14.4: Addressing Runoff and Erosion

Management practices are available to help reduce runoff and soil losses. For example, an Ohio experiment in which runoff from conventionally tilled and no-till continuous-corn fields was monitored showed that over a four-year period, runoff averaged about 7 inches of water each year for conventional tillage and less than 0.1 inch for the no-till planting system. Researchers in the state of Washington found that erosion on winter wheat fields was about 4 tons per acre each year when a sod crop was included in the rotation, compared to about 15 tons when it was not included.

Effective runoff and erosion control is possible without compromising crop productivity. However, it may require a new mindset, considerable investment or different 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 focus on changes in soil and crop management and on using vegetative solutions, such as reduced tillage, 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:

• Agronomic measures help control erosion while also improving soil health and crop productivity.
• Significant advances have been made in farm machinery and methodologies for conservation tillage systems that make use of crop rotations and cover crops.

• Structures generally focus on containing runoff and sediment once erosion has been initiated, i.e., they trap sediment that has already eroded. Conversely, agronomic measures try to prevent erosion from occurring in the first place by decreasing runoff potential.
• Structures are often more expensive to build and maintain (with significant upfront expense) than are agronomic measures, while they also tend to be less effective.

Therefore, 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 are primarily to complement agronomic measures.

Erosion reduction works by either decreasing the shear forces of water and wind or by 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. Live plants are the best way to protect the soil and to stimulate soil health.
• Use management practices that increase aggregation and infiltration.
• Do not disturb the soil unless it is well covered. Loose, exposed 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 tilling 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 greatest 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.
• Evaluate whether areas of erodible land are better taken out of production. Sometimes an economic analysis of field yield patterns (for example, using yield monitor data) shows that yields in these fields or portions of fields are not sufficient to overcome the input costs. If these areas are not profitable, more benefit is gained from government payments as part of conservation reserve or set-aside programs.

Reduced Tillage

Figure 14.3. No-till soybeans with corn residue.

In the past decade it has become clear that the best way to reduce erosion is to keep the soil covered, and the best way to maintain strong aggregates is to disturb the soil as little as possible. Transitioning to tillage systems that increase surface cover and reduce disturbance (Figure 14.3) is therefore the single most effective approach to reducing erosion. Incidentally, reduced tillage also generally provides better economic returns than does conventional tillage. 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 saves considerable labor time and wear and tear on machinery. It also significantly reduces runoff and erosion losses by preventing aggregate dispersion and surface sealing from intense rainfall (see Figure 6.11). 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 on the soil’s ability to withstand water and wind erosion, especially when there is no return of organic materials as manure.

Cover Crops

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 at reducing erosion if they are cut and mulched or rolled and crimped, 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). This practice has revolutionized farming in parts of temperate South America, 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.

Figure 14.4.Field and closeup views of soybeans grown in a black oat cover crop mulch in South America.Photos by Rolf Derpsch.

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 health helps maintain lower erosion and runoff rates during that part of the crop cycle.

Figure 14.5. Corn and alfalfa grown in rotation through alternating strips. Photo by Tim McCabe, USDA-NRCS

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 farms that use both row and sod crops (dairy farms, for example). Each crop may be grown for two to five years on a strip, which is then rotated into the other crop.

Permanent sod, as hayland or pasture, is a good choice for steep soils or other soils that erode easily, although slumping and landslides may become a concern with extremely steep slopes.

Adding Organic Materials

Figure 14.6. Equipment for manure injection with minimal soil disturbance.

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 sealing/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 or manure stimulates earthworm activity and results in larger and more stable soil aggregates.

The adoption rate for no-till practices is lower for livestock-based farms than for grain and fiber farms. Manures often 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 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).