The Nutritional Requirements of the Predominant Bacterial Flora of the Soil

Several aspects of techniques for nutritional classification of soil bacteria have been critically investigated. The methods used by recent Canadian workers are found to be unsatisfactory ; (a) because their seven standard media fail in several respects to give an unequivocal grouping ; (b) because some of the media used contain inadequate concentrations of essential nutrients ; (c) because recommended incubation times are in some cases too short. A simpler method of classification into five nutritional groups, using longer periods of incubation and more satisfactory differential media, has been used to compare soils receiving different manurial treatments. Results indicate that other factors, such as the well-known rapid fluctuation in bacterial numbers which occurs in all soils, may obscure any effects of soil treatment unless a very elaborate system of controls is adopted. It is suggested that an even simpler method of nutritional grouping into three classes may be of value : (a) organisms capable of using ammonia nitrogen : (b) organisms requiring amino-acid nitrogen, with or without vitamin suppleinent,s : (c) organisms requiring more complex nitrogenous materials. The possibility of developing a grouping technique by direct plating on differential solid media has been studied. It is considered that this is not possible; association effects on plates with mixed colonies suffice to obscure differences between a series of media of increasing nutritional complexity. The growth-promoting properties of soil-extract are believed to be due in part to complex nitrogenous substances, which can be replaced by certain concentrations of suitable peptones, and in part to such cations as calcium, magnesium and strontium. The concentration of such cations needed to allow growth of exacting strains is related to the concentration of peptone in the medium and it is suggested that they may act by antagonising in some way the inhibitive effects of certain peptone constituents.