Nutrient dynamics in a climbing bean and sorghum crop rotation in the Central Africa highlands

Bean (Phaseolus vulgaris L.) is a major crop in Eastern and Southern Africa, but yields are low. Climbing bean is much more productive than bush bean and is gaining importance with small scale farmers. Our objectives were to: compare nutrient balances with climbing and bush bean in a grain sorghum (Sorghum bicolor L.) based crop rotation system; and to determine the effect of management alternatives on the productivity of the subsequent crop. Different yearly crop rotations alternating sorghum with another crop were compared over six seasons in the highland area of southwestern Uganda at a steeply sloping, terrace site with clayey, acidic soil. The rotation treatments were climbers, bush bean, non-nodulating bush bean, and wheat (Triticum aestivum L.) with the whole plant harvested, and climbers with only pods harvested. Yield of climbing bean exceeded bush bean yield by 120% over three seasons. Sorghum yielded most grain in the rotation with climbing bean where only pods were harvested, while yield did not differ for the other treatments. Nutrient balances were more negative for the climbing bean rotations than for bush bean rotation. Nitrogen derived from the atmosphere was estimated to be 40% of plant N for both climbing and bush bean using the difference method, but 57% using the15N abundance method. Of the total N removed in crop harvest over the six season period, a greater proportion was estimated to be derived from the atmosphere for the climbing bean rotations than for bush beans.