Chapter 16

The Use of Cover Crops to Manage Soil

Challenges in Growing Cover Crops

Despite the many benefits associated with cover crops, there are also challenges and limitations. Major limitations include nitrogen management, water use, residue equipment interference, delayed or reduced crop emergence, reduced nitrogen-fertilizer efficiency, termination challenges, pest problems, risk of frost damage, and increased cost, labor and management.

Nitrogen Management

Cover crops growing in the fall will capture residual fertilizer nitrogen and plant-available nitrogen that is released from soil organic nitrogen stocks. When the cover crop is terminated, it will decompose and release some nitrogen that will be available to the next cash crop, while other nitrogen will be incorporated into microbes. When the carbon to nitrogen (C:N) ratio is high (about 25:1 or greater), a greater proportion of the nitrogen is incorporated into microbial biomass and less is available for the cash crop.

Soil Moisture Conditions at Planting

One important consideration when using overwintering cover crops is their potential to deplete soil water. Although cover crops can improve water infiltration and soil water-holding capacity, the short-term depletion of soil water in the early spring may affect cash crops in dry springs. In this situation, cover crops may need to be incorporated early to conserve soil water, or irrigation may be required.

Residue Equipment Interference

Thick residue coverage formed by a cover crop can cause planter interference, poor furrow closure, and poor seed to soil contact of spring-planted cash crops. Additional equipment on the planter (e.g., row cleaners or coulters) and/or additional passes with other equipment (e.g., strip tillage equipment, roller crimpers, etc.) may be needed to combat cover crop residue during spring planting. Some farmers create strips with row cleaners a week or two prior to planting and terminate the cover crop near planting to remedy this situation.

Soil Temperature Fluctuations

In high-residue conservation systems, cover crop residues will reduce the amount of solar radiation reaching the soil surface. This results in cooler soils in the spring that are slower to warm up compared with conventionally tilled soils. Delayed soil warming can make planting more difficult, delay time of planting, delay crop emergence, and ultimately reduce cash crop establishment and yield.

Reduce Nitrogen-Fertilizer Efficiency

Cover crops may also reduce nitrogen-fertilizer efficiency in conservation systems, depending on the method of application. Surface applications of urea-containing fertilizers to soils with large amounts of residue can result in large losses of nitrogen. When applied on top of the cover crop residue, urea and urea-ammonium nitrate solutions volatilize more than ammonium nitrate and subsequently lose more nitrogen to the atmosphere.

Termination Challenges

Attributes that make a plant a good cover crop (e.g., root biomass production, aboveground biomass production, rapid establishment and growth, etc.) can also make the cover crop a major pest if not controlled properly.

Pest Problems

Cover crops may attract and provide shelter for pests, potentially increasing the pest and/or disease populations in the field. Consider specific pest/crop interactions that may become a problem. For example, cereal rye or orchardgrass can attract armyworms.

Risk of Frost

A bare, firm, moist orchard or vineyard floor absorbs heat during the day and releases it at night, increasing the air temperature by as much as 3 to 4 degrees F (1.6–2.2°C). A winter cover crop reduces absorption of heat by soil during the day, increasing the risk of frost damage to trees and vines in the spring. In addition, cover crop species with tall growth habits can cool temperatures closer to the fruit zone, causing damage to flowers or fruit.

Increased Cost, Labor, and Management

Cover crops require additional cost and labor for cover crop seed and planting. If the spring cash crop planting requires additional equipment or additional passes of strip tillage or roller crimpers, these are additional costs and labor. If the cover crop requires additional nitrogen or reduces the stand of the cash crop, these are expenses.

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