The Chernozem Family

Abstract

Chernozem are well-drained soils with a thick, black topsoil characterized by an extraordinary accumulation of humus, granular structure that renders them friable even when dry and high base saturation, overlying a limey subsoil. They occupy some 230 million hectares in the steppes of Eurasia and Prairies of North America; they grade to chestnut soils (Kastanozem) in drier regions and to Dark grey forest soils (Phaeozem) in wetter areas. The sub-types of chernozem appear to be an evolutionary as well as geographic (south–north) sequence. Two evolutionary stems may be distinguished, illustrated in Moldova by the sequence in the south of the country from Calcareous to Common chernozem with a side branch to Xerophyte-wooded chernozem; and in the more humid north by the sequence from Typical to Leached chernozem, with a side branch to Podzolized chernozem. Calcareous chernozem (Calcic chernozem in the World Reference Base) is the least developed family member with carbonates, inherited from the loess parent material, throughout the soil profile; compared with other sub-types, it contains less humus and is less resilient to erosion by wind and water. The topsoil of Common chernozem (Haplic chernozem) has more humus, a stronger structure and is free of carbonates; in the subsoil there is secondary carbonate as pseudomycelia as well as calcite inherited from the parent material. Soils buried 2000 years ago beneath Trajan’s Bank are both Calcareous and Common chernozem, just like today’s soils, which suggests that the evolution from Calcareous to Common chernozem is slow. A variant under oak parkland (Vermic chernozem) exhibits a humus content as much as 12% in the topsoil (almost 500 t/ha in total) and extraordinary biological activity. Typical chernozem exhibit the characteristics of chernozem in their most pronounced form: very thick, black topsoil with intensive accumulation of mull humus and well-developed granular structure; carbonates are leached to below 50–90 cm and there is little secondary carbonate. Particle size distribution shows weak clay accumulation but the mineralogy indicates the beginnings of clay destruction. Leached chernozem are close to Typical, distinguished by leaching of carbonate to below 82/125 cm and the beginnings of silt and clay movement from the A to the B horizon. In Podzolized chernozem, carbonates are leached below 75/140 cm; the A horizon has about 85% base saturation, a pH in water of 5.8–6.1 but only 4.5–5.1 in dilute salt solution, reflecting 5–6 meq/100 g exchangeable acidity. All chernozem are very fertile. The bonitet rating compiled by statistical comparison of crop yields, soil type and texture takes the Typical chernozem as the gold standard of 100 points. Points are deducted according to attributes of soil and land that are associated with depression of crop yield compared with the standard; further points are deducted according to the degree of erosion.