Some proven practices are broadly applicable to most sites and farming situations in North Carolina, whereas others are still being tested. The basic soil management principle for minimizing nitrate contamination of groundwater is to minimize the amount of available nitrogen in the root zone except when the crop is actively assimilating it. Although this concept is simple, it is difficult to apply. It requires attempting to match the available nitrogen with crop needs and managing the system so that the nitrogen is there when needed. However, even with the best management some nitrate will not be used by crops and will thus be available for leaching.
Growers can take several steps to help match fertilizer rates to crop needs.
One is to take into account all nitrogen sources when determining the amount of nitrogen to apply to cropland. Organic nitrogen sources are often overlooked. For example, several studies have shown that farmers often ignore nitrogen contributions from legumes and organic wastes when they calculate fertilizer needs. As a result, there is more nitrogen than is needed by the crops, and potential for nitrate to leach into groundwater is increased. Timing of nitrogen applications is important because the greatest amount of nitrate loss occurs between the time the fertilizer is applied and the time the nitrogen is taken up by the crop.