Conservation tillage practices reduce soil loss from wind and water. These practices can also impact water quality. Not only can soil be a pollutant, it can also carry pollutants. This publication defines conservation tillage and its effects on surface and ground water quality.
In the Midwest, where soil erosion by water is a concern, conservation tillage systems leave at least 30 percent of the soil surface covered with crop residues after planting. Western regions of the United States, susceptible to wind erosion, define conservation tillage as any system that leaves on the surface at least 1,000 pounds per acre of flat, small grain residue equivalent throughout the critical wind erosion period.
The most common conservation tillage systems in Indiana use chisel plows, tandem discs, field cultivators and a variety of "one-pass" tools (Figure 1). These systems are also known as "mulch-till" systems. Following heavy residue crops such as corn, wheat or sod, these tools usually leave 30 percent or more residue cover.
Planting systems that leave the soil surface undisturbed until the time of planting include no-till, ridge-till and various forms of strip-tillage. These systems consistently leave the highest levels of crop residue and involve planting or drilling seed into a narrow seedbed prepared by coulters (no-till), ridge-scrapers (ridge-till), disk openers and other attachments (Figure 2).
Every two years the Environmental Protection Agency (EPA) compiles a report on the quality of surface and ground water. The 1992 National Water Quality Inventory listed agriculture as a leading contributor of siltation, nutrients and pesticides in surface waters.
Nutrients - Water quality concerns focus on two crop nutrients - nitrogen (N) and phosphorus (P). Nitrogen, in nitrate form, readily moves down through the soil profile and enters ground water. Nitrate occurs naturally in the soil. Fertilizer and manure applications are other sources of nitrate in soil. Consuming large quantities of nitrate in food and drinking water can cause health problems in humans, particularly infants.
Phosphorus affects surface water as a result of sedimentation. Phosphorus attaches to soil particles. During erosion, soil particles carry the phosphorus into water bodies. Aquatic plants thrive on phosphorus and can grow or spread to levels harmful to desirable fish populations. Large algae blooms result from high levels of phosphorus in surface water.
Siltation - Siltation occurs when sediment, or eroded soil, settles into ditches, streams, rivers, lakes and estuaries (Figure 3). Sediment is the largest pollutant by volume of Midwest surface waters.
Sediment increases the turbidity, or cloudiness, of water. Turbidity reduces light penetration, which impairs photosynthesis, alters oxygen levels and reduces the food supply for certain aquatic organisms. Sediment can cover spawning beds, destroying fish populations. Sedimentation (another word for siltation) causes algae blooms from the increased nutrient levels in the water. Sedimentation slowly fills in lakes and reservoirs, reducing their water holding capacity.
Pesticides - Herbicides are the type of pesticide most frequently detected in water supplies. Herbicides control weeds on over 90 percent of Indiana's cropland. Soil erosion and water runoff from treated fields are the main sources of pesticides in surface water. Ground water can become contaminated from pesticides leaching through the soil profile. Keeping pesticides out of surface and ground water reduces potential human health risks.
Conservation tillage's greatest effect on surface water quality is reduced runoff. Residues protect the soil surface from the impact of raindrops and act like a dam to slow water movement. Rainfall stays in the crop field allowing the soil to absorb it. With conservation tillage less soil and water leave a field.
Table 1 shows the effects of residue cover on surface runoff and soil loss. An increase in residue cover significantly decreases runoff and sediment from a field. Typically, 30 percent residue cover reduces soil erosion rates by 50 to 60 percent compared to the moldboard plow.
|Table 1. Effects of surface residue cover on runoff and soil loss.|
(% of rain)
|Sediment in Runoff|
(% of runoff)
Nitrogen and phosphorus in surface water result from sedimentation and runoff. Ammonium nitrogen loosely attaches to soil particles and nitrate nitrogen dissolves in surface runoff. Phosphorus binds very tightly to soil particles. Small, more chemically active clay particles attract and hold nutrients. These smaller soil particles erode more easily than larger soil particles. Preventing soil erosion reduces the loss of chemicals attached to sediment, but not quite proportionally.
As with nitrogen and phosphorus, some herbicides attach to soil particles or dissolve in surface runoff. Practices that increase water infiltration and reduce surface runoff, such as conservation tillage systems, effectively reduce herbicide runoff. Studies indicate that no-till systems reduce herbicide runoff by up to 70 percent compared to conventional systems. However, in some cases herbicide runoff was greater in no-till. If a large rainstorm occurs soon after herbicide application, the herbicide washes off the soil surface and crop residues before it can infiltrate the soil or contact the target plants.
