IMPROVING GROUNDNUT AND MAIZE YIELDS BY ENHANCING BIOLOGICAL NITROGEN FIXATION OF FERRALSOLS AND ACRISOLS IN WESTERN KENYA

Abstract

Food insecurity in sub - Saharan Africa (SSA) is on the rise and this has become a global concern. One of the major contributing factors to this scenario is soil fertility depletion that culminates to low food productivity. In Kenya, nitrogen is one of the widely deficient nutrients. Biological nitrogen fixation (BNF); a symbiotic process that takes place in most leguminous crops can replenish nitrogen into the soil system. Groundnut is an important crop especially in western Kenya both for its high protein content and its economic value. Groundnut yield in this region however, is very low and there is a general concern on how to raise its production. A study was carried out in acid soils of Koyonzo and Ligala in western Kenya to determine the effectiveness of different inoculants and agricultural lime in enhancing BNF and yields of groundnuts and maize under intercropping system. Red Valencia groundnut variety was intercropped with Hybrid 513D maize variety. Two types of agricultural lime (dolomitic lime, and calcitic lime) and four rhizobia inoculants (A6w, V2w, W1w and biofix) alongside positive and negative controls were tested using the randomised complete block design (RCBD) in a split plot arrangement. There were three replications at each site. A6w, W1w and V2w were the indigenous rhizobia strains that had passed the authentication test. N treatment was the positive control where no inoculation was done but nitrogen was supplied from calcium ammonium nitrate (CAN). Maize monocrop was included in this study to act as reference crop for BNF analysis. Prior to planting, a blanket application of phosphorus and potassium were applied at the rates of 26 kg ha-1 and 50 kg ha-1, respectively. The experiment was carried out during the long and short rainy seasons of 2011. Data collected included; nodule number and weight, soil pH, phosphorus, nitrogen, calcium, magnesium, percent nitrogen derived from the atmosphere (% Ndfa), and groundnut kernel and maize grain weights. All data was subjected to analysis of variance (ANOVA) and treatment means were separated using contrast analyses. The results showed lime significantly increased the soil pH, available phosphorus and soil calcium and magnesium. Inoculation significantly increased nodule number and weight per plant. There were significant differences among indigenous rhizobia in fixing nitrogen (p<0.05). Rhizobia inoculation accounted for 58.91 % and 78.95 % increase in the amount of nitrogen fixed above the control at Koyonzo and Ligala respectively. The strain that fixed the highest amount of nitrogen was A6w at both sites under the dolomitic soil amendment. Dolomitic and calcitic lime did not differ significantly (p<0.05) in affecting the amount of nitrogen fixed. Liming and rhizobia inoculation significantly increased both groundnut and maize yields. Liming accounted for 16.71 % and 10.55 % groundnut yield increase at Koyonzo and Ligala sites, respectively while inoculation alone accounted for 90.57 and 110.67 % groundnut yield increase at Koyonzo and Ligala sites respectively. The best treatment combination was rhizobia A6w with dolomitic lime which gave groundnut yields of 2.01 and 0.98 t ha-1 at Koyonzo and Ligala respectively during the 2011 SRs. Liming significantly increased maize yields. The best inoculant A6w, gave maize yields of 3.76 and 2.78 t ha-1 at Koyonzo and Ligala, respectively. In conclusion, soil amendment with dolomitic lime and inoculating groundnuts with rhizobia strain A6w which can be commercialized resulted in increased groundnut and maize yields. This practice can therefore be adopted by farmers in western Kenya to improve the productivity of the groundnut maize intercropping systems.