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In many tropical drylands, low and variable rainfall severely limits crop production. In arid and semi-arid regions, runoff is reported to increase and stabilize crop production. Runoff however, may adversely affect the nutrient status of the soil.

The present study was made on a runoff irrigation system with Acacia saligna and Sorghum bicolor in the semi-arid zone of northern Kenya. This examines the effect of irrigation water on soil nutrients and studies the influence of trees, and organic and chemical fertilizers on some aspects of nutrient cycling. The nutrient contents of the soil under runoff irrigation and rainfed agriculture were compared before and after irrigation. Foliar nutrient contents of good and poor growing Sorghum were compared outside and inside the irrigation system, in order to identify the nutrients which are limiting crop production. Net nitrification was measured with and without irrigation using resin cores. Nutrient leaching was estimated by analyzing the soil solution obtained by suction cups. A fertilization experiment was conducted applying leaf manure and chemical fertilizers.

N deficiency accounted for most of the yield depression. Foliar N contents of Sorghum are lower under runoff irrigation than under rainfed agriculture. This can be explained by the lower soil NO3 content and the lower net nitrification in the irrigated plots. The total soil N content decreased more than 10% within one year after establishing the runoff irrigation system. N fertilization increased the total grain yield by 4.5 times. Under the tree canopy, N losses are lower than that in soil without vegetation cover.

Leaching of nutrients, especially N, is the main problem limiting crop yield in the studied irrigation system. Runoff irrigation can only be successful if the available nutrients are kept in the system, or if additional nutrients can be provided. Trees, with their continuous and deep root system, can recover nutrients and make them available for the annual crop when the leaves are used as green manure.