Soil Quality in Vineyards - Soils for Fine Wines - page 178

The Sensitivity of Grapevines to Soil Salinity Table 7.3

Reduction in
Vine Yield
(%)

Salinity
Description

EC e
(dS/m) Comments

Nonsaline
Slightly saline

2
2-4

10
10-25

Negligible effect
Vines on own roots begin to be
affected

Moderately saline 4-8 25-50 Own-rooted vines severely affected,

but vines on rootstocks such as
Ramsey (Salt Creek), Rupestris du
Lot, 99R, and 140Ru are more
tolerant

Very saline 8-16 50 Grapevines cannot be grown

successfully
Vines die Highly saline 16 0

Source: Compiled from Neja et al. (1978), May (1994), and Cass (1998)

7.2.2.4 Maintaining a Salt Balance Under Irrigation

EC e is a measure of the salt concentration at the soil's saturation percentage, that
is, the weight of water required to completely saturate 100 g of sieved, air-dry soil.
As soil in the field dries out because of evapotranspiration, the concentration of
dissolved salts in the soil water and hence the effective EC increase markedly. Al-
though the amount of water held at saturation varies two- to threefold between a
sandy soil and a clay, the magnitude of salt concentration increase on drying is
similar, amounting to a fourfold increase between saturation and the PWP . Salts
accumulate over many cycles of irrigation in irrigated vineyards and should be
counteracted by regular leaching to maintain a salt balance. The calculation of a
leaching requirement ( LR ) for salt balance, and how this affects the overall irriga-
tion requirement, is shown in box 7.3.
The LR can be met through the regular irrigation schedule. But with water-
ing strategies such as RDI or PRD that use drip irrigation, there will be little
drainage during the irrigation period. Leaching of accumulated salts should then
occur during the winter dormancy period, as a result of either rainfall or a delib-
erate blanket irrigation.

7.2.2.5 Overall Salinity Management

Good salinity management must take into account not only the soil salt balance,
but also any off-site effects of salt. Unnecessary leaching of salts into drainage wa-
ters creates salinity problems for users downstream. Proper irrigation management
must therefore focus on the dual objective of minimizing saline return flows to
streams and avoiding a build-up of soluble salts in the soil and groundwater im-
mediately below the soil.
In this context, the distinction between LR and LF , the actual fraction of the
applied water that passes through the root zone, is important. Ideally LF should
equal LR , but because of variability in the application and infiltration of water on
a field scale, the average LF is generally greater than LR . This means that more
water is draining through the root zone than is necessary for salinity control in