Volume 16 (2001) Issue 3 Pages 131-140
For many purposes the soil micro-organisms can be considered as a single pool of living soil organic matter (the soil microbial biomass). Current methods to measure the microbial biomass are described and their merits and demerits discussed. The concept of the microbial biomass as a living soil organic matter pool is illustrated by reference to it as a labile reservoir of potentially plant-available nutrients. An estimate of the turnover times of biomass carbon (C) of 0.94 years and of biomass phosphorus (P) of 0.39 years shows that the turnover rates of nutrients within the biomass may be quite different. An understanding of the dynamics of biomass P is important. The lack of P availability in many tropical agricultural soils has been described as `the bottle-neck of world hunger'. Even if P is supplied it may be rapidly and irreversibly fixed in these, usually, strongly P-fixing soils. By adding small rates of animal manures with the fertilizer, more biomass P is formed. During the process of biomass turnover, this P may be released slowly and taken up by the crop more efficiently. Thus, in a Kenyan P-fixing soil, crop yields were much larger when both manure and fertilizer P were given than when either were applied singly.