Nitrate (N03) is the primary source of nitrogen (N) for plants; it is a nutrient they cannot five without. Extensive farming can rob the soil of its natural nitrogen sources, so farmers often add nitrate fertilizers. Properly managed, nitrogen does not endanger health and can increase crop production. However, when more nitrogen is added to the sod than the plants can use, excess nitrate can leach into groundwater supplies and contaminate wells. Because nitrate is converted to a very toxic substance (nitrite) in the digestive systems of human infants and some livestock, nitrate-contaminated water is a serious problem.