Chapter 16

The Use of Cover Crops to Manage Soil

Planting Cover Crops

To establish a cover crop successfully, field preparation is the first critical component to address. Two of the primary issues to consider are weed management and the development of a smooth seed bed for good seed-to-soil contact. Ideally field preparation will mirror that for annual cash crops, but is generally less intensive. Establishment involves identifying the plant hardiness zone, selecting the seed-source, tillage, planting the cover crops, fertilization, and mechanically and chemically controlling the cover crops.

Plant Hardiness Zones

The first step in the selection of a suitable cover crop is identifying its plant hardiness zone of the location and crop tolerance to cold temperatures. While soil, water, and light are critical, low or high temperatures can limit plant growth in a certain location. For a successful cover crop program, farmers will need to select cover crop species that tolerate the temperatures characteristic for the hardiness zone. The USDA Hardiness Zone Map is based on the average annual minimum winter temperature in a geographic location and is based on temperature records (Figure 16.15).

Understanding Seed Tags

Cover crop seed should be purchased from dealers who sell quality seed that has been tested for viability and is free from weed species. Cover crop seed costs vary considerably from year to year and from region to region, but historically, legume cover crops cost more to establish compared to grasses. The law requires that each seed lot offered for sale must be truthfully labeled. This is regulated by the Federal Seed Act as well as state seed laws. In addition, all state certification agencies comply with the minimum requirements and standards of the Association of Official Seed Certification Agencies (AOSCA) to insure uniform testing methods and minimum standards of seed quality.

Seed Treatments and Coatings

Legume Specific Coatings

To maximize nitrogen fixation, inoculate legume seeds with the correct strain of Rhizobium bacteria before planting. Different types of rhizobia bacteria are specific to certain crops. There are different strains for alfalfa, clovers, soybeans, beans, peas, vetch, and cowpeas. Keep in mind there is no such thing as “universal” inoculant. Since each legume needs a specific strain of rhizobia bacteria, be sure to buy the right inoculant. For example, bacteria that is specific to a soybean plant won’t form a relationship with clover or vetch cover crop. Therefore, although many strains of Rhizobium may be present in the soil from previous inoculated legume crops, they may not be compatible with the legume cover crop being planted.

Small Seeds Specific Coatings

Small grass and brassica seeds are often coated with some combination of calcium carbonate, clay, and other chemical compounds. These coatings can more than double the size and weight of seed. For example, small, irregularly shaped seeds (such as ryegrass) are prone to being blown with the wind when broadcast (either with a spreader or aerial applicator).

Planting Methods

There are main methods used in planting cover crops—drilling, broadcasting, aerial seeding, and frost seeding.

Drilling

Usually, the most consistent cover crop stand is obtained by using either a no-till or a conventional drill (Figure 16.16). Conventional drills are designed to plant into a prepared seedbed. In conservation tillage systems, heavier, no-till seeders are required to operate in residues present from these tillage systems. No-till drills can slice through crop residue easier using a coulter mounted in front of the openers combined with the additional weight.

Broadcasting

Broadcasting is faster than drilling and works well if the entire field is being seeded. However, higher seeding rates are needed when broadcasting because seeds are left on the soil surface and are exposed to fluctuating moisture conditions as well as wind. The amount of seed should be increased 20 to 30 percent compared to drilling to ensure an adequate stand.

Aerial Seeding

On larger areas, aerial seeding by fixed-wing aircraft or helicopter in late summer during crop die-down can be effective. As the leaves of the summer crop drop off, they aid germination by covering the seed, retaining moisture and protecting the soil. Aerial seeding is cost-effective when seeding a large number of acres in a timely manner.

Frost Seeding

Frost seeding is an economical no-till cover crop establishment technique that can be done in annual crops (primarily winter cereals) and to renovate pastures. Frost seeding is the broadcasting of seed onto the soil surface during periods of freeze-thaw cycles. Seasonal freeze thaw cycles cause the soil to repeatedly develop small cracks on the surface, allowing the seed to achieve good soil contact for germination.

Fertilization

As with cash crops, ensuring soil fertility and pH are within recommended ranges is essential for optimizing cover crop biomass production and associated benefits. Nitrogen can be limiting for biomass production, particularly for grass cover crops. Some producers are reluctant to apply nitrogen fertilizer to cover crops due to the cost and the absence of a direct monetary return from the cover crop.

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