Vitamin B₁₂ as a Growth Factor for Soil Bacteria

Abstract

STUDIES of the nutritional requirements of soil bacteria isolated on a non-selective basis¹ have shown the presence of a well-defined group of organisms dependent upon soil extract for maximum growth. These bacteria show no growth, or at best sub-maximal growth, in media containing sugars and inorganic salts, supplemented by yeast extract or combinations of amino-acids and vitamins (not including B₁₂). Though the relative incidence of this group may vary with the soil type, soil management and the season², the chief factor affecting the proportion of these organisms is believed to be the growing plant, investigations with a variety of crops^3,4 having shown a lowered relative incidence in the rhizosphere, attributable to a preferential stimulation by the plant root of bacteria with simpler nutritional needs. However, in soil beyond the zone of influence of the root, this group has been found to comprise 8–35 per cent of the bacteria in the soils examined.