Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions

Chapter XXI

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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.

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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Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regionsChapter XXI

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