For the foreseeable future a majority of the population, and almost all the mal- and under-nourished, will continue to be found in the tropics and subtropics. Food security in these parts of the world will have to be met largely from local resources. The productivity of the land is to a large extent determined by the fertility of the soil, which in turn is mostly determined by its organic matter content and stored nutrients. Soil organic matter is readily lost when organic matter inputs are reduced upon cultivation and more so upon intensification. The concomitant loss of topsoil and possible exposure of subsoil acidity may cause further soil degradation. Plant nutrients to replenish what is yearly taken from the soil to meet the demands for food and fibre amount to 230 million tonnes (Mt). Current fertilizer consumption stands at about 130 Mt of N, P2O5, and K2O, supplemented by an estimated 90 Mt of N from biological nitrogen fixation worldwide. Although 80 % of the population lives in the developing world, only half the world's fertilizer is consumed there. Yet, as much as 50 % of the increase in agricultural productivity in the developing world is due to the adoption of fertilizers. World population growth will cause a doubling in these nutrient requirements for the developing world by 2020, which, in the likely case of inadequate production, will need to be met from soil reserves. Because expansion of the cultivable land area is reaching its limits, the reliance on nutrient inputs and their efficient use is bound to grow. With current urban expansion, nutrients in harvested products are increasingly lost from the rural environment as a whole. Estimates of soil nutrient depletion rates for sub-Saharan Africa (SSA) are alarmingly high. The situation may be more favourable in Latin America and Asia where fertilizer inputs are tenfold those of SSA. Closing the nutrient cycle at a community level in rural areas may be tedious; on an inter-regional level it is associated with considerable costs of collection, detoxification and transportation to the farms. Yet, at the rate at which some of the non-renewable resources such as phosphorus and potassium are being exploited, recycling of these nutrients will soon be required.
