Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 293

(wheat, canola) which are not significantly affected by soil salinity levels below 4
dS/m. The rating systems of salinity with greater detail are as follows:

i. Nonsaline (0-2 dS/m)
ii. Slightly saline (2-4 dS/m)
iii. Weakly saline (4-8 dS/m)
iv. Moderately saline (8-15 dS/m)
v. Strongly saline (>15 dS/m)

Soil salinity is typically described and characterized in terms of concentrations
of soluble salts. While a general classification scheme based upon soil EC and SAR
has been accepted, be aware that salinity problems occur on a continuum rather than
at a given threshold. Appropriate management strategies should be evaluated from
the following and other measurements:

Saline = high salt content (EC > 2 dS/m)

Sodic high sodium content (SAR > 13)

Saline-sodic

=

=

high salt, high sodium

=

Basic high pH (problems usually start at pH > 7.8)

8.4 Impact of Salinity and Sodicity

8.4.1 Impact of Salinity on Soil and Crop Production

The consequences of salinity are detrimental effects on plant growth and final yield
damage, damage to infrastructure (roads, bricks, corrosion of pipes and cables),
reduction of water quality for users, sedimentation problems and soil erosion. When
crops are too strongly affected by the amounts of salts, it disrupts the uptake of
water into roots and interferes with the uptake of competitive nutrients. Salinity
causes damage to the soil structure, including dispersion, erosion, and waterlogging.
Salinity reduces plant available water content (PAWC) and root depth for dryland
agriculture.
The salinity of the soil water affects swelling. Clay swelling and dispersion are
the two mechanisms which account for changes in hydraulic properties and soil
structure. Swelling that occurs within a fixed soil volume reduces pore radii, thereby
reducing both saturated and unsaturated hydraulic conductivity. Swelling results in
aggregate breakdown, or slaking, and clay particles movement, which in turn leads
to blockage of conducting pores. Clay swelling occurs because clay particles imbibe
water to lower the exchangeable cation concentration near the negatively charged
surfaces of the clay. Divalent calcium ions are more strongly absorbed to clay sur-
faces than monovalent sodium. Consequently, calcium clays swell les than sodium
clays.