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A method for measuring the amount of P held in soil micro-organisms (biomass P) is described and the assumptions on which it is based are discussed. Biomass P is calculated from the difference between the amount of inorganic P (P i ) extracted by 0.5 (Spm) NaHCO 3 (pH 8.5) from fresh soil fumigated with CHCl 3 and the amount extracted from unfumigated soil. Some CHCl 3 -released P i is sorbed by soil during fumigation and extraction: an approximate allowance for this is made by incorporating a known quantity of P i during extraction and correcting for recovery. Most of the P released is in inorganic form and the proportion increases with duration of fumigation. Non-microbial P is little, if at all, affected by fumigation. Microbial biomass P is calculated from CHCl 3 -released P i by dividing by 0.4, i.e. by assuming that 40% of the P in the biomass is rendered extractable as P i by CHCl 3 . Measurements of biomass P must be done in fresh soil, CHCl 3 releases much less P in air-dry soil.
Brookes et al. (Fri,) studied this question.