Studies of single soil organisms, while useful in specialized cases, e.g. Rhizobia and mycohrrizae, do not yield information on the functioning of the soil ecosystem. This is because most important soil processes, e.g. carbon and nitrogen mineralization, depend upon interactions between entire suites of organisms, many of which still await identification and most of which remain unculturable.

For many purposes, treating the soil microbial community as a single, undifferentiated unit, the soil microbial biomass (defined as all soil organisms <5000 μm3 volume), has much to commend it. It is analogous to studying the forest rather than an individual tree and uniquely permits an understanding of the soil–plant–microbe system as a whole, rather than studying only a small part. The present paper reviews the development of methods to measure microbial dynamics over the last century, the evolution of biomass methodologies and how they have helped the study of crucial soil processes such as nutrient and carbon cycling. Possible future directions for this research are also discussed and an explanation set forth of why the manipulation of this huge population (easily comprising 10 tonnes per hectare of living microbial cells in UK arable soils) has, so far, proved elusive.