ERRATIC NODULATION AND NITROGEN-FIXATION IN FIELD-GROWN PIGEONPEA [CAJANUS-CAJAN (L) MILLSP]

Following numerous reports of nodulation failures in pigeonpea [Cajanus cajan (L.) Millsp.] crops in New South Wales, a series of experiments was conducted in glasshouses and at 6 locations in the field. When inoculated seed was grown in moist vermiculite or in sand beds in the glasshouse, pigeonpea nodulated, and fixed N2, normally; but at 3 sites in the field, we could detect neither nodulation nor N2 fixation, despite adequate inoculation or a population of suitable rhizobia in the soil. At another site there was only sporadic occurrence of effective nodules. Nitrogen was fixed at 2 of the 3 field sites on acid soils, but at 1 site it appeared that nodulation was due to a naturally occurring population of soil rhizobia and not to the inoculant. When comparisons were made, pigeonpea was invariably inferior to symbiotically related legumes, cowpea and adzuki bean, in nodulation and N2 fixation. This inferiority was associated with substantially poorer rhizobial colonisation of pigeonpea rhizospheres. The experimental findings confirmed the anecdotal evidence that pigeonpea is an erratically nodulating grain legume on neutral and alkaline soils.