Soil microbiology and nutrient cycling

Soil organisms play a central role in the recycling of nutrients in soils, making them available to plants, transforming some nutrient elements to gaseous forms which can be lost from soil, and other transformations which predispose nutrients to loss. In this chapter we will illustrate these processes in the context of sustainable crop production. To do so requires some consideration of what is meant by 'sustainable crop production'. To be truly sustainable nutrient supply should correspond to demand, with the rate of nutrient removal from the fields by crops matched by replacement into the plant-soil system. There is no escape from the thermodynamic principles that underlie the law of conservation of mass matter. This implies that for any process occurring in a closed system, the mass of the reactants equals the mass of the products. When viewed from this absolute standpoint, no cropping system would be truly sustainable unless all the residues of the consumers were returned to the fields. The use of composts, manures and slurries on fields used for food production is widely practiced, and in some cases even the collection of human excreta (night soil; Fig. 1) and its application to fields is not unknown. With the increasing urbanization of the human population and the necessary development of waste water and sewerage systems, the return loop for, or connection between, animal (including human) and food wastes and land used for food production, is broken in many parts of the world. This effectively leads to the net transfer of vast quantities of nutrients from sites of food production, which are predominantly rural, to cities, usually to be dissipated to the atmosphere or water courses and the ocean, or locked-up (at least for the short-term) in waste disposal sites, such as land-fill dumps.