Agriculture Reference
In-Depth Information
Assuming a
24
threefold
concentration
of the irrigation
Actual
soil ESP
values
20
water
16
12
Irrigation
water
unchanged
8
The relationship between soil
ESP
4
Figure 7.4
and the
SAR
of irrigation water
(White 1997). Reproduced with
permission of Blackwell Science
0
4
8
12
16
20
24
SAR of irrigation water (mmols
1/2
/L
1/2
)
Ltd.
threshold for Australian soils is attributed to generally low salt concentrations in
the soil solutions and the lack of weatherable minerals that can maintain the salt
concentration during leaching. An
ESP
15 and
EC
e
4 dS/m define a
saline-
sodic
soil in the United States.
Sodic soils present management problems because of the tendency for their
aggregates to slake readily when wet and for the clay to disperse (section 3.2.3).
This tendency is worsened when pressure is applied, such as from the wheels of
heavy machinery in the vine inter-rows, as discussed in section 7.1.2. Many of the
duplex soils in Australian vineyards have sodic subsoils (see fig. 3.2e). Because of
these potential sodicity problems, the ratio of Na to (Ca
Mg) in irrigation wa-
ter is important. The influence of this ratio, the
sodium adsorption ratio
(
SAR
), on
soil cation exchange is explained in box 7.4.
Results from the western United States show an approximate 1:1 relationship
between the
SAR
of irrigation waters and a soil's
ESP
, expressed in cmols charge
(
)/kg soil, as shown in figure 7.4. Thus, in the United States and many other
countries, an
SAR
15 mmols charge
1/2
/L
1/2
measured in the saturation extract
is the accepted criterion of a
sodic soil
. In Australia, when the
SAR
is measured in
a 1:5 soil to solution ratio, the predicted
ESP
is approximately twice the soil's
SAR
. However, problems associated with a high
ESP
are moderated by a high salt
concentration. A saline-sodic soil, for example, can have a stable structure and rea-
sonable permeability, provided that the high salt concentration is maintained. Fig-
ure 7.5 shows how the critical
SAR
for decreased soil permeability and unstable
structure depends on the salt concentration of the leaching water. Because the
ESP
-
SAR
-salinity response varies between soils, the relationship in figure 7.5
should be used as a guide only.
The most effective remedy for sodic soil problems is to apply gypsum
(CaSO
4
.2H
2
O), which dissolves slowly to provide Ca
2
ions. Gypsum should be
applied at vineyard establishment along the rip lines, so that it can be washed into
the subsoil. However, gypsum can also be applied to established vineyards along
the vine row, at rates of 1 t per 100 m of row. The quality of gypsum is based
