Effect of Water Suctions on the Growth in Soil of the Ciliate Colpoda Steini , and the Bacterium Azotobacter Chroococcum

Summary Pure cultures of the bacterium Azotobacter chroococcum , and mixed cultures of A. chroococcum and the ciliate Colpoda steini , were incubated for 35 days in soil samples, which were either saturated or subjected to a constant suction (either pF 1.5, 2.0, or 2.7). Previous to inoculation, the soil samples had been sterilized by y‐irradiation (5 Mrad) and saturated with a solution of glucose and mineral salts. In the samples maintained at pF 2.7, the Colpoda population decreased after inoculation. In the other samples the ciliate populations increased after inoculation, and the duration of each growth period was related to the degree of suction applied. The longer growth periods were associated with the smaller suctions. Twenty‐eight days after inoculation, saturated soil contained the largest ciliate population. Large numbers of Azotobacter cells were present in all samples, although there was a decline in numbers with time. Azotobacter populations in the samples at pF 2.7 did not increase after inoculation, and smaller Azotobacter populations were often present in the mixed compared with the pure cultures at the same pF value.