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]
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.