Agronomic and Economic Potential of Tughutu and Minjingu Phosphate Rock as Alternative Phosphorus Sources for Bean Growers

On-farm experiments were conducted in farmers' fields at 6 different sites in Western Usambara Mountains (WUM) in northern Tanzania during the 2000-2001 cropping season. The objectives were to study the effects of Tughutu (Vernonia subligera O. Hoffn) pruning in combination with Minjingu phosphate rock (MPR) or triple super phosphate (TSP) supply on the concentration of P in the tissues and seed yields of common bean, and to assess the economic returns of these different technologies to farmers. The experimental design was a randomized complete block with each of the 6 farmers' fields used as a replicate. The treatments included: control, MPR or TSP each at 26 kg P ha−1, Tughutu at 2.5 t ha−1, and Tughutu at 2.5 t ha−1 combined with MPR or TSP at 26 kg P ha−1.Addition of MPR or TSP alone significantly raised P concentration in bean shoots from 1.27 mg g−1 to 1.70 and 1.95 mg g−1 respectively. Combining MPR or TSP with Tughutu increased P concentration above the proposed deficiency level of 2 mg g−1. The relative agronomic effectiveness (RAE) of MPR ranged from 12.5% to 45.0%. Seed yields of common bean was markedly increased by 28%-104% from MPR or TSP supply alone, and 148%-219% from Tughutu application combined with 26 kg P ha−1 of MPR or TSP relative to the control. With Tughutu alone, seed yield increased by 53%.From economic analysis, the increase in seed yield with the supply of MPR or TSP combined with Tughutu translated into a significantly (P ≤ 0.001) higher marginal rate of return and dollar profit for common bean farmers in WUM in northern Tanzania. It is concluded that Tughutu and its combined application with MPR or TSP is an appropriate integrated nutrient management strategy that may increase bean yields and dollar profit to the rural poor communities in Tanzania.