Integrated use of organic and inorganic soil nutrient resources for soil fertility Improvement in potato (solanum tuberosum l.) production

Low soil fertility is one of the most important constraints to potato prcxluction in the central Kenya highlands. Specifically, nitrogen arid phosphorus are the most limiting soil nutrients. The farmers tackle this problem through use of inorganic fertilisers and manures. However, the amounts of inorganic fertilisers used are sub-optimal due to the high cost and the manure available is inadequate. During shortrains of 1999 and longrains of 2000, ten on-farm researcher I farmer-designed and farmer-managed trials were conducted at Kianjuki catchment in Embu District. The objective was to use farmers' participatory research approaches and select the most appropriate organic and inorganic fertiliser combinations. Seven treatments were proposed and presented to farmers for discussion and selection of the most relevant treatments to be evaluated. Four treatments namely, inorganic fertiliser (500 kg DAP ha-I), manure applied at 5tha"\\ combination of half rate of manure and half rate of fertiliser and farmers' practice as a control were selected by farmers. Potato var. 'As ante' was planted. Soil and manure analysis showed that the pH of the soils ranged from 4.40 - 5.58, C 2.17 - 2.97, extractable soil P (Modified Olsen) 34.53-102.42 ppm and manure N 0.39 - 1.83 with a mean of 1.22%. During the shortrains of 1999 the average total potato tuber yields from the inorganic fertiliser treatment were 24.4 t ha-I, 17.3 t ha'i from plots planted with manur alone, 22.1t ha-I from plots planted with half fertiliser and half manure rates, and 23.4tha'l from plots planted by farmers using their own practice. Moisture stress markedly affected the trials during the longrains 2000. However, the manure applied at 50kg N ha'i equivalent yielded 6.1 tha'i of potato tubers, inorganic fertiliser alone 8.3 tha'I. combination of manure and fertiliser at half rates 7.2 tha'i and the farmers' practice 6.9 tha'i. Results indicate that the combined use of fertiliser and manure is a feasible option for soil fertility improvement for potato prcxluction in smallholder farms. This was also ranked as the most appropriate combination by the farmers.