Black soils are mineral soils which have a black surface horizon, enriched with organic carbon that is at least 25 cm deep. Two categories of black soils (1st and 2nd categories) are recognized. The categories are distinguished to recognise the higher value, and thus greater need for protection, of some soils (Category 1), while still including a wider range of soils within the overall Black Soil definition (Category 2).

1st category Black Soils (the most vulnerable and endangered, needing the highest rate of protection at a global level) are those having all five properties given below:

• The presence of black or very dark surface horizons typically with a chroma of ≤3 moist, a value of ≤3 moist and ≤5 dry (by Munsell colours); 
• The total thickness of black surface horizons ≥25 cm; 
• Organic carbon content in the upper 25-cm of the black horizons of  ≥1.2% (or ≥ 0.6% for tropical regions) and ≤20%; 
• CEC in the black surface horizons ≥25 cmol/kg; and
• A base saturation in the black surface horizons ≥50%.

Most but not all 1st category Black soils: 

• Have well-developed granular or fine sub-angular structure and high aggregate stability in the black surface horizons that are in a non- or slightly degraded state, or in the humus-rich underlying horizon which has not been subjected to degradation.

2nd category Black soils (mostly endangered at the national level) are those having all three properties given below:

• The presence of black or very dark surface horizons typically with a chroma of ≤3 moist, a value of ≤3 moist and ≤5 dry (by Munsell colours); 
• The total thickness of the black surface horizons of  ≥25 cm; and
• Organic carbon content in the upper 25-cm of the black horizons ≥1.2% (or ≥ 0.6% for tropical regions) and ≤20%.

Black Soils definition endorsed by the 11th Working Session of the Intergovernmental Technical Panel on Soils (21/11/2019, FAO HQ)