Chapter 17

Soil Management for Field Crops

Cover Crops for Sustainable Crop Rotations

Integrating cover crops into a field crop rotation is a key component of sustainable soil management. For example, cover crops can reduce nutrient and soil loss, increase soil organic matter, improve soil physical properties, suppress weeds, and even serve as extra sources of livestock forage. All of these benefits help translate into increased cash crop productivity. Selection of cover crops will depend on the goals of the farmer, which may include providing nitrogen, biomass, insect habitat, weed management, erosion protection, or combinations of these. Species selection will also depend on when the cover crop is planted—cool season or warm season.

Cover Crop Selection for Field Crops

The impact of cover crops can be variable, depending on plant variety and management practices. Choosing the right cover crop is important and deciding what to plant will depend on the on-farm goals. The most commonly used cover crops are grasses and legumes and to some extent brassicas.

Grasses

Grasses, including winter cereals such as rye, wheat, barley and triticale, are the most widely used cover crops in field crop rotation systems. Winter cereals such as rye, wheat, barley, and triticale are typically planted in late summer through late fall and produce a small to moderate amount of root and above-ground biomass before going dormant in the winter. Vigorous growth resumes in early spring, and large amounts of biomass are produced by mid to late spring. In some instances, winter cereals produce more biomass than is easily managed prior to planting the next grain crop.

Legumes

Legumes are valued as cover crops primarily for their ability to fix nitrogen. But legumes need to be grown later in the spring—typically until a few weeks after cereals elongate—to reach the early flowering stage and achieve near-maximum nitrogen fixation.

Brassicas

Brassica cover crops have grown in popularity recently due to their ability to provide many of the same benefits as grasses but with residues that break down more rapidly in the spring. Certain brassicas are also becoming well known for their ability to produce a large taproot that is effective at breaking soil compaction.

Cover Crop C:N Ratio Considerations

The addition of fresh crop residues stimulates growth of soil microbes and increases microbial demand for nutrients, particularly nitrogen. Microorganisms use carbon, nitrogen and other nutrients as a food source in order to break down the residues. If the amount of nitrogen in the residues is too low, the microorganisms use soil nitrogen instead, reducing nitrogen availability to the cash crop. This is called nitrogen immobilization. If the amount of nitrogen in the residues is greater than microbial demand, nitrogen is released and nitrogen availability for plant growth is increased, a process called nitrogen mineralization.

Low C:N Ratio

Assume cover crop residues with a C:N ratio of 12:1 are returned to the soil. This would be similar to killing a hairy vetch cover crop (legume functional group) at mid bloom. These residues contain significantly more nitrogen than soil microbes need in order to digest it.

Medium C:N Ratio

Assume cover crop residues with an intermediate C:N ratio of 24:1 are returned to the soil. This would be similar to killing rye (grass functional group) in the vegetative stage (prior to seed head emergence). In this case, there would be neither an excess nor a shortage of nitrogen for microbial consumption of the residues.

High C:N Ratio

Assume cover crop residues with C:N ratio of 48:1 are returned to the soil. This would be similar to killing a rye cover crop in the grain fill stage. These residues contain significantly less nitrogen than soil microbes need to consume it. The residues would be broken down slowly and protection of the soil surface provided by those residues would be long-lasting. The microbes would take the additional nitrogen needed for digestion of these residues from the pool of available soil nitrogen.

Mixing Cover Crop Functional Groups

Both very high and very low cover crop C:N ratios can be problematic. High C:N ratios in the range of 50:1 or greater can cause severe short-term soil nitrogen immobilization and nitrogen deficiencies in subsequent crops. Low C:N ratios in the range of 15:1 or less can lead to rapid mineralization of cover crop nitrogen. Although it originates from an organic source, this nitrogen can be quickly converted to soluble forms and potentially leached or lost almost as easily as fertilizer nitrogen.

Management Options for Reducing Nitrogen Deficiencies

Consider the following recommendations for reducing the risk of nitrogen deficiencies to corn, soybeans, or other N-fertilized cash crops following a grass cover crop:

Cover Crops for Corn-Soybean Rotation

Rotating corn and soybeans with cover crops is a widely used practice among Midwest farmers. There are many different options when integrating cover crops into a corn-soybean rotation that can be tailored to work in very specific soils, climates, or management styles. Nitrogen management is generally a major factor in cover crop decisions for the corn-soybean rotation. A fall-planted grass or small grain will scavenge leftover nitrogen from the previous corn or soybean crop. Legumes are much less efficient at scavenging nitrogen, but will add nitrogen to the system for the following crop. Legumes must be seeded at least 6 weeks before hard frost to ensure winter survival. Legume/grass mixtures are quite good at both.

Corn-Soybean Rotation with Rye

Cereal rye is a cool-season grass species that provides much-needed diversity to the corn-soybean rotation that consists of two warm-season crops. Planting rye after corn and ahead of soybean is a better fit, because soybeans can tolerate later planting in the spring better than corn, which allows rye to accumulate more spring biomass. One of the most important attributes that winter rye possesses over other winter cereals, like winter wheat or triticale, is its tolerance to extreme cold temperatures.

Cover Crops for Cotton Rotation

In what would otherwise be continuous cotton production, any winter annual cover crop added to the system can add rotation benefits, help maintain soil productivity, and provide the many other benefits of cover crops highlighted in Chapter 16. Wheat and rye are the most commonly used cover crops for cotton. They can be seeded later than legumes and are less susceptible to disease and winter kill. Rye works better than wheat.

Cover Crops for Dryland Rotations

While cover cropping can deliver many positive on-farm benefits, farmers in dry regions face several challenges related to incorporating cover crops into their farming systems. Major challenges include which cover crop to select and availability of water to raise cover crops. Another concern among dryland farmers is whether or not cover crops will extract too much water from the soil profile, thus limiting the water that will become available for the following cash crop. One of the emerging innovative ways for cover cropping in dryland systems is to combine no-till or minimum tillage with cover cropping. Reducing tillage intensity can help improve soil health, promote water infiltration, and reduce evaporative losses during the period that the cover crop is growing.

Cover Crops in Conservation Tillage Systems

To achieve all of the benefits of no-till, it is critical that farmers incorporate cover crops into the crop rotation. Cover crops have the ability to “jump-start” no-till, perhaps eliminating any yield decrease. For example, some farmers have planted a cereal rye cover crop before soybeans and then begin the no-till process in the spring, in this case by no-till planting the soybeans into the rye residue. Changing to no-till without a cover crop would cause less aeration and possibly more initial compaction in a field compared to a conventionally tilled field, but the use of a cover crop offsets these negatives.

Wheat-Fallow Rotation

In dryland cropping systems of the High and Central Great Plains of the U.S., winter wheat is the dominant crop and is traditionally grown with alternating years of bare fallow. These fallows are maintained largely free of growing vegetation for 14 months to store soil moisture and stabilize wheat yields in the face of low and highly variable annual precipitation. However, on average only about 20 to 35 percent of precipitation remains in the soil at the end of the fallow period, with the higher storage water efficiencies under no-till systems.

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