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In tropical drylands, low and variable rainfall severely limits crop production. Runoff irrigation is reported to increase and stabilize crop production in arid and semiarid regions. At the same time, the runoff water may affect the nutrient status of the soil. This study was carried out in a runoff irrigation system with Sorghum bicolor (L.) in semiarid Northern Kenya. In this paper, we examine the effect of runoff water on soil chemical properties and on the mineral nutrition of Sorghum. The nutrient contents of the soil before and after irrigation were compared under runoff irrigation and rainfed agriculture. Plots with good and poor growing Sorghum were selected and analysed for their foliar nutrient contents. The net nitrification was measured with and without irrigation using resin cores. In order to test for N deficiency, a fertilization experiment was conducted with the application of 100 kg N ha(-1). Foliar N contents of sorghum are lower under runoff irrigation than under rainfed agriculture. This can be explained by the lower soil NO, content and the lower net nitrification in the irrigated plots. The total N content decreased more than 10% within one year after establishing the runoff irrigation system. N fertilization increased the thousand grain weight by 33% and the total grain yield by 350%. Leaching of nutrients and especially N is the main problem limiting crop yields in the studied irrigation system. Runoff irrigation can only be successful if the available nutrients are kept in the system or additional nutrients can be provided.