ABSTRACT
Soil is a dynamic three-phase system. The three phases are: solid, which is represented by mineral particles, together with some organic material; liquid, consisting of a solution of various salts in water; and a gas phase, consisting of air with changing amounts of oxygen, carbon dioxide and nitrogen. The equilibrium of these phases changes continuously as, for example, rainfall fills pores or voids and excludes some of the gases. Soil properties vary greatly from place to place, in line with changes in the nature and the relative content of the three phases. The three phases interact greatly, and the nature of the interactions determines the behaviour of the soil in response to external impacts such as farming, drainage, forestry and engineering.
