CHAPTER XXI.. Soils of Arid and Humid Regions (_continued_).

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; 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.

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