Agriculture Reference
In-Depth Information
Bare soil surfaces are the most susceptible, and so areas where the fetch of the
wind is long, that is, where there are no obstacles such as “tree belts” to act as
windbreaks.
Vineyards do not normally suffer from wind erosion, except possibly when
the vines are young. Windblown soil particles can damage young vines, which
should be protected by plastic vine guards around their trunks. In mature vine-
yards during the summer, the leafy canopies of the vine rows provide an excellent
windbreak. During the winter, when the vines are leafless, the soil surface is usu-
ally moist so that even clean-cultivated vineyards are not susceptible to wind ero-
sion. If the soil surface is left reasonably rough (because it is not a seed bed), the
resistance offered to wind movement is high, and susceptibility to erosion is re-
duced.
Control of Soil Erosion in Vineyards
Erosion control in vineyards involves a combination of managing the soil prop-
erties (hence modifying the
K
factor), modifying the slope and length factors (or
L
and
S
combined), and modifying the conservation practice
P
, in some instances
in combination with the crop factor
C
.
Erodibility K
. All the factors that improve soil structure, particularly as dis-
cussed in sections 7.3.1 and 7.3.2 are relevant to making soil intrinsically less sus-
ceptible to erosion. The nonwetting sands of Padthaway in South Australia are
susceptible to erosion because of their low infiltration capacity. This problem is
overcome by mixing subsoil clay with the sandy surface at the rate of 75-100 t/ha.
Polyacrylimide wetting agents can also be applied to the surface.
Slope Length L
. In Australian vineyards on duplex soils, where the A horizon
is shallow or there are drainage problems, the topsoil is formed into ridges along
the rows. If the row direction is across the slope, the ridges have the additional
benefit of preventing downslope runoff and minimizing erosion. But if the rows
run up and down the slope, runoff may be encouraged and the erosion risk in-
creased.
Slope S
. In vineyards, the slope factor is most commonly modified by
ter-
racing
. This involves cutting a series of steps into the hillslope to improve infil-
tration and divert surplus water across the slope rather than down the slope (fig.
7.16). Some of the more famous terraced vineyards, such as those in the Douro
Valley of Portugal, were constructed by hand and are supported by stone walls.
This process is extremely expensive, but nowadays terraces are made by bulldoz-
ers. The practical upper limit of the slope for machine-made terraces is about 20%.
Terraces are not normally constructed on slopes
7% because erosion control can
be achieved more cheaply by contour banks. In the Napa Valley of California,
erosion control measures are required for new vineyards on slopes
5%, because
of the extreme environmental sensitivity about runoff from vineyards (normally
clean-cultivated), which may carry sediment and chemical residues into streams.
Conservation Practice P
. In the Côte d'Or of Burgundy, the most favored
vineyard sites are on east-facing slopes of the limestone escarpment. The vine rows
run up and down the slope, and erosion is a problem (see fig. 7.14). This is an
example, common in viticulture, of a direct clash between preferred growth re-
quirements and the preferred soil conservation practice. However, where vine rows
7.5.4
