Defines a method of estimating microbial biomass of soils by measuring the total extractable organic biomass material largely from freshly killed microorganisms. Also applies to estimating microbial nitrogen and ninhydrin-reactive nitrogen in soil, but covers only measurement of extractable organic carbon. The fumigation-extraction method applies only to aerobic and anaerobic soils which contain actively decomposing substrates and soils supersaturated with potassium sulfate solution.