Chapter 17

Soil Management for Field Crops

Conservation Tillage Effects on Soil Properties

Effects of conservation tillage on soil properties vary, and these variations depend on the particular system chosen. Generally, the changes are greatest for systems that produce the most plant residue (such as corn and small grains in humid regions, especially if accompanied by cover crops), retain the most residue coverage, and cause the least amount of soil disturbance. No-till systems, which maintain high surface soil coverage, have resulted in significant changes in soil properties, especially in the upper five inches. Fields that have been converted to conservation tillage can take up to 5 years before showing signs of better tilth and biological activity. Soils will improve much faster when cover crops are integrated into the rotation. Some of the effects of conservation tillage in its many and varied forms on soil properties include temperature, moisture content, nutrient availability, pH, organic matter, and microbe populations.

Effects of Tillage on Physical Properties

Soil physical properties are generally more favorable with no-till than conventional tillage-based systems. Conservation tillage systems have been found to moderate soil temperature and moisture and minimize soil erosion losses.

Soil Temperature

The amount of residue influences soil temperature. Soil temperature on the surface of conservation tilled soils is typically lower during the day than plowed tilled soils. This is due to the insulation effect of crop residues and the tendency for the mulch to reflect solar radiation. Residues from corn, wheat, and grass sod maintain cooler soil than residue from soybeans and other crops that produce less residue or residue that decomposes rapidly.

Soil Moisture

Soil water content is generally higher with conservation tillage than with conventional tillage because of a decrease in water evaporation from the soil surface. Accumulation of crop residues on the soil surface accounts for this reduction. Crop residues on the surface create an effective barrier that slows water as it runs off the surface and allow more infiltration.

Reduced Soil Erosion

When soil is tilled and loosened, and residue is buried or removed, the potential for erosion increases. Research has proven that switching from conventional intensive tillage systems to conservation systems results in greater water infiltration and reduced soil erosion due to increased crop residue coverage.

Effects of Tillage on Chemical Properties

Soil chemical properties that are usually affected by tillage systems are pH, CEC, exchangeable cations, and soil total nitrogen. Soil chemical properties of the surface layer are generally more favorable under the no-till method than under the tilled soil.

Nutrient Availability

During the initial four to six years of conservation tillage, the buildup of organic matter results in the immobilization of nutrients, especially nitrogen. This is in contrast to the mineralization of nutrients that is encouraged by the decline of soil organic matter under conventional tillage. Eventually, when soil organic matter stabilizes at a new higher level, nutrient mineralization rates under conservation tillage increase.

Soil pH

Numerous studies report a decrease in soil pH after adoption of conservation tillage. When crop residues are left on the soil surface and no mixing of the soil takes place, any acid-producing fertilizers added will be concentrated in the uppermost portion of the soil profile.

Soil Organic Matter and Carbon Sequestration

As mentioned, conservation tillage systems have been reported to increase soil organic matter content and enhance sequestration of carbon. With conservation tillage systems, especially no-till, residue is left on the soil surface where decomposition is slow, which then causes organic matter in the upper few inches to increase. If left undisturbed over the period, soil organic matter can eventually be transformed into long-lasting humus. The increase in soil carbon concentrations with increasing amounts of residue suggests that carbon will be higher at the surface of conservation tilled soils than conventional tilled soils.

Effects of Tillage on Biological Properties

As organic matter, in particular soil organic carbon, increases in conservation tillage systems, soil biological activity increases. This enhances the diversity of organisms and the ecosystem functions they perform. Earthworms and fungi, both important for soil structure, are especially favored. Many studies have shown higher counts of microbes in the surface of conservation tilled soils as compared with conventional tilled soils. Below the surface depth this trend is reversed with equal or greater amounts of microbes existing in conventional tillage at lower depths.

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