ABSTRACT
Most soils consist of four components and three phases (Fig. 2.1). The four components include inorganic solids, organic solids, water, and air. Inorganic components are primary and secondary minerals derived from the parent material. Organic components are derived from plants and animals. The liquid component consists of a dilute aqueous solution of inorganic and organic compounds. The gaseous component includes soil air comprising a mixture of some major (e.g., nitrogen, oxygen) and trace gases (e.g., carbon dioxide, methane, nitrous oxide). Under optimal conditions for growth of upland plants, the solid components (inorganic and organic) constitute about 50% of the total volume, while liquid and gases comprise 25% each (Fig. 2.2a). Rice and other aquatic plants are exceptions to this generalization. The organic component for most mineral soils is about 5% or less. Immediately after rain or irrigation, the entire pore space or the voids in between the solids are completely filled with water, and the soil is saturated (Fig. 2.2b). When completely dry, the water in the pores is replaced by air or gases (Fig. 2.2c). General properties of components and phases are listed in Table 2.1. Under optimal conditions for some engineering functions, such as foundation for buildings and roads or runways, the pore space is deliberately minimized by compaction or compression. For such functions, the solid components may compose 80-90% of the total volume. There must be little if any liquid component for the foundation to be stable. Some industrial functions (e.g., dehalogenation) may require anaerobic conditions, however.
