The Nitrifying Bacteria in Soils from Rothamsted Classical Fields and Elsewhere

S ummary . Ammonia oxidizing autotrophic nitrifying bacteria in soil samples from 3 classical Rothamsted fields and elsewhere were counted by a modified 2‐layered silica gel plating method. Counts ranged from 0‐18000/g of dry soil, the maximum number being found in the Broadbalk farmyard manure plot. Nitrosomonas europaea was isolated in pure culture and detected only in soils treated with dung or other organic fertilizer. Nitrosocystis coccoides and Nitrosospira spp. were found in other soils. Nitrobacter spp. were present in many soils and of the 18 pure cultures isolated there seemed to be 3, or possibly 4, different colony types.