As the active microbial biomass decomposes soil organic matter and fresh plant residues nitrogen can be released in inorganic (mineral) forms that are plant available. Potentially mineralisable nitrogen (or Soil Nitrogen Supply) is a simple laboratory assay that reflects this release of soil nitrogen and thus provides a ranking system for the capacity of a soil to turnover soil organic matter. The higher the number the more likely that a soil will release more nitrogen. The limitation to this measurement is it is not possible to determine over what time period this nitrogen will be released meaning that some of the soil nitrogen supply is likely to be outside of plant nitrogen demand. The descriptive categories provided are based on the likely benefit that a growing plant will have from the level of soil nitrogen supply compared to the potential for excessive soil nitrogen supply to result in movement of nitrate into waterways.