Nitrogen Availability Indexes
The practical value of a method providing an index of the availability of soil-nitrogen (N) has long been appreciated, and many biological and chemical methods have been proposed. The microbiological assay methods are open to a variety of criticisms. The methods involving estimation of the amount of mineral-N formed during incubation have been extensively employed. The methods involving estimation of CO2 have been described as indirect incubation procedures. It is generally accepted that the most satisfactory methods currently available for assessment of the potential ability of soils to provide nitrogen for crop growth are those involving estimation of the mineral-N formed when soil is incubated under conditions which promote mineralization of soil-nitrogen. These methods have a rational basis, because the agents responsible for release of mineral-N during incubation of soils are those which make organic soil-N available for crop growth during the growing season.