14.5: Other Practices and Structures for Soil Conservation

Soil-building management practices are the first approach to runoff and erosion control, but structural measures may still be appropriate. For example, diversion ditches are channels or swales that are constructed across slopes to divert water across the slope to a waterway or pond (Figure 14.7). Their primary purpose is to channel water away from upslope areas and prevent the downslope accumulation and concentration of runoff water that would then generate scouring and gullies.

Figure 14.7. A hillside ditch in Central America channeling runoff water to a waterway on the side of the slope (not visible). A narrow filter strip is located on the upslope edge to remove sediment.

Grassed waterways are a simple and effective way to reduce scouring in areas where runoff water accumulates; they also help prevent surface water pollution by filtering sediments out of runoff (Figure 14.8). They require only small areas to be taken out of production and are used extensively in the Grain Belt region of the United States, where long gentle slopes are common.

Terracing soil in hilly regions is an expensive and labor-intensive practice for conserving soil structure, but it is also one that results in a more gradual slope and reduced erosion. Well constructed and maintained structures can last a long time. Most terraces have been built with significant cost-sharing from government soil conservation programs prior to the widespread adoption of no-tillage and cover cropping systems.

Contour tilling and planting is a simple practice that helps control erosion without equipment investments. It was therefore one of the first conservation practices promoted after the Dust Bowl of the 1930s. When you work along the contour, instead of up- and downslope, wheel tracks and depressions caused by the plow, harrow or planter will retain runoff water in small puddles and allow it to slowly infiltrate. However, this approach is not very effective when dealing with steeper erodible lands, does not have significant soil health benefits and does not eliminate tillage erosion.

There are a number of other practices that can help mitigate the off-site effects of soil erosion but do little to build soil health. Filter strips remove sediment and nutrients before runoff water enters ditches and streams (Figure 14.9). Sediment control basins have been constructed in many agricultural regions to allow sediment to settle before stream water is further discharged; they are often used in areas where conventional soil management systems still generate a lot of erosion (Figure 14.10). For both practices, their effectiveness varies depending on the time of year (less in winter and in the wet season) and whether soil particles readily settle out of runoff water (less settling out for clayey than sandy soils).

Figure 14.8.(Left) A grassed waterway in a Midwestern cornfield safely channels and filters runoff water.Photo by Ann Staudt, Iowa Learning Farms.

Figure 14.9.(Right) Edge-of-field filter strips control sediment losses to streams.Photo courtesy of USDA-NRCS.

Wind erosion is reduced with most of the same practices that control water erosion by keeping the soil covered and increasing aggregation: reduced tillage or no-till, cover cropping and perennial rotation crops. In addition, practices that increase the roughness of the soil surface diminish the effects of wind erosion. The rougher surface increases turbulent air movement near the land surface and reduces the wind’s shear and ability to sweep soil material into the air. Therefore, if fields are tilled and cover crops are not used, it makes sense to leave soil in a rough-tilled state when crops aren’t growing. Also, tree shelterbelts planted at regular distances perpendicular to the main wind direction act as windbreaks and help reduce evaporative demand from dry winds (Figure 14.11). They have recently received new attention as ecological corridors in agricultural landscapes that help increase biodiversity and may fit with the principles of alley cropping (Chapter 11).

Figure 14.10.Left: A sediment control basin in a Central European landscape where conventional tillage is widely used. Right: Sediment regularly fills the basin and needs to be dredged.

Finally, a few words about landslides. They are difficult to control, and unstable steep slopes are therefore best left in forest cover. This is generally the case in most developed countries, but steep slopes are sometimes farmed in poor agricultural regions of the world. A compromise solution is the use of wide-spaced trees that allow for some soil stabilization by roots but that leave enough sunlight for a pasture or crops (Figure 14.12), a form of silviculture we also discussed in Chapter 11. In some cases, horizontal drains are installed in critical zones to allow dewatering and to prevent supersaturation during prolonged rains. But these are expensive to install and are more commonly used in urban areas and along roads where landslides on steep slopes are great hazards.

Figure 14.11.(Left) A field shelterbelt reduces wind erosion and evaporative demand, and increases landscape biodiversity.

Figure 14.12.(Right) An experiment with wide-spaced poplar trees planted in a New Zealand pasture to reduce landslide risk.