122 / Basics of Environmental Science
gullies, but much more productively. During dry weather the gully is widened, soil added, and grass
seed sown beneath a mulch held in place by netting. When the grass is established the waterway will
continue to carry water, but will trap soil, and the grass itself can be grazed or cut for hay or silage.
Netting can also be laid over a bare surface after seed has been sown to achieve temporary erosion
control. This holds the soil in place until plants have emerged to bind it more permanently. The
technique is sometimes used on roadside verges and road-centre reservations, and the netting can be
made from material that slowly decomposes and is incorporated into the soil.
Techniques for preventing wind erosion are designed to minimize particle detachment by reducing
the speed and thus the energy of the wind. Maintaining a vegetative cover achieves this by creating
a relatively calm ground-level microclimate. Strips of short stubble about 3 m wide will catch and
hold almost all the soil lifted by the wind (FOTH AND TURK, 1972, p. 371).
Imagine the direction of the prevailing wind as a slope, as though it were blowing downhill, and a
version of contour ploughing is also very effective. Where rows of crops are planted at right angles
to the prevailing wind, each row shelters those downwind. Crops that are not grown in rows, such as
cereals, can be protected by strips of row crops planted at intervals across the wind. As well as
reducing erosion, this also helps prevent the soil from drying.
On a larger scale, trees and shrubs are often used as shelter belts or windbreaks. As Figure 3.16
shows, the wind is deflected over the top of the windbreak and its speed is reduced for a considerable
distance downwind. The advantage of this is obvious, the disadvantage less so. Reducing wind speed
also reduces the drying and chilling of the soil surface and can produce microclimatic conditions that
vary markedly with distance from the windbreak. The resulting uneven growth and ripening of the
crop can cause serious harvesting difficulties.
Figure 3.15 Two types of terracing for reducing runoff