CRP is the Conservation Reserve Program. The government pays farmers to maintain certain fields in grasses or trees. Often, this is land that is too steep for row cropping without extensive erosion. At first glance, you would think that removing biomass from these fields every year would reduce soil fertility. Actually, this is not necessarily so. Some plants, including most grasses, are stimulated to grow more strongly by mowing or grazing. This greater biological activity leads to tapping more nutrient reserves from deep in the earth and from the air. Bacteria and fungi work in symbiosis with plant roots to extract nitrogen from the air and from certain soil particles. These nutrients are translated into plant mass and greater organic matter in the soil.
Estimating Ethanol Yields from CRP Croplands / March 19, 2010 / Newsfrom the USDA Agricultural Research Service: "This extensive study also shows that CRP lands in the northeastern United States with a high proportion of tall native prairie grasses have the potential to produce more than 600 gallons of ethanol per acre. This energy can be produced while maintaining the ecological benefits of CRP grasslands."