Use of Heavy Metals for Quantification of Rhizobia and Suppression of Bacterial Contaminants in Carrier Based Inoculants

Abstract

Quality analysis of commercial legume inoculants involves determination of number of live rhizobia of

appropriate species per unit weight of carrier. Conventional analysis is performed by means of plate counts of

rhizobia on CRYEMA, but contaminants sometimes suppress/mask the growth of rhizobial colonies and make their

counts unrealistic. Plant infection-most probable number (MPN) assay is considered to be more reliable, but is

relatively expensive and time consuming. In the present study, a selective medium is devised, which permits growth

of selected strains of rhizobia and at the same time refrains the growth of contaminants on plates. To inhibit nonrhizobial

contaminants in carrier based inoculants, heavy metals were added in different combinations. A

combination of Zn and Co at a concentration of 50 ppm each was found to inhibit bacterial contaminants without

impeding the growth of rhizobial colonies on CRYEMA. However, these additions were not effective in refraining the

proliferation of contaminants in charcoal-soil based rhizobial inoculants at this concentration. These metals when

used in higher concentrations in charcoal-soil based inoculants, adversely affected nodulation and the dry matter

yield of soybean.