Nitrogen is applied to fields in various forms such as anhydrous ammonia, ammonium nitrate, urea, ammonium sulfate, and others. While ammonium can be taken up by plants, much of it undergoes nitrification in the soil to produce nitrate. Nitrate does not bind to clay particles and is mobile in the soil. Mobility is both an asset and a detriment for producers. The benefit of high mobility of nitrate is that it readily diffuses through the soil so plant roots do not have to actually intercept it in the soil.
Plants thus expend far less energy developing roots to meet their nitrogen needs.
The disadvantage of high mobility is that nitrate can move with soil moisture beyond the rooting zone of the crop. Once the nitrate is beyond the rooting zone, it is subject to further deep leaching and can contaminate ground water. The report also indicated that nitrate concentrations in groundwater have increased since the 1950s which is prior to intensive, large-scale, irrigated agriculture. Nitrate levels in fallow fields were monitored for one year beginning in March in an associated study. In March, nitrate levels were concentrated in the upper soil profile and low in the deeper profile.