Introduction

The thin layer of soil covering most of the earth's land surface supports a rich and wide diversity of terrestrial life. Formed slowly by physical, chemical, and biological weathering of parent materials, soil particles are moved and sorted by wind and water until, in most natural ecosystems, the counteracting processes of soil formation and loss reach equilibrium (Greenland, 1977a). The equilibrium established, and hence the soil type, is critically influenced by the nature of the climax vegetation which holds the soil in place and plays a central role in the processes of nutrient and organic matter cycling. The ultimate inherent fertility of the soil depends on the factors of soil formation – climate, parent material, and vegetation (Jenny, 1941).

Concern about the state of the world's soil and other resources has grown over the past two decades, culminating in the Brundtland Commission Report – Our Common Future (WCED, 1987a). Major problems – such as the greenhouse effect, caused by increasing levels of atmospheric carbon dioxide, effects of industrial atmospheric pollutants on forest growth, the extinction of irreplaceable animal and plant germplasm, and the impact of deforestation and land degradation – present mankind with hard choices between economic growth, poverty alleviation, and conserving the resource base for future generations. The need for economic development which is sustainable has never been clearer. For most developing countries this means meeting the food needs of a growing population by expanding food production without irreparably damaging the soil which is a key resource for agriculture.