TOWARDS A PRACTICAL APPROACH TO FERTILIZER RECOMMENDATIONS FOR FOOD CROP PRODUCTION IN SMALLHOLDER FARMS IN KENYA

A comprehensive long-term fertilizer trial programme for annual crops was established in 1985 at 70 sites in the high and medium rainfall ar eas of Kenya testing N and P, farmyard manure and other critical nutri ents. Sites were selected according to their representativeness regard ing soils and climate in agro-ecological units. The fertilizer trial p rogramme included all major food crops typical for the respective area , both monocropped and intercropped in the case of cereals. Economic o ptimum fertilizer rates were calculated taking into account input/outp ut price ratios and value-cost ratios. The majority of sites showed re sponses to N or P, only at one site response to both N and P with a si gnificant interaction was found. Price elasticity of crops such as pot atoes and cabbages was higher than that of maize or sorghum. Critical soil P levels were determined for maize at 13 ppm P for modified Olsen extract and 32 ppm P for Mehlich I extract. Variability of rainfall w as found to greatly influence viability of fertilizer recommendations. First attempts to extrapolate fertilizer recommendations from areas o f representativeness to areas with similar conditions by means of GIS and modelling approaches are discussed.