Geoscience Reference
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matter in mg/l can then be determined in the laboratory. The latter
determination is called TDS ( Total Dissolved Solids ).
13.1.2 Exchangeable Sodium: Concept of SAR
Let us take a normal soil that is irrigated with a mediocre-quality water.
The quantity of sodium retained on the exchange complex will increase
to a value in equilibrium with the sodium content of the water. It will
be useful to know this value for, in the end, it is going to control the
agronomic properties of the soil. But it is difficult to measure it directly.
Its value is:
ESP (= Exchangeable Sodium Percentage ) = 100 x Na exch / CEC
This expression is not far from ESP ¢ = 100 x Na exch /( Ca exch + Mg exch )
inasmuch as the sum of Ca + Mg represents the bulk of exchangeable
cations in saline soils. This will not be valid in many other soils in
which the soil solution is poor in bases so that H, Al and Fe could be
present in significant proportions!
It is accepted then, without scientific demonstration to prove so, that
the quantity
[ Na soil
+ ]
_______________________
2 + ] ) , termed Sodium Adsorption Ratio = SAR and
measured in the soil solution with concentrations expressed in meq/l,
mol/l or mmol/l can be linked to ESP¢ and then to ESP by a relation
of the type (Gapon equation):
÷ _____________________
1/2 ◊ ( [ Ca soil
2 + ] + [ Mg soil
ESP = a · SAR + b
However the error in predicting ESP from SAR may sometimes be
as high as 40 per cent (Durand 1983)!
The SAR must be lower than 20 in irrigation water. On the other
hand, the standard ESP exceeds 15 per cent of the exchange capacity
in the definition in WRB of the 'natric' diagnostic horizon, presence of
which serves to identify Solonetz.
13.1.3 Relations Between the Parameters
Relationships can be established between three families of parameters
(Fig. 13.2): conductivity, salt content as determined and osmotic potential
(Servant 1978; Laraque 1991).
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