Investigations into the basis of microbial ecology in soil, illustrated with reference to growth of soil diphtheroids and Azotobacter in a model system

Soil is regarded as consisting of a number of microbial habitats or ecological niches. Each niche is made up of three factors: substrate, environment, and time. A model system was used to simulate a niche reasoned to occur in field soils and to be occupied by heterotrophic bacteria. The system comprised a particulate substrate (dried yeast) mixed in a porous environment (sand) maintained over periods of up to 1 year. Yeast and sand were placed in special culture tubes which were inoculated with known numbers of bacteria suspended in mineral salt solution. This was distributed evenly through the sand by a suction method. The fate of single and mixed populations of three species of soil bacteria was followed by plate counts and microscopical observations. The bacteria were Arthrobacter globiformis, a second soil diphtheroid, and Azotobacter chroococcum. Both diphtheroids grew and persisted well either in single or mixed culture. Populations of azotobacter grown singly initially maintained their numbers, but then declined progressively; they declined more rapidly in association with the second soil diphtheroid, and abruptly to zero with A. globiformis.