Rhizobium Needs Magnesium, not Calcium

Abstract

IN tropical countries there occur abundant examples of legumes that are normally nodulated despite the fact that they grow on acid soils, low in calcium. Observations of some particularly striking examples of this behaviour in the poor ‘Wallum’ soils of south-east Queensland led to a re-examination of the symbiotic system in the legumes¹. It was postulated that the tropical legume equipped with cowpea-type Rhizobium is adapted to these low-calcium environments because they were the conditions under which the symbiosis evolved, while the clovers and clover type Rhizobium are specialized types that have developed calcicole habits. Growth experiments with the bacteria using clays saturated to varying degrees with calcium using McCalla's technique² appeared to support the hypothesis, in that strains known to require lime, such as those from lucerne, gave growth in proportion to the degree of calcium saturation of the clay, whereas cowpea-type strains showed relative indifference to the calcium-level. A basic study of the relative calcium requirements of these types was then begun, using a liquid medium in which calcium was supplied in solution, not absorbed on a clay.