Is it the organic matter? Or is it the minerals? Obviously both are important, and there are other factors, but of these two which one is usually the limiting factor?

the best soils are supposed to be dark brown chernozem soils, they're dark brown even at very deep depths.

"Dark brown" screams organic matter to me. So is the primary determinant of "good" soil simply the organic matter? And most soils are more or less "fine" in terms of minerals? And if this is the case, isn't building soil simply just a matter of defecating more on bad soils?