The role of calcium and magnesium in the nutrition of Rhizobium

Abstract

A critical examination was made of the calcium nutrition of Rhizobium, a large number of strains being selected to represent a fair cross section of Leguminosae. Growth was examined in colloidal clay suspensions, in liquid yeast-extract mannitol medium, in a synthetic liquid medium specially purified of calcium, in tubes of krasnozem soil, and on nutrient agar slopes. Effects on symbiotic behaviour were assessed by periodic testing on suitable host species. It is concluded that Rhizobium is not a calcium-sensitive organism. If it has any calcium need it must be of a secondary nature, satisfied by minute trace amounts of the element. On the other hand it is shown that Rhizobium has an essential need for magnesium. Apparent growth responses to calcium when grown on colloidal clays are actually responses to magnesium made available by cation exchange. Growth of many slow-growing types is improved by omitting calcium compounds from agar culture media and enhancing magnesium supply. The significance of these findings to the growth and maintenance of cultures, to the solution of field nodulation problems, and to the evolutionary history of the symbiosis is discussed.