CHERNOZEM

one of the 30 soil groups in the classification system of the Food and Agriculture Organization (FAO). Chernozems (from the Russian words for "black earth") are humus-rich grassland soils used extensively for growing cereals or for raising livestock. They are found in the middle latitudes of both hemispheres, in zones commonly termed prairie in North America, pampa in Argentina, and steppe in Asia or in eastern Europe. Chernozems account for 1.8 percent of the total continental land area on Earth. Chernozems are characterized by a surface layer that is rich in humus and in available calcium ions bound to soil particles, resulting in a well-aggregated structure with abundant natural grass vegetation. They form in climatic zones with seasonal rainfall of 450-600 mm (18-24 inches) per year, coming in the spring or early summer; with cold winters; and with relatively short, hot summers. In the colder areas of these climatic zones, a natural tall-grass vegetation develops on soil profiles whose surface layers can be as much as two metres (about six feet) thick, with up to 16 percent humus by mass. Lime may accumulate below this layer because of limited downward percolation of calcium salts. Chernozems are closely related to the soils in the Mollisol order of the U.S. Soil Taxonomy that form under tall-grass vegetation. Related FAO soil groups originating in a steppe environment are the Kastanozems and Phaeozems.