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Why is chernozem soil dark and rich in humus?

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Map showing the extent of the Eurasian steppes, highlighted in green, stretching from eastern Europe through Ukraine and southern Russia, across Kazakhstan, and into Mongolia and northern China. The map labels the Western Steppe and Eastern Steppe regions and includes major geographic features, countries, and ancient sites within the steppe zone. — the Steppe Extent of the Eurasian steppes.

Chernozem, a term derived from Russian words meaning "black earth," is the distinctive soil of the steppe. Its dark color and high fertility are primarily the result of significant humus accumulation derived from the region's dense, natural grass cover.12

Several environmental factors facilitate this process:

Inhibited Decomposition: The combination of cold winters and summer droughts limits the activity of organisms that would otherwise break down organic matter, allowing it to accumulate in the soil.2
Limited Leaching: High rates of evaporation prevent the downward leaching of nutrients, keeping the organic material within the upper soil horizons.2
Vegetation Cycle: The thick growth of perennial grasses provides a constant supply of organic material. Because the rate of litter production is high, soil stocks of organic matter increase, creating deep, humus-rich layers.34

These conditions result in a well-aggregated, nutrient-dense soil that is among the most fertile in the world.14