The North-South divide! Organic wastes, or resources for nutrient management?

Advances in our understanding of the management of organic resources are reviewed in relation to management of N demands for crop growth in smallholder agriculture in the tropics. In most tropical countries, organic residues represent an important source of nutrients for crop growth whereas they represent unavoidable but problematic wastes in many developed countries. Sources of N for cropping systems are N 2-fixation by legumes or mineral N fertilizers; recycling of organic resources can improve the overall efficiency of N use within the cropping system. Approaches to manipulating the timing of N availability in the soil in relation to the strength and timing of crop N demand are discussed. A framework is presented, focused around the capture efficiency of N by crops, which links systems of N excess to systems of N limitation in relation to N use and potential N losses.