Soil microbiological research has been directed for the most part towards a study of processes in which micro-organisms are known to participate rather than towards an objective study of soil micro-organisms themselves. While organisms concerned with known processes have been given much study, relatively little attention has been paid to groups of bacteria whose functions are as yet unknown or but little understood, but which are believed to comprise a very large proportion of the micro-population of arable soils. A review is made of investigations based on the biological, as contrasted with the biochemical (or functional) approach to soil microbiology. Qualitative studies of the general soil microflora are regarded as essential to a better understanding of microbiological activity in soil and its relation to practical problems of crop growth, soil borne plant diseases, and general soil fertility.