In vitro Assessment Indicates that Fertilizers Commonly Used in Legume Production Differentially Inhibit Commercial Rhizobial Strains

In-vitro rhizobial inhibition by fertilizers commonly used in legume production was assessed with commercial inoculant strains, <i>Bradyrhizobium lupini</i> WU425, <i>Mesorhizobium ciceri</i> CC1192 and <i>Rhizobium leguminosarum</i> bv. <i>viciae</i> WSM1455, and a commercial granular inoculant (WSM1455). Standard bacterial inhibition assays were used with four fertilizers: single superphosphate (SSP), granulated DAP (based on diammonium phosphate) and MAP (monoammonium phosphate), and a compound fertilizer (MAP with ZnSO₄). WSM1455 was strongly inhibited by DAP and MAP fertilizers, and moderately by SSP, with DAP being non-lethal. However, CC1192 and WU425 were not inhibited by DAP and were less sensitive to MAP fertilizers, with the effects being bacteriostatic rather than bactericidal. Apart from pH effects (particularly SSP), comparisons with analytical grade products indicated that fertilizer impurities might contribute to this inhibition. Granular inoculant mixed with MAP or DAP rapidly lost moisture and viability (75% within 1 h). The fertilizers were found to inhibit rhizobia with strain by fertilizer interactions, with differences in degree and lethality. Consequently, it is suggested that close placement of inoculated seeds near fertilizer at planting or mixing granular inoculants with fertilizers risks poor nodulation outcomes. These in-vitro findings need field validation but may be considered valuable in providing advice to legume growers.