The potentials of rhizobium inoculation of cowpeas (Vigna unguiculata (L.) Walp.) and its contribution
to arresting declining soil fertility and improving farm agro-economic performance were investigated in the
semi-arid Mbeere District of Eastern Kenya with 30 farmers in a farmer field school. The inoculation of
4
cowpeas with Rhizobium and application of triple superphosphate fertiliser, TSP, (T ) resulted in higher agroeconomic
performance and positive soil phosphorus balance than planting the same cowpeas variety with either
TSP 3 2 1
alone (T ), Rhizobium inoculation alone (T ) or without TSP and rhizobium inoculation (T ). The grain
y 4 3 2
ields attained in Rhizobium + TSP (T ), TSP (T ) and Rhizobium (T ) treatments were 54%, 26.1% and 6.8%
h 1
igher than that of the control (T ) respectively. The augmentation of Rhizobia population through inoculation
increased grain yields by 22.5% in treatments with TSP and by 6.8% in non-TSP fertilised plots, demonstrating
that Rhizobium augmentation had a positive effect above the naturally occurring Rhizobium strains. The
performance of the treatments in terms of major economic indicators (gross margins, net cash income and
return to labour) were in the order Rhizobium + TSP > TSP > Rhizobium > control further demonstrating the
positive effects of augmenting Rhizobium population, but with best results from treatments receiving a
combination of Rhizobium inoculation and phosphorus fertilisers.