Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 273

8.2 Development of Soil Salinity and Sodicity

8.2.1 Causes of Salinity Development

Soil salinization is the accumulation of free salts in soil to such an extent that it
leads to degradation of soils and vegetation. The salts originate from the natural
weathering of minerals or from fossil salt deposits left from ancient sea beds. Salts
accumulate in the soil of arid climates as irrigation water or groundwater seepage
evaporates, leaving minerals behind. Irrigation water often contains salts picked up
as water moves across the landscape, or the salts may come from human-induced
sources such as municipal runoff or water treatment. As water is diverted in a basin,
salt levels increase as the water is consumed by transpiration or evaporation.
Salinity problems are caused from the accumulation of soluble salts in the root
zone. The problems include high total salts, excess exchangeable sodium, or both.
Generally, it is a natural process, and may results from the following:

• high levels of salt in the soils

• irrigation with saline water

• shallow saline groundwater

• landscape features that allow salts to become mobile (movement of water table)

• climatic trends that favor accumulation of salts

• land-use practice and rainfall pattern

• man-made activity

• urban area

• seepage salting

8.2.1.1 Salts in Soil

Salt is a natural element of soils and water. Originally salts came from the weath-
ering of rocks that contain salt. In some areas (for example in Australia), salinity is
an inherent situation (enormous amounts of salts are stored in the soils). However,
human practices have increased the salinity of top soils by bringing salt to the
surface through disrupting natural water cycles, by allowing excess recharging of
groundwater and accumulation through concentration. Sodicity is generally depen-
dent on the parent material from which a soil is formed and is found on older soils
where there has been sufficient weathering of clay minerals to cause a dominance of
sodium. The most common salts in soil include chlorides, sulfates, carbonates and
sometimes nitrates of calcium, magnesium, sodium, and potassium. The chemical
composition, common name, and equivalent weight of common salts are given in
Table 8.1 .

8.2.1.2 Salinity from Irrigation Water

Salinity problems can also occur on irrigated land, particularly when irrigation water
quality is marginal or worse. It has been estimated that slightly more than one-fourth