Portion of article by Scott Killpack and Daryl Buchholz, Department of Agronomy, University of Missouri

Most of the nitrogen found in the soil is in numerous different organic forms. However, plants are unable to use organic forms of nitrogen. Normal, rather complex processes in the soil convert nitrogen from organic forms into ammonium.

Nitrogen Mineralization - Immobilization

Q: What affects mineralization and immobilization?

A: Decaying plant material, humus and/or organic matter in the soil are important sources of nitrogen. How much plant residue or organic matter are in the soil, and the types present, can affect the types of nitrogen conversion processes that take place. For example, the presence of a high amount of wheat straw or corn stalks in the soil will result in immobilization processes taking place. Immobilization results in plant usable forms of nitrogen in the soil becoming unavailable for subsequent crop growth. This nitrogen is used by microorganisms in the decomposition process of wheat straw or corn stalks. Once the wheat straw or corn stalks have become highly decayed, immobilization stops and mineralization starts. That is, plant usable forms of nitrogen such as ammonium become available again. Immobilization processes generally do not take place when a legume crop, such as alfalfa or white clover, is plowed under. Mineralization will likely be the dominant process.

Conversion processes of nitrogen to different forms are all accomplished by various groups and types of microorganisms found in the soil. Factors that affect microorganisms, such as temperature, oxygen supply and moisture, can affect the degree to which nitrogen conversions take place.

Both immobilization and mineralization processes can have a direct impact on water quality. Immobilization can result in a reduction of inorganic forms of nitrogen, including nitrate. However, this reduction in nitrate is generally temporary.
Mineralization results in the production of ammonium (NH+). Under favorable conditions, ammonium is further converted by microorganisms to nitrate. Buildup of nitrates in soil followed by heavy rains can move plant available nitrate below the root zone. Nitrate is a form of nitrogen that is highly mobile and easily moves with water.