Agriculture Reference
In-Depth Information
Box 7.3
Leaching Requirements for Salinity Control
A complete
salt balance
in an irrigated soil would take account of inputs from
the atmosphere, salts released by weathering, and fertilizer salts, in addition to salts
in the irrigation water. On the debit side, there would be salts removed in crop
products, those lost in drainage, and any insoluble carbonates and sulfates
precipitated in the soil. However, the effect of inputs other than through irrigation
and outputs other than through drainage tend to balance out, so the salt balance
simplifies to
Salt input from irrigation
Salt output in drainage below the root zone
(B7.3.1)
That is,
EC
iw
d
iw
EC
dw
d
dw
(B7.3.2)
where the subscripts
iw
and
dw
refer to irrigation and drainage water, respectively,
d
is the volume of water per unit area (mm), and
EC
measures the salt
concentration in the water. Rearranging equation B7.3.2 gives
EC
iw
d
dw
Leaching requirement LR
(B7.3.3)
EC
dw
d
iw
The
LR
is therefore defined as the fraction of the irrigation water
(conductivity
EC
iw
) that must pass through the soil to maintain the
EC
at the
bottom of the root zone (
EC
dw
) at a specified value. The critical value for
EC
dw
is
set by the vine's tolerance of salinity, expressed in
EC
e
values (see table 7.3) and is
calculated from the approximate expression (Rhoades and Miyamato 1990)
Critical EC
dw
EC
iw
(B7.3.4)
Thus, if the vine tolerance
EC
e
is 2.5 dS/m and
EC
iw
of the irrigation water is
1 dS/m, we have
5
EC
e
1
0.09
LR
(B7.3.5)
1
This calculation indicates that about 9% of the irrigation water should drain
below the root zone to avoid salinity problems. The total irrigation water
requirement is then calculated as
d
cw
2.5
5
d
iw
(B7.3.6)
LR
where
d
cw
is the vines' water requirement (mm) as determined by
evapotranspiration during the growing season.
1
the soil. Critical aspects of overall salinity management under vineyard irrigation
are therefore to
• keep the
LR
as low as possible (
0.05) by using water of low
EC
iw
,
• schedule the amount and timing of water application by monitoring soil
water status or evapotranspiration, so that the actual
LF
is kept as low as
possible (
LR
), and
• use salt-tolerant rootstocks where necessary.
