Chemistry Reference
In-Depth Information
drainage, has been associated with the decline of several past civilisations,
including those of Babylon, Carthage and the Hohokam Indians.
36
Salts,
particularly of sodium, magnesium and calcium, accumulate in soils because
plant roots are selective in the ions that they allow into the plant, and these ions
are required in smaller quantities than many others. In many circumstances this
is not a problem if there is sufficient rainfall (or irrigation) to leach the
accumulated salts from the soil, but in arid regions, or in areas where irrigation
with water-containing dissolved salts is practised, salts build up.
Saline soils are those for which the electrical conductivity of a saturated
paste extract exceeds 4 dS m
21
, corresponding to an osmotic potential of about
2145 kPa or a total cation concentration of about 40 mmol l
21
.
25
This
approximates to values at which the growth of many plants is reduced by salt
accumulation. In practice, measuring electrical conductivity of saturated paste
extracts is laborious, so a soil-water (1 : 5) extract is normally used and a
conversion factor applied. In contrast to saline soils, there is no universally
applied definition of what constitutes a sodic soil, although the key factor is a
high proportion of sodium relative to other cations. Sodicity is typically
defined in relation to the exchangeable sodium percentage, which expresses the
sodium on exchange sites as a percentage of the total exchangeable cations. In
the USA, a value of 15% is typically used to define a sodic soil, but in Australia
a value of 6% is common. The difference arises because the property of
agricultural importance most frequently associated with sodicity, the disper-
sion of soil, varies considerably with soil type and is not uniquely related to
exchangeable sodium percentage.
Naturally occurring saline soils are widespread in arid and semi-arid regions
where low rainfall and no, or limited, leaching occurs to remove salts from
soils (in many ways, these are the opposite conditions from those that lead to
the formation of the acidic soils described in section 2.1). In many Middle
Eastern countries, saline soils have formed as a consequence of irrigation with
water over many hundreds of years, but in Australia dryland salinity is a more
complex phenomenon.
37
Salt has accumulated deep in soils of many parts of
Australia for prolonged periods, but this was not a problem until large-scale
land clearance and deforestation replaced deep-rooted perennial species with
shallow-rooted annual crops, allowing increased drainage of water to the
watertable. This led, in turn, to rising watertables and the upward movement
of salts through the soil profile into the rooting zone of the annual crops and
pastures, with severe consequences for productivity.
38
Irrigation-induced salinity is increasing along with the introduction of new
irrigation schemes. Most commonly, it arises because of inattention to the need
for adequate drainage systems without which the consequences are combina-
tions of rising watertables, upward movement of salts from naturally-occurring
saline soil layers, and accumulation of toxic concentrations of salts. Even when
drainage is adequate, inadequate leaching can result in the build-up of high salt
concentrations, with deleterious effects on plant growth. With pressure to use
effluents and ''grey water'' for irrigation increasing around the world, many of
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