Enhancing the potential of organic and mineral fertilizers for bean production on contrasting soils
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Smallholder common bean (Phaseolus vulgaris L.) farmers on Buganda Catena of Uganda predominantly use indigenous knowledge to describe and manage their soils. Low and declining soil fertility which varies along topo-sequence, limits bean production, hence farmers have to improve fertilizer use and grain yield on contrasting soils. The objectives of this study were (i) to characterize three local soil types commonly cropped to beans and (ii) establish the suitable combinations of organic and mineral N & P fertilizers for increasing bean production on different soils. It was hypothesized that (i) the three local soils cropped to beans have similar properties and (ii) combined application of organic and mineral fertilizers to the three local soils gives equal yields. Together with farmers in Masaka district, three local soils mainly cultivated to beans (Lidugavu-black, Limyufumyufu-reddish, and Luyinjayinja-Gravelly) were identified, characterized and scientifically classified. On-farm experiments were carried out during 2015A and B seasons consisting of N & P each at 0, 7.5, and 15 kg ha-1 applied as urea and TSP, respectively, and with three levels of manure (0, 2.5 and 5.0 t ha-1) replicated three times in a factorial RCBD. High and low fertility on black and red soils, respectively, observed qualitatively by farmers was in agreement with results from scientific field and laboratory characterisation. These were differentiated by variability in pH, N, P, Ca and Mg. Highly limiting major nutrients were N and P. Soils characterized as different by farmers were classified as Phaeozem (black), Cambisol (Red) and Umbrisol (Gravelly), topographically located at the bottom, middle and crest, respectively. There were significant (P<0.05) differences in yield in response to combinations of N, P and manure on the different soils. Highest grain yield was on Umbrisol (2495 kg ha-1) at 5.0 t manure ha-1, although this was not significantly different (P>0.05) from that observed at [7.5 kg N+15 kg P +5.0 t manure]. Phaeozem at [7.5kg (N+P) ha-1 +2.5 t manure ha-1] gave yield similar to the highest on Umbrisol. Highest yield on Cambisol (2023.1 kg ha-1) was at [15kg N ha-1 +5.0 t manure ha-1] and this was not significantly (P>0.05) different from that obtained with [15kg (N+P) ha-1 +2.5 t manure ha-1]. Relative yield increase from fertilizer application reached 36% mainly on Umbrisol and Phaeozem. Yield due to combining fertilizers did not necessarily increase significantly beyond what were observed when either input was applied separately at a nutrient rate equivalent to the combination. Resulting yield increase up to 476 kg ha-1 and 291 kg ha-1 on Cambisol and Phaeozem, respectively, were under different fertilizer rates. These results show best fit combinations of organic and inorganic N & P fertilizers to be more effective than application of either of the materials separately. However, determining optimum supplementary nutrients beyond N&P for better yields require an economic assessment for improved farmer decisions.