Composition of Black Soil: Natural Ways to Amend It Without Agrochemicals

This paper explains black soil's characteristic dark color as the product of four overlapping factors: iron oxide (Fe2O3) and manganese oxide (MnO2) coating soil particles, the chemical weathering of dark, iron- and magnesium-rich volcanic basalt rock over time, the darkening effect of even small amounts of decomposed organic matter (humus), and a moisture-retention property that makes the soil appear darker still when wet.

A compositional breakdown is provided by approximate percentage: clay 40-60% (holds moisture, makes soil heavy), silt 20-30% (contributes to fertility), sand 10-20% (improves drainage), calcium carbonate 5-10% (alkaline, aids nutrient availability), magnesium carbonate 1-5% (supports plant metabolism), iron oxide trace-3% (gives dark color, aids photosynthesis), alumina 5-15% (involved in nutrient retention), and organic matter at a comparatively low 1-2%.

That low organic-matter figure is presented as the actual lever for improvement: black soil's mineral wealth (calcium, magnesium, iron, alumina) is already abundant, but its low humus content limits the biological activity needed to make those minerals available to crops.

The paper closes with a four-step PQNK restoration sequence: prepare the field for irrigation, break the hardpan, form permanent raised beds, and restore the soil surface with cover-crop roots left in place and mulched, rather than relying on any purchased mineral or organic amendment.