Nutrient interactions of alley cropped Sorghum bicolor and Acacia saligna in a runoff irrigation system in Northern Kenya

In a runoff irrigation system in Northern Kenya, we studied the nutrient in teractions of sole cropped and alley cropped Sorghum bicolor (L.) Moench an d Acacia saligna (Labill.) H.L. Wendl. The trees were pruned once before th e cropping season and the biomass was used as fodder for animals. The nutri ent contents in leaf tissue, soil and soil solution were monitored and the uptake of applied tracers ((1)5N, Sr) was followed. The grain yield of alle y cropped sorghum was similar to or slightly higher than in monoculture and did not decrease near the tree-crop interface. Foliar N and Ca contents of the crop were higher in the agroforestry combination than in monoculture, corresponding to higher soil N and Ca contents. Soil solution and soil mine ral N dynamics indicate an increase of N under the tree row and unused soil N at the topsoil in the alley of the sole cropped trees as well as below 6 0 cm depth in the crop monoculture. The N use efficiency of the tree+crop c ombination was higher than the sole cropped trees or crops. Competition was observed for Zn and Mn of both tree and crop whereas for Ca only the tree contents decreased. P, K, Mg and Fe dynamics were not affected by alley cro pping at our site. The lower uptake of applied Sr by trees in alley croppin g compared to those of the monoculture stand suggested a lower competitiven ess of the acacia than sorghum, which did not show lower Sr contents when i ntercropped. The study showed the usefulness of combining soil and plant an alyses together with tracer techniques identifying nutrient competition, nu trient transfer processes and the complementary use of soil nutrients, as t he main features of the tree-crop combination.