Natural Sequence Farming

The book mentions a number of times to concentrate organic compounds at the surface for that is where grass obtains most of its fertility from. I thought that grass roots went many feet below the surface. So I'm not sure why there is a concerted advocacy in the book for keeping the fertility at the surface?

Grasses are the final succession species and prefer high fertility. The high fertility zone of the Ao horizon, is where the interface of air, water and fertility is the favoured zone of most grasses. Deep rooted perennial grass mine minerals from deep in the soil.