Introduction

Salinity

The term salinity describes the presence of soluble salts in soil or aqueous solution. Whilst the ions Na+ and Cl- of sodium chloride are often dominant contributors to salinity, other ions can also provide an important addition to the solute concentration in soils. As the concentration of saline salts in soil increases, plant or tree growth is inhibited, and, at sufficiently high concentrations, death may occur (see the frontispiece to this chapter, showing a saline stream, Quairading, Western Australia). Physical characteristics of soil are also deleteriously affected by salinity: hydraulic conductivity can decline dramatically, and, on drying, soils can become very hard and unsuitable for plant growth.

Especially in those regions of the world where annual rainfall is low and evaporation rates potentially high, areas of naturally saline soil can be found. This is referred to as ‘primary salinity’, and a salt concentration by weight of only some 0.1% to 0.3% in surface soils can substantially reduce the growth of most plants and trees. It has been estimated that about 1000 million hectares, or 7% of the world's land area, is affected by salt to some extent (Dudal and Parnell, 1986).

However, what has emerged, particularly in recent times, as a major environmental issue in sustainable land management is the large-scale increase in salt-affected land caused by human activity. This is referred to as ‘secondary salinity’.