Responses of Irish potatoes (<i>Solanum tuberosum</i> L.) to mineral and organic fertilizer in various agro-ecological environments in Kenya

Abstract

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 P 2 O 5 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 K 2 O ha −1 . Farmyard manure was applied at 0 or 5 t ha −1 with or without N and/or P 2 O 5 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 P 2 O 5 and at 9 sites to N application. A significant response to K 2 O application was found at three out of the 17 sites. Nutrient use efficiency ranged from 13 to 214 kg tubers per kg P 2 O 5 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 P 2 O 5 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 P 2 O 5 ha −1 and farmyard manure, although significant differences were achieved in only one out of the 19 sites.