INOCULATION OF GROUNDNUT (PEANUT) IN SUDAN1

Abstract

Field inoculation of groundnut (Arachis Hypogaea L.) with effective strains of Rhizobium is complicated because many tropical soils contain indigenous cowpea rhizobia capable of forming nodules on this host. We evaluated inoculation in an attempt to improve nitrogen fixation in a sub-Sahara soil of Sudan. Greenhouse screening of rhizobia on two Sudanese groundnut cultivars, Barberton (Spanish type) and Ashford (Virginia type), indicated significant differences among the 10 strains tested, mainly in host-plant color ratings and nodule numbers. Two strains, 8A11 (from the Nitragin Co.) and TAL 309 (from NifTAL), were selected for testing in the field. The strains were applied as single or mixed inoculants, formulated (a) in peat and placed with the seed or 5 and 10 cm below the seed, and (b) in oil and applied to the seed. The soil (Entic Pellusterts) at the test site contained 2.1 x 104 cowpea rhizobia g−1. When inoculants were added in a peat carrier at seedling depth, the only nitrogen-fixing trait significantly improved (P < 0.01), compared with the uninoculated controls, was nodule mass. The TAL 309 strain was serologically distinct from the indigenous cowpea rhizobia, and agglutination reactions indicated that approximately 40% of the main- and lateral-root nodules were occupied by this strain where applied. Unfortunately, this did not result in improved yields. With inoculation in an oil carrier, both nodule mass and pod yield of Barberton were significantly reduced (P < 0.01). The Barberton cultivar evidently was sensitive to the presence of oil. As peat inoculant was placed deeper into the soil, both the linear and quadratic terms were significant (P < 0.06) for improved top dry weights, whereas other traits were unaffected. The nitrogen control (120 kg N ha−1 as ammonium sulfate) significantly depressed nodule numbers and nodule mass and improved top dry weights and tissue nitrogen. The Ashford and Barberton cultivars were significantly different in nodule numbers, top dry weights, pod yields, and nodule mass.