Effect of selected site factors with ten tree species not fixing nitrogen on the yield of important field crops under agroforestry conditions of cultivation. An example from Butare/Rwanda (East/Central Africa).
Details are given of investigations since 1982 of agroforestry trials in Rwanda with 10 different tree species commonly used in agroforestry programmes: Acacia elata, Acrocarpus fraxinifolius, Casuarina littoralis, Cedrela serrata, Croton megalocarpus, Entandrophragma excelsum, Grevillea robusta, Maesopsis eminii, Podocarpus falcatus, and Polyscias fulva-none of which fix N. The field crops growth were maize, sweet potato, bushbeans, soyabeans, and sorghum millet, and the investigations also covered the light requirements of these five crops without interference by the roots and crowns of the trees. Bushbeans did well to moderately well with all but three of the 10 tree species, even sometimes under heavy shade. Maize did well to moderately well under slight shade. Sorghum millet grew well under slight shade, and showed a particularly positive response with Podocarpus falcatus. Soyabeans grew prolifically on the Entandrophragma excelsum sites. Sweet potato gave average yields only at the end of the first growing season in a year when precipitation was high. The results confirm the tolerance of bushbeans to shade and their suitability for cultivation under agroforestry conditions. The other field crops require water and light to an extent which may not be generally available in agroforestry systems. Of the 10 species of trees investigated, Podocarpus falcatus gave a positive effect on the soil, and the decrease in soil fertility with Acrocarpus fraxinifolius, Croton megalocarpus and Entandrophragma excelsum was relatively slight. In order to maintain and especially to increase soil fertility, other measures are required, viz. regular additions of manure and compost, more fallow periods, mineral fertilizers, and mulching, as well as pruning. Widespread use of agroforestry systems could lead to an increase of ≥3 m3/ha in wood yield.