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What is a black soil?

Black Soils refer to many different soil types that have:

- a well-structured, dark coloured surface horizon due to their enrichment of high-quality humus down to a depth of more than 40 cm - mostly 60 to 80 cm;

- a high base saturation (i.e. a high percentage of the cation exchange capacity is occupied by the basic cations Ca2+, Mg2+ and K+) and;

- a moderate to high content in organic matter.

The natural areas from which black soils developed are the prairies and steppes that experience summer-dry and freezing conditions. In the World Reference Base (WRB) for soil resources, black soils include Chernozems, Kastanozems and Phaeozems. In the USDA classification, black soils correspond to Mollisols.

Mollisols have a high soil organic matter (SOM) content. According to Buringh (1984), the average organic carbon content of a Mollisol is around 130 t/ha in croplands and 160 t/ha in grasslands. However, figures on organic carbon and organic matter are highly variable according to land-use and land-cover.

Chernozems are made up of very dark topsoil and secondary carbonates. Their mollic horizons are rich in organic matter (10–16%), are highly saturated with bases (Base saturation over 50%) and react neutrally in terms of soil pH. Chernozems are known for being well-tructured, chemical properties beneficial to biological activity and plant growth and inherent fertility. These soils are distinguished by a very deep (more than 1 m) organic matter-enriched layer, perfectly expressed granular structure, almost optimal bulk density, and a satisfactory stock of nutrients. Kastanozems have a similar profile to that of Chernozems, but the organic matter-rich surface horizon is thinner and not as dark, and they show more prominent accumulation of secondary carbonates. These soils are potentially fertile soils but suffer from periodic low soil moisture. Finally, Phaeozems are similar to Chernozems and Kastanozems, but they are leached more intensively. Consequently, they have dark, organic matter-rich surface horizons that are less rich in bases. Phaeozems are either free of secondary carbonates or have them only at greater depths. They all have a high base saturation in the upper meter of the soil.

Furthermore, Black Soils are included in many national soil classification systems. For example in India the soil group of Black soils was used for soils on the Decan plateau that are nowadays classified as Vertisols. In China, Black soils correspond with soils originally very rich in organic matter (6 – 8%) and are subject to a significant loss of soil carbon due to soil erosion. In Bangladesh, Black Terai soils are of alluvial origin classified in the Soil Taxonomy as Humic Dystrudepts.