QUALITATIVE STUDIES OF SOIL MICROORGANISMS

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Abstract
On the basis of growth requirements, a classification was made of soil bacteria isolated by nonselective methods. Seven main nutritional groups were recognized, ranging from those capable of max. growth in a simple basal medium to those requiring soil extract. Bacteria having more complex nutritional needs are relatively more abundant in more fertile soils. Organisms requiring amino acids and growth factors show wide variation in specific needs, suggesting that the functions of soil organisms may be of a highly specialized nature. No clear-cut relationship was noted between nutritional requirements and morphological type, though certain trends were noted. Pleomorphic forms comprised a greater proportion of bacteria with more complex requirements. Soil extract was required for 19% of the isolates. For the great majority of these the growth-promoting properties of soil extract were dependent upon a factor or factors capable of adsorption by charcoal and recovery by elution. For certain organisms requiring soil extract, the nutrilite effect of the latter could be replaced by a filtrate of cultures of other soil bacteria capable of growth in the basal medium. For other organisms similar filtrates were ineffective, indicating different growth requirements, and suggesting that soil extract contains > 1 growth factor.