Organic and inorganic solids, liquids of various kinds and gases in intimate contact with the solid phase comprise the three-phase system that is soil. Soil management includes management of all three phases but emphasis is typically on the solid and liquid phases. The liquids and gases occupy the soil pores, which are the spaces between the solids. Soil management is challenging because of the great spatial and temporal variability of the liquid and gas phases that are in constant flux. Unless disturbed by growing roots, burrowing animals, or humans, the temporal changes to the solid phase are slow to occur but spatial variability is often great. Physical degradation of soil most often reflects a change in the total volume or size distribution of the pores brought about by reorganization of the solid phase. When reorganization occurs in the subsoil, it is most often called compaction and when it occurs on the surface it is often referred to as soil sealing or crusting. Compaction, sealing and crusting are significant soil physical management problems worldwide. The objective of this chapter is to describe some of these problems, the processes of soil physical degradation, their impacts on wind and water erosion and control of the degradation.

Soil hydrology is controlled for the most part by the total volume of pores and the pore size distribution, hence a decrease in either of these has the primary effect of reducing the rate of fluid entry into the soil and the rate of fluid flow and chemicals through the soil.