Table 2 summarizes the effects of conservation tillage on surface water quality. Conservation tillage significantly reduces surface runoff, decreasing the amount of sediment, nutrients and pesticides in surface waters.
|Table 2. Summary of conservation tillage effects on sediment and
chemicals in surface runoff. |
(adapted from Sims et al., 1994)
|Sediment||Surface water runoff decreases with reduced tillage, in less soil leaving the field.|
|Nitrogen||Runoff decreases with reduced tillage, resulting in less nitrogen loss, although runoff concentrations may be higher.|
|Phosphorus||Phosphorus is mainly associated with sediment. Conservation tillage typically decreases sediment losses resulting in less phosphorus lost in runoff.|
|Pesticides||Loss of sediment-associated materials decreases with reduced tillage. Runoff losses of pesticides appear to decrease with use of conservation tillage, although initial runoff concentrations may be higher.|
Overtime, conservation tillage systems change the soil's physical properties, such as the development of macropores. Macropores are small, open channels in the soil created by earthworm activity, soil cracking and root growth. Tillage mixes or disturbs the upper soil profile, destroying macropores. More macropores exist in long-term no-tillage systems than fields managed with other conservation tillage systems.
Macropores allow larger quantities of water to rapidly infiltrate the soil, increasing the possibility of leaching. Several studies have monitored the movement of nitrogen (nitrate form) through no-till and moldboard plowed fields. Data indicate that in some cases, nitrogen does move deeper into the soil in no-till fields; other studies show the reverse. Even if nitrogen moves deeper into the soil, it may not enter ground water. The fate of chemicals deep in the soil profile is poorly understood.
The rapid adoption rate of no-tillage systems has raised the concern over the herbicide applications required by no-till. While some no-till farmers use more active ingredients per acre, evidence suggests that no-tillage systems do not require significantly more chemicals for weed control when compared to conventional tillage systems (e.g., moldboard plow).
With no-tillage farming, management practices play an important role in protecting surface and ground water quality. For example, USDA Agricultural Research Service at Cochocton, Ohio offers the following two scenarios:
Placed in perspective, the potential for ground water contamination equals the chance of a heavy, intense rainfall happening shortly after chemical application. Obviously, very few farmers would apply chemicals knowing severe thunderstorms were minutes away. Table 3 provides a summary of the effects of conservation tillage on ground water quality.
|Table 3. Summary of conservation tillage effects on chemical movement
through soil. |
(adapted from Sims et al., 1994)
|Nitrogen||Nitrogen leaching appears to increase with reduced tillage.|
|Phosphorus||Little leaching of phosphorus occurs regardless of tillage treatment.|
|Pesticides||Leaching of highly mobile pesticides appears to increase with use of conservation tillage. Extent of tillage effects and leaching is closely related to the amount of rain following pesticide application.|
The adoption of conservation tillage systems continues to increase rapidly in Indiana and much of the United States. Since 1990, there has been over a 25 percent increase in total no-till acreage, accounting for nearly 3.5 million acres in Indiana. Other data indicate that mulch-till systems (chisel plows, discs, field cultivators, etc.) are also increasing (CTIC, 1993).
In summary, the effects of conservation tillage on surface and ground water quality include:
1993 National Crop Residue Management Survey Report. Conservation Technology Information Center, National Association of Conservation Districts, West Lafayette, IN 47907.
Hill, P.R., C.L. Schoon, J.E. Lake, M.G. Evans, K.J. Eck, J.F. Bowen, and P.J. Hess. 1994. Indiana's T-by-2000 tillage and crop residue cover survey results. Agricultural Experiment Station Bulletin 689. Media Distribution Center, Purdue University, 301 S. 2nd Street, Lafayette, IN 47906.
Sims, G.K., D.D. Buhler, and R.F. Turco. Residue management impacts on the environment, in Managing Agricultural Residues. P.W. Unger, editor. CRC Press, Inc., Boca Raton, FL, 1994, pp 77-98.
U.S. Environmental Protection Agency. 1994. The quality of our nation's water. A summary of the 1992 National Water Quality Inventory. Office of Water, Washington, D.C.
Cooperative Extension work in Agriculture and Home Economics, state of Indiana, Purdue University, and U.S. Department of Agriculture cooperating; H.A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. The Cooperative Extension Service of Purdue University is an affirmative action/equal opportunity institution.