Sustainable Agriculture Research and Education - Publications - Building Soils for Better Crops- Chapter 13

So long! It's been good to know you.
This dusty old dust is a gettin' my home.
And I've got to be drifting along.
Woody Guthrie, 1940

The dust storms that hit the center of the U.S. during the 1930s were responsible for one of the great migrations in our history. As Woody Guthrie pointed out in his songs, soil erosion was so bad that people saw little alternative to abandoning their farms. They moved to other parts of the country in search of work. Although changed climatic conditions and agricultural practices improved the situation for a time, there was another period of accelerated wind and water erosion during the 1970s and 1980s.

Erosion by wind and water has occurred since the beginning of time. Although we should expect some soil loss to occur on almost all soils, agriculture often increases erosion. Erosion is the major hazard or limitation to the use of about one-half of all cropland in the United States! On much of this land, erosion is occurring fast enough to reduce future productivity. As we discussed earlier, erosion is also an organic matter issue, because it removes the soil layer highest in organic matter, the topsoil. The soil removed from fields also has huge negative effects off the farm, as sediment accumulates in streams, rivers, and reservoirs or blowing dust reaches towns and cities.

A small amount of erosion is acceptable, as long as new topsoil can be created as rapidly as soil is lost. The maximum amount of soil that can be lost to erosion each year, while still maintaining reasonable productivity is called the soil loss tolerance or T value. For a deep soil with a rooting depth of greater than 5 feet, the T value is 5 tons per acre each year. Although this sounds like a huge amount of soil loss, keep in mind that the weight of an acre of soil to 6 inches depth is about 2 million pounds, or 1,000 tons. So 5 tons is equivalent to about .03 inches ([5/1,000] x 6 inches = 0.03 inch), enough to fill 200 bushel baskets with soil. If soil loss continues at this rate, at the end of 33 years about 1 inch will be lost. On deep soils with good management of organic matter, the rate of topsoil creation can balance this loss. The soil loss tolerance amount is gradually reduced for soils with less rooting depth. When the rooting depth is shallower than 10 inches, the T value is about 1 ton per acre each year. This is the same as 0.006 inch per year and is equivalent to 1 inch of loss in 167 years.

When your soil loss is greater than the tolerance value, productivity suffers in the long run. Yearly losses of 10 or 15 tons or more per acre occur in many fields. Management practices are available to help reduce runoff and soil losses. For example, researchers in Washington state found that erosion on winter wheat fields was about 4 tons each year when a sod crop was included in the rotation, compared to about 15 tons when sod was not included. An Ohio experiment where 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 one- tenth (0.1) inch for the no-till planting system.

Effective erosion control is possible without compromising crop productivity. However, controlling erosion is not always easy. It may require considerable investment (as with terracing) or new management strategies (as with no-till systems). The numerous approaches to controlling erosion can be generally grouped into structural solutions and agronomic management practices. Structures for reducing erosion generally involve engineering practices, where an initial investment is made to build terraces, diversion ditches, drop structures, etc. Agronomic practices to reduce erosion focus on changes in soil and crop management. Appropriate conservation methods may vary among fields and farms. Recently, there has been a clear trend away from structural measures in favor of agronomic management practices. The primary reasons for this change are:

Management measures help control erosion, while also improving soil quality 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 management measures try to prevent erosion from occurring in the first place.
Structures are often expensive to build and maintain.
Most structures do not reduce tillage erosion.

For long-term sustainability of crop production, use of agronomic management practices is usually preferred, although structural measures can effectively complement them.

Erosion reduction works by either decreasing the shear forces of water and wind or by keeping soil in a condition that can't erode easily. Many conservation practices actually provide both. The soil organic matter management practices we discussed in the earlier chapters all reduce erosion. We'll also briefly cover other important practices for keeping erosion to a minimum.

Reduced Tillage
Transition to tillage systems that increase surface cover (chapter 15) is probably the single most effective and economic approach to reducing erosion. Restricted and no-till regimes succeed in many cropping systems by providing similar or even better economic returns than conventional tillage, while providing excellent erosion control. Maintenance of residues on the soil surface and the lack 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 slows down the loss of soil organic matter and aggregation. Less tillage promotes higher infiltration rates and reduces runoff and erosion. 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.

Reducing or eliminating tillage also diminishes tillage erosion and keeps soil from being moved downhill. The gradual losses of soil from upslope areas exposes denser subsoil and may in many cases further aggravate runoff and erosion. It is, however, possible to gradually reverse the effects of tillage erosion caused by using a moldboard plow. Because the plow moves soil forward and to the side, topsoil can be gradually moved back up the slope, if plowing is performed diagonally to the slope in the uphill direction, with the soil being thrown 45 degrees to the front/right of its original location. Of course, this approach may not give good soil inversion during plowing and does not address water and wind erosion concerns.

