Monitoring nutrient¯ows and economic performance in African farming systems (NUTMON). III. Monitoring nutrient¯ows and balances in three districts in Kenya

Abstract A nutrient balance study was carried out for 26 farms in three different districts in Kenya. Balances for the major nutrients nitrogen, phosphorus and potassium were calculated for individual activities within the farms and for the entire farms, using Farm-NUTMON, a research tool that combines calculation of nutrient¯ows and balances with economic farm analysis. Four ows into the farm (chemical fertilizer, organic fertilizer and feeds, atmospheric deposition, nitrogen ®xation), six¯ows out of the farm (farm products, other organic outputs, leaching, gaseous losses, erosion and human excreta) and six internal¯ows (consumption of external feeds, household waste, crop residues, grazing, animal manure, and home consumption of farm products) were considered. Data on fertilizer use, use of organic materials, yields, home consumption and the management of residues, manure and household waste were gathered by interviewing the farmer. Assumptions for deposition, leaching and gaseous losses were made by using empirical relationships based on literature data (transfer functions). Erosion was estimated relating the overall slopes of the farms to soil loss ®gures. The mean balance of all farms was À71 kg N, 3 kg P and À9 kg K ha À1 yr À1 , with large variations between farms and little variation between districts. Emissions (leaching, gaseous losses and erosion) were estimated to be much higher than immissions (atmospheric deposition and N-®xation). In¯ows and out¯ows at ®eld level were much higher for cash crops than for food crops. Soil nutrient mining under napier grass (Pennisetum purpereum) was severe from high estimated losses in the napier-livestock-manure cycle. Based on an evaluation of the results of this case study, recommendations were made for improvement to the approach. Developing a sustainability indicator for soil fertility requires the nutrient balance to be linked to the actual soil nutrient stocks and other soil quality indicators.