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Soil Preparation

• Operations and Timing
• Equipment Used
• Potential Environmental Problems (Soil Erosion)
• Best Management Practices

Prior to planting, the soil needs to be prepared, usually by some form of tillage or chemical "burn-down" to kill the weeds in the seedbed that would crowd out the crop or compete with it for water and nutrients. Tillage methods can be divided into three major categories, depending on the amount of crop residue they leave on the surface. Residue slows the flow of runoff that can displace and carry away soil particles.

• Conventional tillage - Until the last decade or so the standard tillage practice for corn was use of the moldboard plow for primary tillage followed by several secondary tillages and mechanical cultivation after the crop was up. Now about two-thirds of row crops are planted without use of the moldboard plow (Allmaras et al., 1997), and mechanical cultivation is often limited to one, or no operations.
• Reduced tillage is usually done with a chisel plow and leaves 15% to 30% residue coverage on the soil.
• Conservation tillage leaves at least 30% residue coverage on the soil. Conservation tillage methods include no-till, where no tillage is done at all and seeds are placed directly into the previous season's crop residue; strip-till, in which only the narrow strip of land needed for the crop row is tilled; ridge till; and mulch till.

Herbicides are used in all these methods to kill weeds. In no-till systems, the herbicide is applied directly on last season's crop residue. In the other methods, some soil preparation takes place before the herbicide is applied. A common myth is that more herbicide is used with conservation tillage methods, but in fact farmers rely on herbicides for weed control under all tillage systems, and the amount used is more or less independent of tillage method.

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Soil Preparation Operations and Timing

Tillage can occur anytime between harvest of the previous year's crop and spring planting. In the eastern Corn Belt, most tillage is usually done between March and May for corn, and can be as late as early June for soybeans. In some cases, tillage is done in the fall, after harvest. In southern states, planting can be considerably earlier or later because of their longer growing season. The optimum time for tillage (to prevent soil erosion) is just before planting. However, wet spring weather can often make it difficult to get equipment into the field as early as needed to optimize yield. Late planting can seriously reduce yields. For example, in the eastern corn belt, corn yields are reduced by 1 bu/acre for each day after May 1 that planting is delayed.

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Equipment Used for Soil Preparation

Tractor - a traction machine that provides mechanical, hydraulic, and/or electrical power to implements to perform a wide range of crop production and handling operations. Tractors are most often used to perform drawbar work (pulling equipment through the field) and PTO (power take-off) (power to rotate equipment components) work. Tractors can be equipped with rubber tires, rubber belts, or steel tracks. A modern farm tractor is almost always equipped with a diesel engine and tractor size is measured by the amount of power that the tractor can produce at the PTO. Tractor sizes range from those with less than 40 PTO horsepower to ones that produce more than 400 horsepower. The cost of a large modern tractor can be well over $200,000.

Plow - an implement used to perform primary tillage. A number of types of plows are in common use including the moldboard plow, the chisel plow, and the disk plow.

The moldboard plow has a large frame that is equipped with a series of "bottoms," each of which consists of a steel coulter to slice through residue followed closely by a steel share that cuts the soil and an attached moldboard that is used to raise and turn over the cut "slice" of soil.

Disk plows work in a similar manner to laterally displace and invert soil through the use of concave steel disk blades.

Chisel plows use curved shanks to penetrate and "stir" the soil without inverting a soil layer. Chisel plows cause less residue disturbance than moldboard plows and are often used in conservation tillage systems.

Disk Harrows (or Disk) - are implements that uses steel blades to slice through crop residues and soil. Disk blades are mounted in groups or gangs that rotate as they move forward through the soil. Front gangs move soil toward the outside of the disk while rear gangs move soil back toward the center of the disk. A disk can be used for primary or secondary tillage.

Field Cultivator -an implement used to perform secondary tillage operations such as seedbed preparation and weed eradication. Field cultivators are equipped with steel shanks that are typically spring mounted to permit the shank to move within the soil and shatter clods. Field cultivators are constructed similarly to chisel plows, but are more lightly built. Large chisel plows can exceed 50 feet in width in the field.

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Environmental Concerns Related to Soil Preparation: Soil Erosion

The major environmental concern related to soil preparation is erosion. Soil erosion is a natural process that occurs when the actions of water and/or wind cause topsoil to be removed and carried elsewhere.

Soil erosion can be caused by either water or wind. In many agricultural areas, soil is eroding at a rate of several tons of soil per acre per year or higher. The map shows an estimate of total soil erosion on agricultural areas in 1992. This includes both cropland and set-aside land in the Conservation Reserve Program. Forested and urban land is not included in the map.

