Researchers compared different types of fertile black soils from various soil classification systems to see how they relate to each other. They used data on soil properties to calculate how similar or different these soils are. They found that some black soils are closely related, like Mollisols in the US, while others, like Vertisols and Andisols, are quite different. The Russian black soils were similar to certain groups in the World Reference Base. Overall, black soils form a distinct group similar to Mollisols, but there is significant diversity within this group, adding to the variety of soil types in black soil areas.