When possible, the farmer should rotate crops such as corn and grain sorghum that heavily use nitrogen with crops such as soybeans, alfalfa and clover. Aside from reducing fertilizer nitrogen requirements, crop rotations provide other proven benefits in terms of reduced insect and weed infestation levels and disease pressure. The nitrogen credit to corn after soybeans is not because of the additional nitrogen in the soil from the soybeans, but because the low C:N ratio of soybean residue immobilizes less soil nitrogen and mineralizes nitrogen from residue sooner the next season. This allows more soil nitrogen to be available to the subsequent crop. Legumes such as alfalfa or clover that are tilled in prior to planting corn increase the level of available nitrogen in the soil as the legume residue mineralizes. Legumes are efficient scavengers of soil nitrate and can substantially reduce soil nitrate levels after corn. Figure 5.6 illustrates the effect of an annual corn-soybean rotation on the amount of nitrate in the vadose zone. This example, from vadose zone soil cores taken in 1992 from the Long-Term Tillage Study at the University of Nebraska South Central Research and Extension Center Farm, shows the long-term effect of implementing a corn-soybean rotation in 1984.