We often refer to the “white” knolls in rolling land as eroded knolls because the topsoil (A Horizon) is thin. Erosion has usually occurred on a knoll but the original A Horizon was very thin and the B Horizon is absent.
When cultivated, the A and Cca are mixed together so the topsoil has lime present. The lime can tie up a lot of phosphorus (P). That is why extra P fertilizer on the “limy” knolls is a good bet. If we ever get this year’s crop off I hope to do just that on Blackstrap farm this fall.