Assessment of the potential to improve N fixation by cowpea(Vigna unguiculata (L.) Walp.) in Ghanaian soils

The potential to improve N fixation by cowpea in Ghanaian soils was examined through: (1) assessment of the natural nodulation of 45 cowpea cultivars in 20 soils sampled from 5 ecological zones; (2) determination of the numbers of cowpea bradyrhizobial isolates in the soils; and (3) determination of the response of cowpea to N fertilization. The ability of 45 cowpea cultivars to nodulate naturally in the various soils showed wide cultural adaptability. Counts of indigenous bradyrhizobia showed that most soils in Ghana contained large populations capable of nodulating cowpea. These ranged from 0.6 × 10 bradyrhizobia cells g soil −1 to 3.1 × 10 4 cells g soil −1 , with 60% of the soils containing more than 10 3 cells g soil −1 . Response of cowpea to N fertilization differed according to soil type. In general all cowpea cultivars showed significant response to increasing N fertilizer applications, indicating that N fixation was not providing the plants with sufficient N for maximum growth and yield. This study suggests that inoculation of cowpea with effective indigenous strains of bradyrhizobial species has considerable potential to improve this situation.