14| 13.76 | 10.28 Phosphoric acid (P₂O₅) | .11 | .05 | .06| Trace. | .00 Sulfuric acid (SO₃) | | | | Trace. | Trace. Carbonic acid (CO₂) | .09 | .11 | 1.62| .91 | 1.88 Water and org. matter | 6.09 | 1.94 | 6.58| 5.85 | 3.96 +------+------+------+----------+------- Total |100.00|100.00|100.00| 100.00 | 99.27 | | | | | Nitrogen | .037| .015| .03| .024 | .012 ----------------------------+------+------+------+----------+-------

The Brahmaputra Alluvium in Assam.—Aside from the immediate alluvium of the Indus, of which no definite data are available, the Indo-Gangetic plain represents the drainage of the southern slope of the Himalaya chain. That of most of the northern slope is represented by the Brahmaputra, which not only originates in a region of heavy precipitation—Thibet—but continues in the same throughout its course, and rounding the easternmost spur of the Himalaya range enters, in southern Assam, upon the region of the maximum rainfall known. Its alluvial deposits should therefore show the reverse characteristics of those of the Ganges; they should, as thoroughly leached soils, be poor in lime, magnesia and potash. We have fortunately on this subject the excellent work done by Mr. H. H. Mann for the Indian Tea Association, the report of which was published in 1901, and contains, besides a large number of analyses, good descriptions of the general soil and cultural conditions of the Assam tea districts, with suggestions for their improvement.

The tea plantations of Assam are located almost wholly on the new and old alluvium of the Brahmaputra river, bordered on the north by the eastern spur of the Himalayas, on the south by the low ranges of the Khasia hills. The soil is mostly quite sandy, the late alluvium gray in color, the older reddish and more loamy. Of the four analyses given in the table and fairly representing the average character of these soils, the two first are from the north side, the latter two from the south side of the river.

It will be noted that the prominent feature of all these soils is an extremely low percentage of lime, the general average being about .08% as against nearly 1.0% in the average Indo-Gangetic soils. In the latter, potash ranges between .65 and .70%; in the Assam soils between .25 and .35. Magnesia averages nearly 1.3 in the Indo-Gangetic, against about .50 in the Assam tea soils. It is thus apparent that the same general facts as regards the leaching-out of soil ingredients already shown for eastern and western North America are strikingly verified in northern India; but reversed as regards the points of the compass. The preferential leaching-out of lime as compared with magnesia and potash, is here again well exemplified. It would be interesting to have an analysis of the Brahmaputra water to compare with that of the Ganges. That tea should flourish for twenty to thirty years in such soils, is a good indication of one cause at least of the total failure of tea culture in California, where tea plants are difficult to maintain alive, and after 25 years form rounded, scrubby bushes not over four feet high. Similar failures of tea on calcareous soils are on record from India. The low lime-content of the Assam soils, then, does not necessarily imply that these soils should be limed to maintain tea production. According to Mann, the main deficiency is in nitrogen, as the figures imply; but whether his recommendation of green-manuring with leguminous crops to increase the nitrogen-supply is practicable without first supplying more lime to the Assam soils, is questionable. Since phosphoric acid is also low, his recommendation to use freely the basic or Thomas slag is doubtless a good one, since lime would thus also be moderately increased.

Bamber gives a number of analyses of tea soils from low ground in Assam, which are very rich in vegetable matter and quite acid. Like those reported by Leather, these “bhil” soils are very poor in lime and nitrogen, but fairly supplied with potash and phosphoric acid.

The Regur or Black Cotton Soils of Southern India.—The second-greatest reasonably uniform soil-area of India is that covered by the regur, or black cotton soils, in south central India, notably the Deccan, where these soils are said to have been cultivated without fertilization for 2000 years and are still fairly productive.[159] Both in their physical character, chemical composition, and cultural characteristics, these regur soils are very similar to the “prairie soils” of the Cotton states and especially to the “black adobe” of California. Like the latter they are of unusual depth without change of tint; they crack wide open during the dry season on account of their high clay content; and the soil is thus partly inverted by the surface soil falling into the cracks. To the latter fact Leather ascribes, in part, the long duration of fertility in the regur lands. The regur also contains fragments of calcareous hardpan (here called guvarayi), just as in the Great Valley of California. The eighteen analyses of regur given by Leather agree so nearly in their essential points that it is admissible to average them; two other examples are however also given in the table.

