The functioning of soils and their ability to supply nutrients, store water, release greenhouse gases, modify pollutants, resist physical degradation and produce crops within a sustainable management system is profoundly influenced by their organic matter content. This volume contains papers from an international conference held by the British Society of Soil Science in Edinburgh in September 1999. It explores the results of recent research studies examining how organic matter functions in soils, factors affecting organic matter quality and quantity and how management of organic matter can be optimized in order to achieve sustainable farming practices.