Nodulation, Nitrogen Fixation and Productivity of Cowpea (Vigna unguiculata) Varieties as Influenced by Rhizobial Inoculation and Phosphorus Application on Farmers’ Fields in Minna, Southern Guinea Savanna of Nigeria.

Abstract

Cowpea is a very important crop in Nigeria but its yield on farmers’ fields have remained very low due to poor soil fertility among other reasons. This study therefore aimed at exploiting varietal differences, rhizobial inoculation and phosphorus application as means of improving nitrogen (N) fixation and productivity of cowpea. The treatments consisted of three rhizobial inoculation levels (uninoculated, inoculated with either USDA 3384 or USDA 3451 rhizobial strain), three phosphorus rates (0, 20 and 40 kg P ha-1) and three cowpea varieties (IT93K-452-1, IT99K-573-1-1, TVX3236) sown on three farmers’ fields in Minna, southern Guinea Savanna of Nigeria. Each farmer’s field served as a replicate. Data were collected on nodule number, nodule dry weight, total N in plant tissue, N-fixed, %N derived from the atmosphere (NDFA), shoot biomass yield and grain yield. The results showed that the highest and lowest values of all the parameters measured were obtained at 40 kg P ha-1 and 0 kg P ha-1 respectively. However, there was no significant difference (P>0.05) between the values obtained at 20 and 40 kg P ha-1 for most of the parameters measured. Though inoculated plants produced more nodules at lower phosphorus rate, they were not effective enough to fix higher nitrogen than the uninoculated plants. There was no significant difference between the inoculated and uninoculated plants in respect of nodule dry weight, total N in plant tissue and grain yield. Variety IT93K-452-1 recorded the highest tissue N-content and N-fixed; however, the highest grain yield was obtained in IT99K-573-1-1. It could be concluded from the results that the exotic rhizobial strains used in this study were not better than the indigenous rhizobial strains in fixing higher nitrogen and improving the productivity of the crop. Furthermore, application of 20 kg P ha-1was sufficient for optimum performance of cowpea in the study area and variety IT99K-573-1-1 can be recommended for optimum grain yield