The response of potatoes (Solanum tuberosum L.) to application of inorganic and organic fertilizer was studied in long-term field experiments between 1987 and 1994. Responses to phosphorus and nitrogen were tested at 27 sites and to potassium at 17 sites. The sites represented various agro-ecological zones in Kenya. Phosphate (triple superphosphate, TSP) and nitrogen (calcium ammonium nitrate, CAN) fertilizer were applied 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 levels of K in the soil at rates of 0 or 50 kg K2O ha−1. Farmyard manure was applied at 0 or 5 t ha−1 with or without N and/or P2O5 fertilizer.

Over the study period average yields varied considerably between sites and agro-ecological zones. At 16 of the 27 sites, potatoes responded strongly to the application of P2O5 and at 9 sites to N application. A significant response to K2O application was found at three out of the 17 sites. Nutrient use efficiency ranged from 13 to 214 kg tubers 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 function 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 potatoes.

The critical soil P value was about 15 ppm (mod. Olsen) for optimal potato nutrition; on soils with higher P levels generally no P response was found. The respectively critical soil K value was found to be about 0.55 meq 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 potato yields with a combination of 50 kg P2O5 ha−1 and farmyard manure, although significant differences were achieved in only one out of the 19 sites.