ABSTRACT
In terms of its constituents, the soil can be viewed as a three-phase system comprising solid, liquid and gaseous constituents (Bonneau and Souchier 1982). The solid phase consists of both mineral and organic material; the mineral fraction is derived largely from the parent material and the organic fraction largely from vegetation growing in and above the soil. Individual particles and fragments often join together to form larger units known as aggregates or peds, and these play a crucial role in the development of many physical and chemical characteristics of soils. Between the solid material there are usually spaces known as pores or voids, and these are occupied by the liquid and gaseous phases. The liquid component, or soil water, derived from precipitation and ground-water sources, is able to transport material through the soil in both suspended and dissolved forms, and is often referred to as the soil solution. The gaseous component, known as the soil atmosphere or soil air, consists of a mixture of gases derived from the above-ground atmosphere and from the respiration of soil organisms. The relative proportion of constituents in a typical topsoil is shown in Figure 2.1, although the proportions can vary widely depending on the time of sample collection, soil type and environmental conditions.