Significance of Soil Residues

Reduced-tillage and no-tillage practices result in less soil disturbance and leave significant quantities of residue on the surface. Surface residues are important because they intercept raindrops and can also slow down water running over the surface. The amount of residue on the surface may be close to zero for the moldboard plow while continuous no-till planting may leave 90 percent or more of the surface covered. Other reduced-tillage systems, such as chiseling and disking (as a primary tillage operation), typically leave more than 30 percent of the surface covered.

Adding Organic Materials
Maintaining good soil organic matter levels helps keep topsoil in place. A soil with more organic matter usually has better tilth and less surface crusting. This means that more water is able to infiltrate into 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.

Adding organic materials regularly to soils also results in larger and more stable soil aggregates. Larger aggregates are not eroded by wind or water as easily as smaller ones. Surface residue mulches provide both physical protection of the soil surface from raindrop impact, as well as food for large numbers of earthworms.

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 sometimes resulting from use of heavy liquid manure spreaders on very moist soils may need to be lessened by tillage. Direct injection of liquid organic materials in a zone or no-till system is generally an option but requires additional equipment investments.

Cover Crops
Cover crops decrease erosion and increase water infiltration in a number of ways. Cover crops add organic residues to the soil and help maintain tilth and organic matter levels. Cover crops frequently grow during seasons when the soil is especially susceptible to erosion, such as the early spring. 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. See chapter 10 for more information about 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. Ideally, such rotations are combined with reduced and no-tillage practices for the annual crops. Permanent sod is a very good choice for steep soils or other soils that erode easily.

Other Practices and Structures For Soil Conservation
Diversion ditches are frequently helpful for channeling water away from the field without flowing over the entire area. Grassed waterways for diversion ditches and other field water channels do not reduce erosion from all of the field, but they do keep sediments on the field and reduce scouring of the channels. Grassed waterways help prevent surface water pollution by filtering sediments out of runoff before they reach a stream or pond.

Tilling and planting along the contour is a simple practice that helps control erosion. When you work along the contour, instead of up and down slope, 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 and also does not reduce tillage erosion.

Alternating strips of row crops and perennial forages along the contour, referred to as strip cropping, is an effective way of reducing erosion losses. In this system, erosion from the row crop is not allowed to worsen over long, unprotected slopes because the sediments are filtered out of runoff when the water reaches the sod of the forage crop. This conservation system is generally effective in fields with moderate erosion potential, and on farms with use for both row and sod crops (for example, dairy farms). Research indicates that crop yields may be slightly higher when crops are grown in strips, rather than in the entire field. The increase in yield is probably due to better use of light and soil where the different strips meet.

Terracing soil in hilly regions is an expensive practice, but one which results in a more gradual slope and greatly reduced erosion. Well-constructed and maintained structures can last a long time, frequently making the high initial investment worthwhile.

Wind erosion is reduced by most of the same practices that reduce water erosion 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 resulting increase in turbulent air movement near the land surface reduces the wind's shear and its ability to sweep up soil material. Therefore, fields subjected to wind erosion may be rough-tilled. Also, tree shelterbelts planted at regular distances perpendicular to the main wind direction act as windbreaks and are very effective in reducing wind erosion losses.

Sources
American Society of Agricultural Engineers. 1985. Erosion and Soil Productivity. Proceedings of the national symposium on erosion and soil productivity, December 1011, 1984, New Orleans, Louisiana. American Society of Agricultural Engineers Publication 8-85. St. Joseph, MI.

Edwards, W.M. 1992. Soil structure: Processes and management. pp. 714. In Soil Management for Sustainability (R. Lal and F.J. Pierce, eds.). Soil and Water Conservation Society. Ankeny, IA. This is the reference for the Ohio experiment on the monitoring of runoff.

Lal, R., and F.J. Pierce (eds.). 1991. Soil Management for Sustainability. Soil and Water Conservation Society. Ankeny, IA.

Ontario Ministry of Agriculture, Food, and Rural Affairs. 1997. Soil Management. Best Management Practices Series. Available from the Ontario Federation of Agriculture, Toronto, Ontario (Canada).

Reganold, J.P., L.F. Elliott, and Y.L. Unger. 1987. Long-term effects of organic and conventional farming on soil erosion. Nature 330:370372. This is the reference for the Washington State study of erosion.

Smith, P.R. and M.A. Smith. 1998. Strip intercropping corn and alfalfa. Journal of Production Agriculture 10:345353.

Soil and Water Conservation Society. 1991. Crop Residue Management for Conservation. Proceedings of a national conference, August 89, 1991, Lexington, KY. Soil and Water Conservation Society. Ankeny, IA.

United States Department of Agriculture. 1989. The Second RCA Appraisal: Soil Water, and Related Resources on Nonfederal Land in the United States, Analysis of Conditions and Trends. Government Printing Office. Washington, DC.