Effect of inoculation method on rhizobium survival and plant nodulation under adverse conditions

Abstract

In a series of experiments, different inoculation methods applied to seeds of Glycine wightii were compared for their effect on survival of rhizobia (Rhizobium sp.) under various conditions: high temperature with and without high relative humidity, exposure to sunlight, and contact with superphosphate. Seeds were inoculated with peat inoculum in gum arabic (gum treatment), in gum arabic plus a coating of lime or rock phosphate, or with peat inoculum in water (slurry). When subjected to 50¦C for eight hours a day, survival of the cowpea-type Rhizobium strain CB756 was poorer at 75 per cent relative humidity than at 40 per cent. CB756 survived better than the clover strain TA1 under high temperature and high relative humidity, and neither lime nor phosphate pelleting increased survival of the two strains over the gum treatment. Poorest survival was with phosphate for TA1 and lime for CB756. When exposed to sunlight, numbers of CB756 fell most rapidly in the gum treatment but, after a longer period in a glasshouse at about 30¦C, survival was poorest in the slurry treatment. When mixed with superphosphate, lime pelleting resulted in best survival of both strains. In pot experiments, lime pelleting resulted in a higher percentage of nodulated plants and higher plant weights of Trifolium subterraneum and Glycine wightii, the advantage being greatest when low numbers of rhizobia were on the seed at sowing.