[159] That is to say, they now produce about 600 pounds, or 10 bushels of wheat per acre, as do the Rothamstead soils after fifty years’ exhaustive cultivation. Probably both have come down to the permanent level of production corresponding to the amount of plant-food made currently available each year by the fallowing process in originally very rich soils. The present product of cotton on the regur lands does not seem to be on record; judging by the wheat product it should not be over one hundred pounds of lint per acre.

It will be noted that while the contents of lime, magnesia and alumina are uniformly high, the content of potash has a wide range; it rises very high (1.14%) in the maximum, while the average is fair.

One conspicuous defect of these soils is their extremely low content of nitrogen, in view of which their lasting productiveness is difficult to understand; unless it be that, as in California, their high lime-content causes a copious crop of leguminous weeds, constantly replacing the nitrogen supply.[160] Unfortunately we have no determinations of humus or of its nitrogen-content. Leather attributes the black color of the regur to some mineral substance rather than to humus; but his arguments are not quite convincing, so long as the Grandeau test has not been made. In view of the low rainfall and the closeness of the texture of regur, it is probable that little if any nitrates are currently washed out of the black cotton lands.

[160] See Voelcker, Report on the Improvement of Indian Agriculture, 1892, p. 46, par. 60.

The regur soil-sheet seems to be underlaid over the greater part of its area by a basaltic eruptive sheet (not by metamorphic rocks, as stated by Leather), and it is not easy to conceive how such a soil stratum can have been formed from such rocks as a sedentary formation. Elsewhere such soils are usually rather light and porous, as is the case in the Hawaiian and Samoan islands; and very high in iron-content. The regur has the character of an alluvial backwater or lake deposit; but how such a formation can have occurred on the Deccan plateau, is a question not easily answered.

Red Soils of the Madras Region.—Interspersed with and to seaward of the regur lands there are in the Madras presidency considerable bodies of “red” lands, which appear to be sedentary soils formed from underlying dark-colored, mostly eruptive rocks. Some of these are very rich in lime and potash, others very poor, and it seems impossible to classify them under any definite category either from the chemical or physical point of view, except as to their red tint. Even this tint, however, is not always found associated with exceptionally high contents of iron oxid, but due rather to its fine diffusion in the soil mass. As compared with the regur, with which the “red ” areas are interspersed, these soils contain, on the average, less lime, potash and ferric oxid; and phosphoric acid is uniformly low. The alluvial (brown and black) soils from the same region, exemplified in the table, are doubtless derived partly from the regur, and their color and composition varies accordingly.

“Laterite Soils.”—These are defined by Wohltmann (Tropische Agricultur, 1892) as being “the characteristic sedentary soils (Verwitterungsböden) of the tropics, formed under the influence of heavy precipitation, high temperatures and drought.” This definition does not indicate their derivation from any particular rock, such as laterite is supposed to be; but its definition puzzles even geologists, and so, as Leather observes, the definition of laterite soils will naturally puzzle agricultural chemists. Accordingly it is difficult to deduce from the analyses given any definite common characters. Leather describes those analyzed by him as red or reddish, sandy and gravelly, the gravel or cobbles often incrusted with a dark-smooth crust of limonite, which to the uninitiated looks as though the rock itself had been fused and vitrified. The samples from Lohardaga and Singhbhum show the effects of these limonite crusts upon the composition of the soils, which resembles that of the Hawaiian soils mentioned above; but in the latter the iron oxid is wholly pulverulent. But it is probable that, as in the case of the latter, the high content of phosphoric acid shown in the statement (.64 for the Lohardaga soil) is tightly locked up in the insoluble form of ferric phosphate. Wohltmann’s definition of laterite soils seems best represented by the “terra roxa” of Brazil, which as he states has .02 to.08% of potash, .02 to.10% of lime, and .045 to .10% of phosphoric acid. Humus and nitrogen are very deficient in all these soils.

