The ammonium form is held in the soil by negatively charged soil clays or colloids. Because soils have this natural negative charge, the nitrate form is not held by the soil. Forms of available soil nitrogen. Nitrogen reactions in the soils Symbiotic nitrogen fixation. Legumes are able to fix enough nitrogen for their needs so that additional nitrogen fertilizer is not necessary to grow these crops.
Many legumes also fix enough nitrogen to supply some nitrogen for succeeding crops.
Soil bacteria decompose organic nitrogen forms in soil to the ammonium form. This process is referred to as ammonification. The amount of nitrogen released for plant uptake by this process is most directly related to the organic matter content. The initial breakdown of a urea fertilizer may also be termed as an ammonification process. Ammonification reactions in soils.