523some in the natural condition and others through land management. More than 2 M
ha of farmland in Australia were estimated to be affected by dryland salinity, with
more than half in WA (Australian Bureau of Statistics 2002 ). Ghassemi et al. ( 1995 )
described irrigation and dryland salinisation in eleven countries but an updated ver-
sion of such comprehensive reviews is still to be repeated. Their estimate of dryland
salinity at the time was 31.2 M ha, which is a similar number to the FAO ( 2015 ).
3 Salinity, Waterlogging and Sodicity – Impacts on Plants
The severity of salinity can be classified according to the electrical conductivity of
the soil saturation extract (ECe). Soils with ECe values of 0–2 dS m−^1 are non-
saline, 2–4 dS m−^1 are slightly saline, 4–8 dS m−^1 are moderately saline, 8–16 dS
m−^1 are highly saline, 16–32 dS m−^1 are severely saline and >32 dS m−^1 are extremely
saline (Barrett-Lennard et al. 2008 ). However, it should be recognised that the
impacts of salinity on crops is not affected by the concentration of salt in soil (of
which ECe is a measure) but rather the concentration of salt in the soil solution:
growth will therefore be strongly affected also by the soil moisture contents.
Salinity impacts the plants because electrolytes in the rooting medium have
osmotic and specific ion effects (Greenway and Munns 1980 ). The osmotic effects
occur because water movement across cellular membranes is proportional to the
concentration gradient of electrolytes and organic solutes. If concentrations are
higher on the outside than the inside of the plant, then the plant may wilt and die;
if the reverse is true, then the plant can take up water and may grow. One strategy
that plants can use to overcome the ‘osmotic shock’ caused by salinity is to increase
concentrations of solutes within cells by either the synthesis or transport of osmot-
ica inside the cells: this process is termed ‘osmotic adjustment’ and it enables
water uptake to resume. Specific ion effects occur because metabolic processes
generally require K+ (and Na+ inhibits the uptake of this ion) and because Na+ and
Cl− have toxic effects on metabolism. For plants suddenly affected by salinity,
osmotic effects are immediate whereas specific ion effects take some time to
impact (Munns 1993 ).
Table 1 Area of salt-affected and sodic soils in the world in million of hectares
Regions Total area Saline soils % Sodic soils %
Africa 1899.1 38.7 2.0 33.5 1.8
Asia and the Pacific and Australia 3107.2 195.1 6.3 248.6 8.0
Europe 2010.8 6.7 0.3 72.7 3.6
Latin America 2038.6 60.5 3.0 50.9 2.5
Near East 1801.9 91.5 5.1 14.1 0.8
North America 1923.7 4.6 0.2 14.5 0.8
Total 12781.3 397.1 3.1 434.3 3.4Source: FAO ( 2015 )
Salinity in Dryland Agricultural Systems: Challenges and Opportunities