Effect of inorganic fertilizer application regimes and rhizobium inoculation on growth, yield and quality of SNAP bean (Phaseolus vulgaris L.) varieties in Central Kenya
Smallholder irrigated farms in central Kenya are characterized by low nutrient levels, particularly N. Besides, there is dependence on few commercial snap bean varieties whose availability has recently become unreliable. Hence, there is need for cost-effective fertilizer management options and alternative varieties that would improve snap bean productivity. Two on-farm trials were established under furrow irrigation between early March 2010 and February 2011 at Kimbimbi in Kirinyaga South district to evaluate the effect of inorganic fertilizer application regimes and Rhizobium inoculation on growth, yield and quality of snap beans. In the first trial, fertilizer treatments comprised: (i) application of di-ammonium phosphate (DAP) at planting and topdressing with Calcium ammonium nitrate (CAN) at 21 days after planting (dap); (ii) application of DAP at planting and topdressing with NPK (17:17:17) at 21 dap; (iii) application of DAP at planting, topdressing with CAN at 21 dap and topdressing with NPK-17: 17: 17 at 35 dap; (iv) application of CAN at planting, topdressing with CAN at 21 dap and then topdressing with CAN at 35 dap; (v) control (no fertilizer added). The varieties tested were Amy and Serengeti. The trial was laid out in RCBD with a factorial arrangement and replicated three times. In the second trial, treatments comprising inoculation with a commercial strain of Rhizobium, application of 91 kg Nlha and a control (0 kg Nlha, no inoculation) were tested against snap bean varieties Amy, Serengeti and Tana. The treatments were laid out in a randomized complete block design with a factorial arrangement and replicated three times. The data collected in both trials included crop emergence, shoot biomass, root biomass, nodule numbers, nodule dry weight, pod yield and yield components. The results of the first trial showed that nitrogen fertilizer regimes delayed seedling emergence while Serengeti took a longer time to emerge and flower than Amy. Fertilizer application depressed nodulation relative to the control. Generally, application of DAP- CAN and DAP-NPK significantly increased shoot dry weight while CAN alone had no effect. Serengeti had higher shoot dry matter than Amy. In Amy variety, only DAP-CAN and DAP-NPK had higher extra fine pods than the control, while all the fertilizer treatments increased extra fine pods in Serengeti. Serengeti had significantly higher total extra fine pods than Amy in all treated plots, but not in control plots. All treatments except DAP-NPK had significantly higher fine pods than the control in Amy while all fertilizer treatments improved fine pods in Serengeti. DAP-CAN and DAP-CAN-NPK fertilizer application regimes had the most positive profitability effect in both Amy and Serengeti. Rhizobium inoculation and application of 91 kg N/ha had no effect on nodule numbers. Nitrogen fertilizer treatment significantly reduced the nodule dry weight of Serengeti than in inoculated and control plots but had no effect on nodule dry weight in Amy and Tana. Plots treated with 91 kg N/ha had higher shoot biomass than control and Rhizobium treated plots. Serengeti had significantly higher extra fine pod yield than Amy and Tana. Generally, N treated plots had higher extra fine, fine and total pod yields than the control and Rhizobium inoculated plots. Application of 91 kg N/ha gave higher profit for Serengeti than for Amy whose profit was higher than for Tana. Generally, nitrogen treated plants performed better in vegetative growth and pod yields than inoculated and untreated plants. Snap bean growth and pod yield responses is dependent on genotype. Use of DAP-CAN fertilizer with variety Serengeti is the most profitable option for smallholder farmers in Mwea. Failure of Rhizobium inoculation to elicit positive growth and pod yield responses in snap bean calls for evaluation of different commercial Rhizobium strains with a wide range of commercial snap bean varieties and locally bred snap bean genotypes.