Agriculture Reference
In-Depth Information
Box 4.11 Salinity Control by Leaching
Tomaintainsaltbalanceinthesoil,theinputandoutputofsaltsmustbeequal.
Foranirrigatedvineyard,accessionofsaltsinrainfallandfromfertilizeris
approximatelycanceledoutbythesaltsremovedinfruit(andpossiblyprunings)
and precipitation of insoluble compounds, so the salt balance simplifies to
Salts from irrigation water
=
Salts in drainage below the ro
ot zone
(B4.11.1)
Because
EC
is a surrogate measure of salt concentration, and volume per unit area
isequivalenttoadepth
d
ofwater(inmillimeters),thesaltbalancecanbewrittenas
=
EC d Cd
iw iw
(B4.11.2)
dw dw
where the subscripts
iw
and
dw
refer to irrigation and drainage water,
respectively.Byrearrangingthisequation,theleachingrequirement(
LR
)canbe
equatedtoaratioof
EC
values; that is,
=
=
LR ddEC
/
/
EC
(B4.11.3)
dw
iw
iw
dw
hus
LR
isdeinedasthefractionofirrigationwater(conductivity
EC
iw
)
thatmustdrainoutoftherootzonetomaintainthe
EC
inthatzone(
EC
dw
)ator
below a critical value. Experiments indicate the critical saturation extract
EC
(
EC
e
)
for
Vitus vinifera
is1.8dS/m(see“SoilTestingforSalinity,”chapter3),whichis
approximatelyequaltoan
EC
dw
of3.6dS/m.Hence,forirrigationwaterof0.8
dS/m,wecancalculatethe
LR
as
08
36
.
.
(B4.11.4)
LR
==
022
.
For vines on a salt-tolerant rootstock, the
LR
would be smaller because the
critical
EC
e
is higher. Conversely, for the same
LR
, water of higher
EC
iw
could be
used.
If one assumes no effective winter rainfall, this calculation shows that about
one-ifthoftheirrigationwatershoulddrainbelowtherootzonetoavoidsalt
accumulation.Inthiscase,foravineyardwaterrequirementof,say,300mm
duringthegrowingseason,thetotalamountofirrigationrequiredtosatisfythe
vines and to provide an
LR
would be
300
1022
(B4.11.5)
d
iw
=
=
385
mm
−
.
The additional 85 mm of drainage would normally be provided by winter
rainfall, so only when winter rainfall is too low would additional irrigation be
requiredduringwintertoprovidethe
LR
. The value of
EC
dw
should be monitored
over several seasons to check that leaching is effective in maintaining a salt
balance.
