The black earths, or mollisols (Seventh Approximation), are the richest soils on the planet. Known as chernozems in the Russian language (chernaya, meaning "black," and zemlya, meaning "earth"), they are found in semiarid grasslands, or steppes, which are wedged between arid deserts and humid forests. In the Soviet successor states, black earths stretch west to east from Moldavia and Volyno-Podolia in western Ukraine to the Russian North Caucasus and deep into Siberia as a steadily narrowing wedge to Irkutsk near Lake Baikal. Transitional between areas with a soil moisture surplus (forests) and areas with a conspicuous soil moisture deficit (deserts), grassland soils are only slightly leached during sporadic thunderstorms. The relative lack of precipitation ensures that solubles like calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg) are accessible to the uppermost humus layer (horizon) of the soil. The A-horizon consists of grass litter and extensive root systems that draw on a thick black to chestnut-brown humus zone that is rich in ionized colloids and natural fertility. The underlying B-horizon often possesses nodules of calcium carbonate, which during frequent droughts rises to the A-horizon through capillary action. Windblown silts known as loess further enrich chernozems by imparting a loamy soil texture.
Chernozems form in areas of cold winters and hot summers that are conducive to rapid evaporation. The resultant imbalance encourages the capillary rise of soluble nutrients from the B- to the A-horizon. Grasses thrive in these conditions, but their matted root systems create a sod that could not be breached by early wooden plows. Accordingly, until the invention of the steel-tipped plow in the 1800s, settlers considered grasslands useless. Requiring irrigation, the black earths now make up the great commercial grain belts.
See also: agriculture; climate; geography
Strahler, Arthur N. (1969). Physical Geography, 3rd ed. New York: Wiley.
Victor L. Mote