Nitrogen can be added to soil in many forms. It can be applied as nitrates, ammonium, urea, etc. Soon after the nitrogen is added and if there is some moisture present, the indiginous bacteria called nitrosomonas and nitrobacter convert all forms of nitrogen to the nitrate form. The nitrate form of nitrogen is an ion with a negative charge (NO3-).
The clays in South Carolina soils also have a negative charge and since similar charges repel, the nitrates are not attracted to the clays like the posit ively charged calcium, magnesium and potassium ions. Nitrates are also very soluble in water. As a result, the nitrate ions are very mobile in the soil and move through quite readily as water percolates down into the soil. If a soil is sandy, nitrates move through the soil even faster due to the large pore space and faster percolation of water. These factors are the reason for environmental and health concerns regarding nitrate movement into groundwater.
Because of nitrogen's rapid conversion to the nitrate form and its subsequent movement through the soil, it makes the soil test for nitrogen quite difficult to interpret. By the time you receive soil test results for nitrate nitrogen, it may have alre ady moved through the soil if some rain had fallen between sample collection and reporting of lab results. Some soil tests for nitrogen are done for certain crops but it is still in an experimental stage and not widely used or accepted in our region. Re searchers will collect soil samples at several depth intervals to monitor the nitrate movement during a crop's growing season.
Because of the mobility of nitrogen in the soil, most states in the Eastern region of the U.S. just make a blanket recommendation for each crop with the assumption that most of the residual nitrogen from the previous crop has moved past the root zone f or the new crop. You will notice that for many crops, nitrogen is recommended as a split application. If all of the nitrogen was added at the beginning of a growing season, much of it would move past the root zone before the crop matures. Splitting app lications allows the nitrogen to be applied more in accordance to the crop's needs at different stages of growth and thus reduce leaching.