Geoscience Reference
In-Depth Information
Generally, whichever the plant, the yield decreases more or less linearly
with increase in measured electrical conductivity (Fig. 13.18).
Reduction in yield
100% = maximum possible yield
From which the model (Maas and
Hoffman 1977; François and Maas 1994):
=
Yield
100% - b EC - a
(
)
where
: salinity threshold, mmhos/cm
: reduction in yield per additional
mmho/cm
a
b
a
Electrical conductivity
Fig. 13.18
Relationship between electrical conductivity and expected yield, in percentage
of the maximum possible yield in nonsaline soils.
For example, the yield of lucerne drops by 7.3 per cent per additional
mmho/cm when the conductivity exceeds 2.0 mmhos/cm. For a soil
with EC = 5.4, we get
Yield = 100 - 7.3
(
5.4 - 2.0
)
= 75%
The correlations are so good that tables have been drawn up
for predicting the yield, plant by plant, in relation to soil salinity
(Bresler
et al
. 1982).
Decrease in yield is noticeable much before toxicity symptoms appear.
In fact, sensitivity to salt depends not only on species but also on variety
or cultivar and growth stage.
Absorption of salts can take place directly through the leaves if
sprinkler irrigation is done with saline water. In this case, scorching
appears on the leaves and the tolerance threshold is no higher than 5
meq/l, sometimes lower if the water gets concentrated by evaporation.
Fruits are also affected. For avoiding this, irrigation at night is
desirable.
13.6.2
Fundamentals of Irrigation of Saline Soils
For a saline soil, the chief principle is to irrigate not when the soil water
reserves fall below a limiting threshold but when the concentration of salts
in the soil becomes too high following evapotranspiration phenomena.
The desire is to stay below the salinity threshold beyond which there is
noticeable reduction in yield of the planned crop. Therefore, a
leaching
fraction
is defined. It is intended to remove the excess salts and is
Calculation of irrigation doses
