(2) Glacial Soils.—In glaciated regions such as the eastern United States north of the Ohio River, the soil was distinctly improved by the advance of the ice which probably culminated 25,000 to 30,000 years ago. In some places, to be sure, the ice scraped off the soil and left little in its place, and in others it deposited sand, pebbles, and boulders. Elsewhere, however, it mixed many different kinds of rock debris together; it ground the material very fine; and it scoured from the rocks much new material which has not yet been leached of its plant food. Thus the soils of many states like New York, Ohio, Michigan, Illinois, Wisconsin, and Iowa were decidedly improved by the advance of the ice. For example in Wisconsin, Professor Whitbeck estimates that glaciatimi has caused the average value of all crops per square mile to show the following values compared with the crops in similar non glaciated areas: Similar benefit arises from glaciation in parts of Europe, especially the Black Earth region of Russia, where materials washed out from the ice seem to have found a final resting-place. Some glacial soils, especi ally on the borders of the glaciated areas where the soil is deep, fine grained and level, have gradually been so much improved by the decay of vegetation and the ac cumulation of humus, that their fertility is second only to that of the soils of the desert. Such dark soils are found in the American prairie states and in south central Russia.
(3) Soils of Moist Re gions. — The third great type of soils occurs in moist regions which have not been glaciated. Such soils vary from the fairly good quality of regions like eastern Pennsylvania with moderate rainfall and cold winters, to the red laterites of moist., hot regions in the torrid zone. These soils are usually old and well leached, but in middle and high latitudes they are usually supposed to be better than in low lati-. tudes, for the plant foods are not leached away so fast. Moreover, some humus accumulates, whereas in tropical regions the dying plants usually decay so rapidly that little or no humus is formed and the nitrogen furnished by the plants is quickly washed away. The tropical laterites are called by Marbut " the end stage or death stage of soils." Their lime, potash, and nitrates are much depleted. Thus on the whole, as one goes from a glaciated region where the soil is new and unleached t o warm regions where the soil is old and thor oughly leached, or from a desert to a region of great humidity, the fertility of the soil de clines.
This does not mean that there is a de cline in the amount of vege tation, for a relatively poor soil in a warm moist climate bears more vegetation than the richest soil in a cold climate or a desert. (Figs. 12 to 15.) It merely means if all soils were given the same amount of water and sunshine, and the same tem perature, desert soils would stand first; then fine gla cial soils where humus has ac cumulated, and next those of non - glaciated regions which are cool enough so that leach ing and decay are checked in winter and the accumulation of humus is possible. Below these stand the leached soils without humus, which
require constant fertilization and deep plowing in order to raise good crops for more than a few seasons. These prevail in the southeastern United States and still more in the moist parts of the tropics. Last of all come the almost completely leached tropical soils of the laterite type where practically all the plant food is gone and agriculture is extremely difficult. An acre of irrigated desert land may produce per haps ten times as much as an acre of laterite, and will retain its fertility many years.
The Utilization of New Soil.—As population increases there is a constant demand for larger supplies of food and raw materials. So far as the soil is concerned this demand can be met in two chief ways: (1) by using soils that are now uncultivated; (2) by more careful culti vation of soils now in use. Uncultivated soil is found in four kinds of localities: (1) in well-populated regions considerable areas are often uncultivated because too poor or too rugged to pay for the work needed to make them productive. (2) Tropical regions contain vast areas of unused soil which is rather poor in quality; (3) Unused soil is equally abundant in cold countries. In glaciated regions some of this is of excellent quality although very irregular in thickness. (4) Dry regions contain enormous areas of wonderfully rich soil.
Poor Tracts in Well Populated Areas.—Let us see which of these four kinds can be most profitably used. To begin near home, we have seen that in rugged regions like New England, the Appalachian states, and California, many unused tracts lie upon slopes among hills and mountains. We have also seen that in places like Long Island a rela tively poor soil may be worth cultivating because of its nearness to a good market. As time goes on many other regions which are not now worth cultivating are sure to be used. Their use is partly a question of fertilizers, which will be discussed later, partly of finding cheap means of making terraces and using machinery on slopes, and partly of the development of markets not far away. In all the mountainous or hilly parts of the United States there is much good soil which could be profitably used if there were machines which could work on a slope as well as on a level, or which were as effective on small areas as on large. But there are no such machines. Until someone invents them the great improvements in modern transportation are likely to make it more profitable to develop distant regions where the relief is not a hindrance.