Effective runoff and erosion control is possible without compromising crop productivity. However, it may require considerable investment or new management. The numerous methods of controlling soil and water can be grouped into two general approaches: structural measures and agronomic practices. Creating structures for reducing erosion generally involves engineering practices, in which an initial investment is made to build terraces, diversion ditches, drop structures, etc. Agronomic practices that reduce erosion focus on changes in soil and crop management, such as reduced tillage and cover cropping, and planting vegetation in critical areas. Appropriate conservation methods may vary among fields and farms, but recently there has been a clear trend away from structural measures in favor of agronomic practices. The primary reasons for this change are as follows:
- Management measures help control erosion, while also improving soil health and crop productivity.
- Significant advances have been made in farm machinery and methodologies for alternative soil and crop management.
- Structures generally focus on containing runoff and sediment once erosion has been initiated, whereas agronomic measures try to prevent erosion from occurring in the first place by decreasing runoff potential.
- Structures are often expensive to build and maintain.
- Most structures do not reduce tillage erosion.
The use of soil-building conservation management practices is preferred for long-term sustainability of crop production, and they are also the first choice for controlling runoff and erosion. Structural measures still have a place, but that is primarily to complement agronomic measures. Erosion reduction works by either decreasing the shear forces of water and wind or keeping soil in a condition in which it can’t easily erode. Many conservation practices actually reduce erosion by using both approaches. In general, the following are good principles:
- Keep the soil covered; water and wind erosion occur almost exclusively when the soil is exposed.
- Use management practices that increase aggregation and infiltration.
- Do not loosen the soil unless it is well covered. Loose soil is more erodible than stable soil, like in no-till systems. Loosening may initially reduce runoff potential but this effect is generally short-lived, as the soil will settle. If loosening is required to reduce compaction, do it with tools that limit disturbance (e.g., zone builders or strip tillers). Soil disturbance is also the single cause of tillage erosion.
- Take a landscape-scale approach for additional control. Focus on areas with high risk, those where runoff water concentrates, and maximize the use of inexpensive biological approaches like grass seeding in waterways and filter strips.
- Focus on critical periods. For example, in temperate areas the soil is most susceptible after the winter fallow, and in semiarid regions it is most fragile after the dry period when heavy rains begin and there is little surface cover. In some regions, heavy rainfall is associated with hurricane or monsoon seasons.
Transition to tillage systems that increase surface cover and reduce disturbance is probably the single most effective and economical approach to reducing erosion. Restricted and no-till regimes succeed in many cropping systems by providing better economic returns than conventional tillage, while also providing excellent runoff and erosion control. Maintaining residues on the soil surface (figure 14.3) and eliminating the problem of soil loosening by tillage greatly reduce dispersion of surface aggregates by raindrops and runoff waters. The effects of wind on surface soil are also greatly reduced by leaving crop stubble on untilled soil and anchoring the soil with roots. These measures facilitate infiltration of precipitation where it falls, thereby reducing runoff and increasing plant water availability.
In cases where tillage is necessary, reducing its intensity and leaving some residue on the surface minimizes the loss of soil organic matter and aggregation. Leaving a rougher soil surface by eliminating secondary tillage passes and packers that crush natural soil aggregates may significantly reduce runoff and erosion losses by preventing surface sealing after intense rain (see figure 6.9). Reducing or eliminating tillage also diminishes tillage erosion and keeps soil from being moved downhill. The gradual losses of soil from upslope areas expose subsoil and may in many cases further aggravate runoff and erosion. We discuss tillage practices further in chapter 16.
Significance of Plant Residues and Competing Uses
Reduced-tillage and no-tillage practices result in less soil disturbance and leave significant quantities of crop residue on the surface. Surface residues are important because they intercept raindrops and can slow down water running over the surface. The amount of residue on the surface may be less than 5% for the moldboard plow, while continuous no-till planting may leave 90% or more of the surface covered by crop residues. Other reduced-tillage systems, such as chiseling and disking (as a primary tillage operation), typically leave more than 30% of the surface covered by crop residues. Research has shown that 100% soil cover virtually eliminates runoff and erosion on most agricultural lands. Even 30% soil cover reduces erosion by 70%.
As discussed in chapter 9, there are many competing uses for crop residues as fuel sources, as well as building materials. Unfortunately, permanent removal of large quantities of crop residues will have a detrimental effect on soil health and the soil’s ability to withstand water and wind erosion.
