Organic material flows within a smallholder highland farming system of South West Uganda

It is now recognised that nutrient losses from the steeply sloping hillsides of the tropics and subtropics occur not only through soil erosion, but through the net transfer of annual crop residues to more profitable parts of the farming system. Studies of soil nutrient balances across Africa are showing evidence of widespread mining of the soil resource within the smallholder farming sector, as the organic matter and nutrient source is not replenished in annually cropped hillside fields. This paper presents information that is central to the understanding of the farming systems employed by smallholder farmers within the highlands of South West Uganda. A time static model of organic resource flows was developed with a smallholder farming community, using visible flow data from farm surveys and semi-structured interviews, to describe this situation. The model explores the sources, whereabouts and current management strategies of organic resources and defines their flow around the farming system. Results confirm a net transfer of 24 Mg ha(-1) yr(-1) (P < 0.01) of organic material, mainly crop yields and residues, from the annually cropped hillsides (covering an area of 0.6 ha per farm (P < 0.001)) to other parts of the farming system. The stover from the annual crop is used almost exclusively as mulch in banana (Musa sp.) plantations. As a consequence, the soils on the hillsides are gradually becoming depleted of nutrients, as farmers' place little value on improving the nutrient status of hillside fields distant from homesteads. Households, as is the case with most African subsistence farmers, would rather concentrate their limited labour and organic residue resources in maintaining the fertility/productivity of the more profitable parts of the farming system, in this instance banana plantations and annual fields close to homesteads. Consequently, in the short term the perennial banana system maintains a balanced flux of organic resources at the expense of hillside soil fertility. Unfortunately, over the longer term the current system will inevitably lead to a severe reduction in mulch availability, which will mean perennial crop yields will eventually decline, leading potentially, towards an unsustainable farming system. Fortunately, however, there are under-exploited organic resources within the existing farming system, that if fully utilised and could help sustain and even improve the yields of both annual and perennial crops. The whereabouts, management and value of these organic resources need to be highlighted to farmers so that alternative management strategies for organic residues can be developed, that are both economically appropriate to the fanner and the resources available, at farm level. (C) 2002 Elsevier Science B.V. All rights reserved.