Agriculture Reference
In-Depth Information
40
30
Decreasing permeability,
unstable structure
20
Stable
structure
10
The relationship between the
critical SAR for soil structural
stability and the concentration of
Figure 7.5
0
5
10
15
20
Leaching water concentration (mmol/L)
the irrigation water (White 1997).
on its S content (
14% S as SO
4
2
is suitable) and its particle size (80%
2
mm diameter).
Another effective way to treat sodic soil problems is to grow a permanent
grass-based cover crop. This crop helps build up the soil organic matter, encour-
ages earthworms, and generally improves the soil's structure.
Waterlogging and Drainage
Problems associated with soil waterlogging are discussed in section 5.6. Transient
waterlogging may occur in duplex soils as a result of saturation of the soil above
a relatively impermeable B horizon, especially during wet periods in winter. A
more serious problem for vineyards occurs when groundwater is present over a
large area and the water table occurs near the vine root zone. A predominance of
winter rain, together with low evaporation rates, results in excess water draining
to the groundwater. In soils of low subsoil permeability, the water drains slowly
and the soil may remain saturated for some time. Even soils of higher permeabil-
ity may become waterlogged because the excess drainage causes the water table to
rise. If the water table comes within 1-2 m of the soil surface, water rises by cap-
illary action to the surface, as described in box 7.5. High water tables can also oc-
cur in irrigated areas where the application of irrigation water is not properly man-
aged (section 6.6.2).
The Carneros region at the southern end of the Napa Valley in California
has heavy clay soils that are poorly drained and tend to waterlog during the win-
ter. Many of the Red Brown Earth soils in the Griffith irrigation area of New
South Wales have poor subsoil drainage and water tables that must be controlled.
The condition of
restricted spring growth
(
RSG
), which is evidenced by poor shoot
growth and die-back in the spring, is more common on heavy textured soils in
low-lying sites with poor drainage. Outbreaks of
RSG
have occurred irregularly in
Australia since 1940 and also in California, especially after cold, dry winters when
the vines have not been irrigated after harvest.
The preferred method of controlling the water table depends mainly on the
soil's texture, as discussed subsequently.
7.2.4
