The study of agricultural soils, that is, of the chemical and lithological constitution of the surface layer, is very advanced in America and the Soils Office has published an excellent atlas.
In the NE region. and in the area of the Great Lakes, where the Laurentian forest extended, mixed transitional soils prevail and next to the so-called podzool soils of the forest areas of the high latitudes, in which the superficial humus, whitish and powdery, deprived of clayey particles, is certainly among the poorest in North America, there are areas with gray-brown soils, very fertile, belonging to the Appalachian forest.
In the Appalachian Forest area, extending from the NE. to SW. among the Appalachians, Ohio and Mississippi, despite the extreme diversity of the constituent rocks – limestone, marl, sandstone, crystalline rocks, moraines and löss – the soils have rather uniform characters. Trees with broad deciduous leaves provide abundant organic matter, and chemical transformations are intense thanks to the mild temperature and heavy rainfall; the deeply worked humus merges with the surface layer of the soil and gives it a gray-brown tint. These lands proved to be very suitable for agriculture, and remarkably fertile, as well as resistant to agricultural exploitation.
In the SE zone, which corresponds to the South Atlantic forest, the abundant rains, the high temperature and the leniency of the winter cause an intense washing that takes away all the soluble elements and clayey particles: the result is a lean soil with a pale hue and not very consistent, while the subsoil, rich in clay and iron oxides, is compact and colored in yellow and red. In Georgia and Florida the phenomenon is accentuated and the accumulation of iron in the subsoil seems to denounce the beginning of the lateritic process typical of tropical countries.
In the central region of the prairies and steppes the soils vary according to the climate and rainfall: in the eastern part, which is more rainy, the constantly humid subsoil is rich in organic matter coming from the deep roots of the grasses, but towards the west, where the rains are very scarce, the subsoil remains constantly dry and, under the thin vegetal layer of the surface, there is a crust of carbonates sometimes very thick called caliche. The chemical constitution also varies, but if the rainfall is sufficient, the agricultural value is good; in fact, the prairie has become the most important cereal region in the United States.
In the region occupied by the Pacific wet forest, thanks to the humid and mild climate, there are gray-brown soils similar to those of the Appalachian forest and equally suitable for agriculture. The soils of the western steppes resemble those of the prairies due to the richness of soluble elements and the presence of a more or less deep layer of carbonates. In the southernmost region, on the closed plateaus of the interior, such as in the Great Basin, and in all areas with very little rainfall, the soils are highly mineralized and contain, at a small depth, alkali so that the vegetation is reduced to sparse patches of artemisie or is even missing in the lower clayey-saline soils.