Because nitrogen (N) is essential to crop plant growth and development, farmers have used N fertilizers increasingly to make crop production profitable. But in many cases, 50 percent or less of this fertilizer is actually used by the crop in a given year. The remainder either percolates (leaches) out of the soil to groundwater, washes off, evaporates or dissolves in water in a process called denitrification, or stays in the soil where it may be useful for a later crop.