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.
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.
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 |