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Sodicity and Salinity
Kristie Watling
NRW, Toowoomba
Ph: 4688 1092
E-mail: [email protected]
• Salinity is the presence of
excess salt (influences the
growth of plants)
• Sodicity is the presence of
excess sodium in the soil
(influences how soil
behaves)
Impacts of Sodicity
• Sodicity affects soil by degrading its
structure, creating poor conditions for plant
growth, and making the soil susceptible to
surface crusting and erosion
• Sodium bound to clay particles displaces
more useful cations such as calcium
• The sodium weakens the bonds between
the soil particles when wetted, resulting in
clay swelling and dispersion (particles
becoming detached)
• Dispersed clay particles can move through
the soil, clogging pores
• Both swelling and dispersion decrease
permeability of soils, thus reducing
infiltration, aeration and drainage
Dis
per
sibl
Sur
fac
e
e la
yer
Rainfall
run
o ff
Entry of
runoff
Old
rootline
Impermeable layer
Dispersible clay becomes mobile
Subsurface collapses
forming ‘pipes’
Sediment fan
appears on surface
Sodicity is often indicated by crusting in the soil
surface and hardsetting of subsoils
• Soil sodicity is a natural feature of many
Australian soils, both in surface soils and
subsoils
• Soil sodicity can be measured by doing
simple field tests to see what happens to
the soil when water is added
• If the clay particles break apart, the soil is
said to be ‘dispersive’
• Soil sodicity can also be measured in a
laboratory by determining the
Exchangeable Sodium Percentage (ESP)
which measures how much sodium is in
the soil, compared to other cations like
calcium and magnesium
• Soils are considered sodic once the ESP
is above 6%
• Water sodicity can be measured by
determining the Sodium Absorption Ratio
(SAR)
• To understand how sodicity impacts on soils,
remember that soils consist of sheets of clay
arranged on top of each other
• The clay attracts and holds onto water and
nutrients because clay surfaces are charged
• Different clay types hold different amounts of
water and nutrients
• Sodicity is often confused with salinity, because
both salinity and sodicity are associated with
sodium
Exchangeable cations
- implications for physical characteristics of soil
Ca2+ is a desirable cation (promoting aggregation –
clay particles are attracted to each other)
Na+ is undesirable (causes dispersion – clay
particles are repelled by each other)
Flocculated clay (lots of calcium on the clay surfaces)
Solid soil
Soil solution (water)
Clay particle
Nutrients (e.g. cations, fertilisers etc)
-
Ca2+
Ca2+
K+
K+
Mg2+
Mg2+
Negative charge on
clay surface
Nutrients (e.g. cations) in the soil water
Salinity v. Sodicity
Non-saline
-
Non-sodic
Ca2+
Ca2+
K+
K+
Mg2+
Mg2+
Salinity v. Sodicity
Non-saline
-
Na+
Sodic
Ca2+
- Na+
-
Na+
K+
Mg2+
Mg2+
Salinity v. Sodicity
Saline
-
Ca2+
K+
Ca2+
K+
Mg2+
K+
Mg2+
K+
Na+
Ca2+
Mg2+
-
Non-sodic
Ca2+
Ca2+
Mg2+
K+
Salinity v. Sodicity
Saline
-
K+
-
Na+
-
Na+
-
Na+
Na+
Na+
K+
Mg2+
Mg2+
K+
Na+
Na+ Na+
-
Sodic
Ca2+
Ca2+
Ca2+
Mg2+
Na+
Managing Sodic/Dispersive Soils
• The best way to manage these soils is not
to disturb them
• Gypsum and/or applying organic matter is
often used in agricultural situations
• Managing water flows over and through
these soils is very important
• During construction, the dispersive soils
can be buried beneath a stable soil cover
• Maintaining vegetation on the surface is
also important
Application of gypsum can control surface sodicity,
it is more difficult to treat subsoil sodicity