The good news is that soil erosion in the U.S. is decreasing. From 1982-1997, soil erosion declined about 40% in the U.S., due to government conservation programs, technological advances, and extension education efforts.

Water erosion is caused by the erosive power of raindrops falling on the soil (particularly if the soil is not covered by vegetation or residue) or by surface runoff. Raindrops cause the less severe forms of erosion (know as sheet and interrill erosion). Severe erosion problems such as rill erosion, channel erosion, and gully erosion can result from concentrated overland flow of water.

Wind erosion is particularly a problem in windy areas when the soil is not protected by residue cover. Wind erosion in the United States is most widespread in the Great Plains states, as can be seen in the map at right. Wind erosion is a serious problem on cultivated organic soils, sandy coastal areas, alluvial soils along river bottoms, and other areas in the United States.
Impacts of soil erosion

Soil erosion has both on-farm impacts (reduction in yield and farm income) and off-farm impacts (contaminated water due to the sediment and associated contamination from nutrients and pesticides carried on the soil particle).

On-farm impacts due to the loss of soil and nutrients include:

• lower fertility levels
• development of rills and gullies in the field
• poorer crop yields
• less water infiltration into the soil
• more soil crusting
• more runoff in the spring and after storms

When fertile topsoil is lost, nutrients and organic matter needed by crops often are removed along with it. Erosion tends to remove the less dense soil constituents such as organic matter, clays, and silts, which are often the most fertile part of the soil. However, the loss in productivity caused by erosion has not been so evident in many parts of the U.S., since it has been compensated for over the years by improved crop varieties and increased fertilization.

Soils can tolerate a certain amount of erosion without adverse effects on soil quality or long-term productivity, because new soil is constantly formed to replace lost soil. This tolerable level is known as "T" and generally ranges from 3 to 5 tons per acre per year. Goals for reducing soil erosion often use the "T" value as a target, because erosion rates below T should maintain long-term productivity of the soil.

Off-farm impacts occur when the eroded soil is deposited elsewhere, along with nutrients, pesticides or pathogens that may be attached to the soil. The tolerable"T" value described above does not take into consideration the off-farm or downstream impacts. Soil eroded by water has effects such as:

• eroded soil deposited in depressions and adjacent fields
• decreased water quality downstream
• decline of downstream aquatic ecosystems because of sedimentation and the addition of nutrients, pesticides, and bacteria associated with the soil
• clogged drainage ditches and other costly problems

Off-farm impacts of wind erosion are due to the blowing soil, which can reduce seedling survival and growth (seed cover), increase the susceptibility of plants to certain types of stress, contribute to transmission of some plant pathogens, and reduce crop yields. Dust affects air quality, obscures visibility which can cause automobile accidents, clogs machinery, and deposits in road ditches, where it can impact water quality.

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Best Management Practices to Reduce Erosion

Conservation tillage leaves at least 30% residue cover on the ground. This simple, low-cost practice can have a huge impact on the amount of soil eroded. Because of energy savings and obvious improvements in soil quality that can result from conservation tillage, it has been widely adopted across the Midwest. In Indiana, for example, conservation tillage was used on 50% of corn and 80% of soybean acres in 2000, a dramatic improvement from 10 years earlier. There is still room for improvement, however. This map shows the percent of U.S. crop land currently in conservation tillage. Percentages are generally higher for soybeans than for corn or other crops.

Contour farming and strip cropping is the practice of planting along the slope instead of up-and-down slopes, and planting strips of grass between row crops.

Cover crops are crops such as rye that grow in late fall and provide soil cover during winter. By providing a cover to the soil, winter soil erosion from both air and water can be greatly reduced.

Grassed waterways protect soil against the erosive forces of concentrated runoff from sloping lands. By collecting and concentrating overland flow, waterways absorb the destructive energy that would otherwise cause channel erosion and gully formation.

Terraces are structural practices that can reduce erosion by holding back the water and routing it along a channel at a lower velocity to where it can be safely discharged, usually into a grassed waterway.
Windbreaks are the best way to protect soil from wind erosion. They can be in the form of rows of shrubs or trees.
Windbreaks
Grass barriers can prevent wind erosion by slowing the wind.
"Living snow fences" prevent wind erosion by slowing the wind.

Photos courtesy of USDA-Natural Resources Conservation Service, except for the four pictures for practices controlling wind erosion, from the ARS Wind Erosion Unit.

Maps from USDA-NRCS "State of the Land."

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