The consensus on the black soils of the U.S. midwest is that they were formed by the massive root systems of (especially tallgrass) prairie and savanna grasses and other plants, and the residue of thousands of years of the seasonal growth and decay of the roots of these plant communities, which are maintained by fire.
The fire is encouraged by seasonal dryness, and by cyclical droughts every few decades, which also kill trees.
Such grasses are adept at drawing stored nutrients into the roots for the dormant season leaving mostly cellulose above ground, to minimize the loss from burning.
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Doug Woodard
St. Catharines, Ontario, Canada
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