|Food from Dryland Gardens - An Ecological, Nutritional, and Social Approach to Small Scale Household Food Production (CPFE, 1991)|
|Part II - Garden management|
|9. Soils in the garden|
|9.7 Preventing soil erosion|
Wind is a major cause of erosion, especially in drylands where surface vegetation is sparse. It carries fine soil particles away, bounces heavier particles of sand along the surface and into plants, and adds to the erosive power of raindrops by increasing their speed at impact.
A small reduction in wind speed results in a proportionately much larger reduction in erosion. For example, a 13% reduction in wind speed results in approximately a 50% reduction in erosion.44 Wind erosion of soil and abrasion of plants with windblown soil particles can be reduced with windbreaks. Windbreaks can improve yields by protecting a plant, a garden bed, or one or more entire gardens (section 10.8.3). They should be placed perpendicular to the prevailing direction of the strongest wind. If strong winds come from different directions at different seasons of the year, then windbreaks may be needed in more than one direction.
Windbreaks should have from 20-50% porosity, that is allow 20-50% of the wind to pass through them. If porosity is low, the windbreak will offer too much resistance and there is a good chance it will be blown over. The lower the porosity, the greater the reduction in erosion closest to the windbreak, but the shorter the distance over which this effect will be felt. Also, with lower porosity the speed and erosiveness of the wind as it passes around the ends of windbreaks will be increased, and these areas should be protected, for example, with perennial crops or stone mulch.
The effect of a windbreak is described in terms of distances equal to its height (H). Tall windbreaks of several rows of trees can produce a 50% reduction from 16H leeward (the direction the wind is blowing toward, that is, downwind from the windbreak) to 2H windward (the direction the wind is coming from, that is, upwind from the windbreak).
Figure 9.24 Protecting Soil from Raindrops
Large windbreaks can protect the whole garden, a group of gardens, or adjacent farmland. Within the garden, smaller windbreaks are appropriate to protect single beds or even plants (Figure 10.12 in section 10.8.3). They can be made from living plants, mats, or stalks of bamboo, sorghum, or other grass, or walls of stone or mud brick. In Egypt, farmers and gardeners place lines of stalks in the ground to protect newly emerged seedlings. The Hopi in southwestern North America do the same, and also use empty tin cans to protect individual plants, and pile flat slabs of sandstone around the base of fruit trees to keep the wind from blowing the soil away from the roots. Wind-breaks can also provide shade or serve as fences, protecting the garden from animals (section 13.3.3).