ABSTRACT
Soil is the loose surface layer of the earth's crust in which plant roots develop. It is a three-phase medium, consisting of a solid porous phase, the pores being filled with a soil solution (liquid phase) comprising water and dissolved substances, and a gaseous phase-a mixture of nitrogen, oxygen, carbon dioxide and water vapour (Fig. 1.1). The proportion of these different phases varies with the type of soil and also over time depending on climatic conditions.
