ABSTRACT A significant proportion of agricultural productivity is consumed by the human population and the nutrients thus exported from the farm will be found in human excreta. Nutrients present in human excreta can therefore make a substantial contribution to the nutrient demand in agriculture. The low cost of chemical fertilizers and a pricing policy supportive of high agricultural productivity not only stimulates high rates of fertilizer usage to the extent of causing increasing pollution of, in particular, aquatic ecosystems and drinking water supplies, it also discourages the development of a farming system based on the efficient use and recycling of its wastes. Efficient recycling of human excreta in agriculture is restricted in most industrialized countries by the contamination of sewage sludge by industrial wastes, in particular heavy metals. High costs of chemical fertilizers and the absence of industrial pollutants in night soil and sewage sludge in many developing countries, create an environme...