The Effect of Bentonite on the Survival of Azotobacter chroococcum in Sandy Soil in a Long-Term Plot Experiment

In a 38-year microplot experiment, very poor sandy soil deprived of the humus layer was amended with waste bentonite (BNT) in four doses of 0, 30, 60, and 120 t ha(-1) in order to improve the properties of the soil. During the first 30 years, the soils were fertilized with organic and mineral fertilizers and planted with various crops, while during the next eight years they were exposed to bare fallowing without fertilization. At the beginning of the experiment, the soils were inoculated with Azotobacter chroococcum. During the next 12 consecutive years we observed a gradual decrease of colony forming units (CFU) of these bacteria. The decrease of the CFU numbers of A. chroococcum was the fastest in the control soil (after 10 years A. chroococcum was not found in this soil). BNT significantly slowed down this decrease (1.7-3.3-times), and the effect was dose-dependent. The CFU numbers were strongly positively correlated with soil pH. After 7/8-year fallowing, when pH of the soils drastically decreased (especially in the 5-30 cm layer), A. chroococcum was found only in a 30-55 cm layer in the soil with 120 t ha(-1) BNT, where soil pH(H2O) was above 6. In a four-year field experiment with another sandy soil, BNT addition increased the CFU number of native Azotobacter spp. (30, 80, and 900-times for 30, 60, and 120 t ha(-1) of BNT, respectively).