Determination of the number of root nodule bacteria in the presence of other organisms

Use of the plant-dilution method for the determination of the number of root-nodule bacteria in nonsterile carrier materials has been described for both small and large-seeded legume hosts. Calculation of the most probable number of rhizobia is described using values based on the appropriate tables of Fisher and Yates, and approximate factors of error for 95 per cent fiducial limits are stated. It was found that the plant-dilution method was likely to underestimate the number of rhizobia by a factor not greater than 2, when comparisons were made with pure cultures. However for non-sterile peat the tendency was for the plate count to give the lower estimate. This latter effect was no doubt due to the presence of other organisms and therefore the plant-dilution method is to be preferred in such cases.