CHERNOZEM, also called BLACK EARTH, grassland soil with a dark humic horizon (a layer with at least one percent carbon) more than 25 cm (10 inches) thick. Chernozems are neutral soils, and they contain no layers of alumina-rich clays or layers in which the clay has been transported away by water (eluviated). The chernozems on the Russian steppes, where they were first identified, were formed from calcium carbonate-rich material and therefore have a calcareous layer that underlies or overlaps the humic layer. The leaching rainfall is low, and clay seldom is accumulated by aqueous transport. In the United States, Canada, and Argentina, however, these soils have formed from noncalcareous materials rich in sodium, magnesium, and silica. These areas often have a greater leaching rainfall, and the accumulation of clay is favored.