While most prominent in the coast region of Bengal, they also occur not only near Madras (Saidapet) but also in the belt of high rainfall on the Malabar (western) coast of the Indian peninsula.

The productiveness of the laterite soils seems throughout to be only moderate, yet much higher than would be expected of soils of similar composition in the temperate zones, where the rate of soil-formation is so much slower than in the tropics.

From the analyses of “coffee soils” from Yarcand in the Sheveroy hills, north of Madras, we learn that coffee does well with a fairly liberal supply of lime (.30 to.44%) and phosphoric acid, but is satisfied with a much smaller amount of potash than is found in the tea soils of Assam.

A farther systematic investigation of the soils of India, with simultaneous accurate observations on their depth, subsoil, geological derivation, topographical location and relations to rainfall, could not fail to yield very important practical results. The examination of samples collected and sent in by persons unfamiliar with the proper mode of taking soil specimens, and the information which should accompany them, always involves a great deal of uncertainty and waste of labor, and indefiniteness of results.

INFLUENCE OF ARIDITY UPON CIVILIZATION.

In connection with the facts given and discussed above, as to the relative productive capacity of lands of the humid and arid regions, it becomes of interest to consider what influence, if any, these differences may have had in determining the choice of the majority of the ancient civilizations in favor of countries where nature imposes upon the husbandman, who supplies the prime necessaries of life, the onerous condition of artificial irrigation.[161]

[161] Verhandlungen der Deutschen Physiologischen Gesellschaft in Berlin, December, 1892; North American Review, September, 1902.

Preference of Ancient Civilizations for Arid Countries.—A brief review suffices to establish the fact of such choice. Aside from Egypt, the permanent fertility of which is ascribed to the inundations of the Nile, we find to westward the oases of the Libyan and Sahara deserts, the high fertility of which has become proverbial and has caused them to be cultivated from ancient times to the present. Similarly, on both sides of the Mediterranean Sea, we find that, instead of the humid forest country, it was in the arid but irrigable coast countries, such as the vegas of Valencia, Alicante, Granada, Malaga, and the even more arid domain of which Carthage was the metropolis; and farther east, in the Graeco-Syrian archipelago and the adjacent coasts, that noted centers of civilization were developed and maintained. Thence the arid belt requiring irrigation extends from Egypt and Arabia to Palestine, Syria, Assyria, Mesopotamia and Persia, and across the Indus through the anciently recognized regions of Indian civilization—Sindh, the Panjab, Rajputana and the Northwestern provinces—to the Ganges, embracing such well-known centers as Lahore, Delhi, Meerut, Agra, etc., inhabited by much more hardy and progressive races than the humid and highly productive tropical portions of the Indian peninsula. Throughout the extensive and important portion of northern India, irrigation is necessary to maintain regular production; and in default of it, periodic famines ravage the country. Thousands of years ago, millions upon millions of treasure were expended there upon irrigation works, as has again been done in modern times; yet in the rainy, forested districts we still find large areas practically tenanted by wild beasts. In Asia Minor, as well as in Central Asia, the remains of ancient cities once surrounded by richly productive irrigated fields, are found where at present only the herds of nomads pasture. The Khanates of southern Turkestan with their historic cities, illustrate the same obstinate bias in favor of arid climates. Similarly, in the New World, it was not in the moist and exuberantly fertile forest lands of the Orinoco and Amazon, but on the arid western slopes of the Andes, that the civilization of the Incas was developed. In Mexico, also, it was the high central, arid plateau, not the bountifully productive tierra caliente, over which the Aztecs chose to establish the main centers of their empire. Even to northward, the inhabitants of the high, dry plains of Arizona and New Mexico were, as their descendants of the Pueblos are to-day, superior in social development to their forest-dwelling neighbors of the Algonquin race. From time immemorial they have practiced irrigation in connection with cultivation, maintaining a comparatively dense population on very limited areas.

It might be thought that the desire to avoid the labor of clearing the forest ground was the motive which guided the choice of the ancient nations toward the cheerless-looking, treeless regions.

