Exploring niches for enhanced soyabean productivity in smallholder farms of Northern and Eastern Uganda
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Most agricultural soils in Sub-Saharan Africa are highly heterogeneous and deficient in phosphorus (P), an important nutrient limiting productivity of soyabean. Heterogeneity in soil fertility is an opportunity for guiding appropriate targeting of soyabean genotypes and P fertilizer application for increased productivity in smallholder farms. On-farm trials were established on 36 farms in each of the Northern Moist Farmlands (NMF) and South and Eastern Lake Kyoga Basin (SELKB) agro-ecological zones (AEZs) of Uganda during seasons 2011A, 2011B and 2012A). The aim of the study was to identify niches for soyabean genotypes and appropriate P levels for increased productivity. Specific objectives were to determine grain yield response, N2 fixation and agronomic P use efficiency (PUE) of soyabean genotypes on fields of good, medium and poor fertility amended with P (as TSP) at 0, 5, 10 and 20 kg P ha-1 and inoculation with Rhizobia. Grain yield, N2 fixation and PUE significantly (P<0.001) differed between the two AEZs with genotypes performing best under medium fertility fields for grain yield in SELKB. Using GGE analysis on grain yield and stability, NGDT 8.11 + Rhiz +20 kg P ha-1 had a wide adaptability on poor field types across the two AEZs. However, specific adaptability to field types per AEZ were: Maksoy 2N without inoculation or P (good fields); NGDT 8.11+Rhiz +20 kg P ha-1 on both medium and poor fertility fields in the NMF AEZ and Maksoy2N +Rhiz + 5 kg P ha-1 (good fertility fields), Maksoy 3N+20 kg P ha-1 (medium fertility fields) and NGDT 8.11 (poor fields) in SELKB. The most efficient and responsive genotypes to P application on good, medium and poor fertility fields in both agro-ecological zones were Namsoy 4M, Maksoy 2N and NGDT 8.11 respectively across the two AEZ . N2 fixation was generally higher in the NMF than SELKB. Combined application of P and inoculation (I+P) resulted in higher N2 fixation than from only P application. More genotypes responded to I+P on good than poor field types in the NMF but the converse was true in the SELKB due to differences in symbiotic efficiency of native Rhizobial populations. This study established niches for targeting different genotypes and management for different goals; yield, PUE or N2fixation and can be exploited in intensification of soyabean production in smallholder systems.