RESPONSES OF IRISH POTATOES (SOLANUM-TUBEROSUM L) TO MINERAL AND ORGANIC FERTILIZER IN VARIOUS AGROECOLOGICAL ENVIRONMENTS IN KENYA

The response of potatoes (Solanum tuberosum L.) to application of inor ganic and organic fertilizer was studied in long-term field experiment s between 1987 and 1994. Responses to phosphorus and nitrogen were tes ted at 27 sites and to potassium at 17 sites. The sites represented va rious agro-ecological zones in Kenya. Phosphate (triple superphosphate , TSP) and nitrogen (calcium ammonium nitrate, CAN) fertilizer were ap plied at 0, 25, 50 or 75 kg P2O5 ha(-1) and 0, 25, 50 or 75 kg N ha(-1 ). Potassium (muriate of potash) was tested mainly on sites with low l evels of K in the soil at rates of 0 or 50 kg K2O ha(-1). Farmyard man ure was applied at 0 or 5 t ha(-1) with or without N and/or P2O5 ferti lizer. Over the study period average yields varied considerably betwee n sites and agro-ecological zones. At 16 of the 27 sites, potatoes res ponded strongly to the application of P2O5 and at 9 sites to N applica tion. A significant response to K2O application was found at three out of the 17 sites. Nutrient use efficiency ranged from 13 to 214 kg tub ers per kg P2O5 and from 20 to 113 kg tubers per kg N applied. In the majority of the sites, both N and P response followed a Linear functio n which suggests that application rates higher than 75 kg N ha(-1) and /or 75 kg P2O5 ha(-1) could be beneficial. Economic analysis revealed that in most cases fertilizer application was highly profitable for po tatoes. The critical soil P value was about 15 ppm (mod. Olsen) for op timal potato nutrition; on soils with higher P levels generally no P r esponse was found. The respective critical soil K value was found to b e about 0.55 meg 100 g(-1) (modified Olsen). Farmyard manure at 5 t ha (-1) significantly increased yields of potatoes at 9 of the 27 sites. On soils with P levels below 12 ppm, there was a positive effect on po tato yields with a combination of 50 kg P2O5 ha(-1) and farmyard manur e, although significant differences were achieved in only one out of t he 19 sites.