Soil fertility management in west Kenya: dynamic simulation of productivity, profitability and sustainability at different resource endowment levels

A farm simulation model was designed to assess the long-term impact of existing soil management strategies, on farm productivity, profitability and sustainability. The model, which runs in time units of 1 year, links soil management practices, nutrient availability, plant and livestock productivity, and farm economics A case study is presented of the application of the model to existing, mixed farm systems in Vihiga district, in the highlands of western Kenya. Three representative farm types were developed using participatory techniques to reflect differences in resource endowments and constraints faced by farmers. The model was used to assess the sustainability of the existing systems for the three farm types as a basis for recommending improved practices for each. A summary model for calculating new sustainability indicators of soil productivity is presented. The low (LRE) and medium (MRE) resource endowment farms, which comprise about 90% of the farms in the area, have declining soil organic matter and low productivity and profitability. In contrast, the high resource endowment category of farms (HRE) have increasing soil organic matter, low soil nutrient losses and are productive and profitable. Crop nutrient yields were 17, 19 and 86 kg N ha(-1) year(-1) on LRE, MRE and HRE farms, respectively. Soil C, N and P budgets were negative in LRE and MRE but positive in HRE. Farm revenue in LRE and MRE was 2-13% of farm revenue in HRE. It comprised 7% of household income in LRE compared with 25% in MRE and 63% in HRE. It is concluded that low land and capital resources constrain the adoption of ecologically and economically sustainable soil management practices on the majority of farms in the area. Strategies are needed to (i) increase the value of farm output (ii) increase high quality nutrient inputs at low cash and labour costs to the farmer, and (iii) increase off-farm income. (C) 1998 Elsevier Science B.V. Al rights reserved.