But if we consider the cost and labor of establishing and maintaining irrigation ditches, it certainly seems that a stronger motive, based on the intrinsic nature of the case, must have influenced their selection. Neither can we with any degree of plausibility ascribe the preference for the arid open country to the fear of enemies lurking in the forest, since war was in early times practically the normal condition of mankind, and was waged with little hesitation wherever booty was in sight. It has also been asked how the ancients could have known of the high productive capacity of arid lands; but no one who has ever seen the springing-up of luxuriant vegetation after the periodic overflows of the arid-region streams, or the same surrounding the springs in the deserts, would ask that question.

Irrigation necessitates Co-operation.—Irrigation enterprises can be accomplished in a very limited degree only by individuals or even families. Its permanently successful execution requires the co-operation of at least several social groups, ultimately of communities and states, if it is not to give rise to acrimonious contentions or actual warfare; witness the “shotgun policy” resorted to in the arid West in times not very remote. Irrigation, in other words, compels co-operative social organization quite different from and far in advance of that necessary in humid countries. And such organization is manifestly conducive to the preservation and development of the arts of peace, which means civilization. The most ancient systematic code of laws known to us is that of Hammurabi, the king of arid Assyria.

The high and permanent productiveness of arid soils induces permanence of civil organization.—In humid countries, as is well known, cultivation can only in exceptional cases be continued profitably for many years without fertilization. But fertilization requires a somewhat protracted development of agriculture to be rationally and successfully applied in the humid regions, and the Germanic tribes, like the North-American Indians, seem to have shifted their culture grounds frequently in their migrations. No such need was felt by the inhabitants of the arid regions for centuries, for the native fertility of their soils, coupled with the fertilizing effects of irrigation water bringing plant-food from afar, relieved them of the need of continuous fertilization; while in the humid regions, the fertility of the land is currently carried into the sea by the drainage waters, through the streams and rivers, causing a chronic depletion which has to be made up for by artificial and costly means. What with the greater intrinsic fertility and the great depth of soil available for plant growth, much smaller units of land will suffice for the maintenance of a family in arid countries; a fact which is even now being illustrated in the irrigated region of the United States, where ten acres of irrigated land instead of 40 or 160, as in the East, form the unit.

The arid regions were, therefore, specially conducive to the establishment of the highly complex polities and high culture, of which the vestiges are now being unearthed in what we are in the habit of calling “deserts;” the very sands of which usually need only the life-giving effects of water to transform them into fruitful fields and gardens. It is also quite natural that the wealthy and prosperous communities so formed should in the course of time have excited the cupidity of the “barbarous” forest-inhabiting races, and as history records, have been over and again overwhelmed by them—a similar fate often afterwards overtaking the conquerors in their turn, after the Capuan ease of their existence had weakened their warlike prowess. At the present time, the arid regions of the old world are still largely suffering from having been overrun by the nomadic Turanians, whose original habitat—Mongolia and Turkestan—while also arid, does not permit of the ready realization of the advantages above outlined, on account of the rigorous climate brought about by altitude. Mahometanism first expelled, and has since repelled, occidental civilization from the arid regions of the Old World, remaining to-day as an obstacle to its progress. The peaceful aggression of railroads and telegraphs now seems likely to gradually overcome this repulsion; and when Constantinople and Bagdad shall be linked together by the steel bands, the desert will lose its terrors, and Mesopotamia and Babylonia will again become garden lands, as of old, under the abundant waters of the Euphrates and Tigris. Until the water-supplies of the arid countries shall have been more definitely gauged, it is impossible to foretell to what extent food-production may be increased by their cultivation under irrigation, after the relief from political misrule shall have rendered such undertakings safe. But it can even now be foreseen that with improved modern methods of cultivation, the productive area of the world can be vastly increased by the utilization of the countries where, as the Turcomans say, “the salt is the life of the land.”

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Chapter XIX

XX. SOILS OF THE ARID AND HUMID[125] REGIONS

XXI. SOILS OF ARID AND HUMID REGIONS (Continued)