The numbers of bacterial cells in field soils, as estimated by the ratio method

Abstract

A technique is described for the enumeration of bacterial cells in soil; it consists in determining, in random microscope fields from a parallel series of stained films, the ratio between the number of bacteria and the number of indigo particles, of which a counted suspension has previously been added to a given mass of soil. The bacterial numbers calculated from such ratios are, of course, independent of the mass of soil on the film. The ratios obtained from parallel microscope fields are distributed at random, although counts of bacteria taken by themselves from the same fields are less uniform. The bacterial numbers found in Rothamsted field soils by this method range from 1000 to 4000 millions per gm. of soil. Samples taken from some of the Hoos Field plots showed a relationship between the total bacterial numbers and the av. yield of straw taken over a number of yrs. Caution is necessary in discussing results obtained from samples taken on only a single occasion, since there is evidence of rapid changes in numbers of bacteria with time. Successive samples taken from garden soil showed significant changes in total bacterial numbers during the course of a day.