Of the zonal soils in a semi-arid climate among the most distinctive and widely distributed types are the chernozem soils, or black earths. A typical chernozem profile appears to consists essentially of two layers. Immediately beneath a grass sod is a black layer, the A horizon, 2 to 3 ft (0.6 to 1 m) thick and rich in humus. In this layer the structure is a crumb or nut. The A horizon grades downward into a B-horizon of brown or yellowish-brown colour then, with a sharp line of demarcation, into a light-coloured horizon. As in the podzolic soils, in the B horizon colloids and bases accumulate by downward percolation through the A horizon, but unlike the podzols, the chernozem soils have no leached A1 horizon.
Chernozem soils are rich in calcium, which appears in excess in calcium carbonate precipitated in the lower B-horizon or just beneath the B-horizon. It has been noted that chernozem soils develop in parent material rich in calcium carbonate. The origin and distribution of chernozems soils have long attracted the soil scientist.
Chernozem soils are important in the United States and Canada, where they form a north-south belt starting in Alberta and Saskatchewan and running through the Great Plains of the United States to central Texas. A similar area lies in Argentina. Other areas are mapped in Australia and Manchuria.
Climate has long been thought to be a determining factor in the development of chernozem soils, Comparison of soil and climate maps shows that the middle-latitude chernozems, in the Americas and Europe, lie on the more arid western side of the humid continental climates and with decreasing latitude extend over into the middle-latitude steppe climates. Aridity is therefore a definite contributing cause. The continental location of chernozem areas makes for hot summers and cold winters. Drought periods with strong evaporation dry out the soil, and forests cannot exist. Instead, grasses, which can withstand drought readily and which are tolerant to soils with excesses of mineral salts, flourish on chernozem soils. Steppe grasslands and prairies are the natural vegetation of the middle-latitude chernozem soils.
An important factor in development of the middle-latitude chernozem soils is their occurrence on loess, the wind-transported dust so extensively deposited during the glacial period. Though chernozem soils are not limited to such areas, the texture and lime content of loess, together with the plains topography, have been especially favorable to chernozem development.
Geographically, perhaps the outstanding point of importance regarding the chernozem soils is their productivity for small grain crops-wheat, oats, barley, and rye. Great grain surpluses are exported from chernozem areas in the United States, Canada, and the Ukraine and Argentina, causing them to be described as breadbaskets of the world.
Where forests invade the chernozem soil grasslands, some influence of podzolization is felt and there develops a faint A horizon. Such soils are known as degraded chernozem soils. They are transitional to the gray-brown podzolic soils and occupy a geographical position adjacent to them. One particularly large region of degraded chernozem soils lies northwest of the Black Sea and extends from the Danube River of Romania to the southern Ukraine.
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