Ammonium is available to plants. Nitrate is the principle form of nitrogen used by plants. The nitrate form of nitrogen is a major concern in pollution. Soil microorganisms use nitrate and ammonium nitrogen when decomposing plant residues. This can be a major concern if crop residues are high in carbon relative to nitrogen.
Examples are wheat straw, corn stalks and sawdust.
The addition of 20 to 70 pounds of nitrogen per ton of these residues is needed to prevent this transformation. After the residues are decomposed, the microbial population begins to die back and processes 1 and 2 take place. These gases escape to the atmosphere and are not available to plants. This transformation can occur within two or three days in poorly aerated soil and can result in large loses of nitrate-type fertilizers. To minimize these losses, incorporate solid ammonium-type fertilizers, urea and anhydrous ammonia below the surface of a moist soil.