Cover crops result in decreased erosion and increased water infiltration in a number of ways. They add organic residues to the soil and help maintain soil aggregation and levels of organic matter. Cover crops frequently can be grown during seasons when the soil is especially susceptible to erosion, such as the winter and early spring in temperate climates, or early dry seasons in semiarid climates. Their roots help to bind soil and hold it in place. Because raindrops lose most of their energy when they hit leaves and drip to the ground, less soil crusting occurs. Cover crops are especially effective in reducing erosion if they are cut and mulched, rather than incorporated. Ideally, this is done when the cover crop has nearly matured (typically, milk stage)—that is, when it is somewhat lignified but seeds are not yet viable and C:N ratios are not so high as to cause nutrient immobilization. In recent years, new methods of cover cropping, mulching, and no-tillage crop production, often jointly referred to as conservation agriculture, have been worked out by innovative farmers in several regions of the world (figure 14.4; see also the farmer case study at the end of this chapter). In parts of temperate South America this practice has revolutionized farming with rapid and widespread adoption in recent years. It has been shown to virtually eliminate runoff and erosion and also appears to have great benefits for moisture conservation, nitrogen cycling, weed control, reduced fuel consumption, and time savings, which altogether can result in significant increases in farm profitability. See chapter 10 for more information on cover crops.
Perennial Rotation Crops
Grass and legume forage crops can help lessen erosion because they maintain a cover on most of the soil surface for the whole year. Their extensive root systems hold soil in place. When they are rotated with annual row crops, the increased soil quality will reduce erosion and runoff potential during that part of the crop cycle. Benefits are greatest when such rotations are combined with reduced and no-tillage practices for the annual crops. Perennial crops like alfalfa and grass are often rotated with row crops, and that rotation can be readily combined with the practice of strip cropping (figure 14.5). In such a system, strips of perennial sod crops and row crops are laid out across the slope, and erosion from the row crop is filtered out when the water reaches the sod strip. This conservation system is quite effective in fields with moderate erosion potential and on operations that use both row and sod crops (for example, dairy farms). Each crop may be grown for two to five years on a strip, which is then rotated into the other crop.
Permanent sod, often as pasture, is a good choice for steep soils or other soils that erode easily, although slumping and landslides may be a concern under extreme conditions.
Adding Organic Materials
Maintaining good soil organic matter levels helps keep topsoil in place. A soil with more organic matter usually has better soil aggregation and less surface crusting. These conditions ensure that more water is able to infiltrate the soil instead of running off the field, taking soil with it. When you build up organic matter, you help control erosion by making it easier for rainfall to enter the soil. Reduced tillage and the use of cover crops already help build organic matter levels, but regularly providing additional organic materials like compost and manure results in larger and more stable soil aggregates and stimulates earthworm activity.
The adoption rate for no-till practices is lower for livestock-based farms than for grain and fiber farms. Manures may need to be incorporated into the soil for best use of nitrogen, protection from runoff, and odor control. Also, the severe compaction resulting from the use of heavy manure spreaders on very moist soils may need to be relieved by tillage. Direct injection of liquid organic materials in a zone-till or no-till system is a recent approach that allows for reduced soil disturbance and minimal concerns about manure runoff and odor problems (figure 14.6).
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 from upslope areas away and prevent the downslope accumulation of runoff water that would then generate increased scouring and gullies.
Grassed waterways are field water channels that 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 midwestern U.S. grain belt region, where long gentle slopes are common.
Terracing soil in hilly regions is an expensive and labor-intensive practice, 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.
Tilling and planting along the contour is a simple practice that helps control erosion. 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. This approach is not very effective when dealing with steep erodible lands, however, and also does not reduce tillage erosion.
There are a number of other practices that do little to reduce runoff and erosion or build soil health but can decrease channel erosion and sediment losses. 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).
Wind erosion is reduced by 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 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 subject to wind erosion in a rough-tilled state when crops aren’t growing. Also, tree shelter belts 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 landscape biodiversity.
Finally, a few words about landslides. They are difficult to control, and unstable steep slopes are best left in forest cover. A compromise solution is the use of wide-spaced trees that allow for some soil stabilization by roots but leave enough sunlight for a pasture or crops (figure 14.12). In some cases, horizontal drains are installed in critical zones to allow dewatering and prevent supersaturation during prolonged rains, but these are generally expensive to install.