QUALITATIVE STUDIES OF SOIL MICROORGANISMS

Abstract

Bacteria isolated from the rhizospheres of wheat, oats, red clover, timothy, alfalfa, and flax, as well as from uncropped soils, were grouped according to their nutritional requirements. A general rhizo-sphere effect shown by all crops at 2 stages of growth was indicated by a much higher percentage, in the rhizosphere, of bacteria with the simplest nutritional requirements than in soil without crops. The situation was reversed with regard to bacteria requiring the complex nutrients of soil extract. Significant differences between certain crops were noted for bacteria giving max. growth response in a glucose-inorganic-salts medium and in that medium supplemented with amino acids, growth factors, amino acids and growth factors, and yeast extract, respectively. In the main, these specific differences existed between red clover, alfalfa, and flax on the one hand and wheat, oats and timothy on the other. The legumes, however, differed from flax with respect to bacteria responding to amino acids and amino acids plus growth factors. Significant differences were found in the rhizosphere effect on 5 of the bacterial groups between plants at the seedling and flowering stages, but these were limited, in the case of each group, to 1 or 2 crops.