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Enhance Soil Armor

9 Spoke Wheel

The ninth spoke is soil armor. It may be broken down into two separate categories (See figure to the right). First is passive armor; this is the dead plant residue that remains on the soil surface following crop harvest or crop termination. Second is active armor; typically represented by an actively growing economic or cover crop.Soil Armor with caption

Maximizing soil armor will weatherize cropping systems against the effects of drought, excess rainfall, and extremes in temperature. Maximizing soil armor is accomplished by leaving all crop residue remaining after grain harvest. The residue needs to be either left standing or be spread evenly over the surface of the field for success. If residue is clumped or concentrated in swaths, it will compromise establishment of the following crop, create non-uniform soil temperature and moisture conditions, and provide non-uniformity in food sources for biological life.

It is best to leave as much residue standing as possible during harvest so that there will not be a thick blanket of loose residue through which to plant. When residue is left standing, the soil warms up quicker in the spring and dries out faster than if the residue remained on the soil surface as mulch. The standing residue will be anchored in the soil, which facilitates planting of the next crop. Eventually, the dead stalks will fall and decompose on the soil surface. In situations where crop residues must be removed to meet the operation's objectives, it is essential to plant cover crops in a timely manner to compensate for the removed residue. Individuals who provide the best overall soil armor have learned how to combine both passive and active armor.

Some no-tillers experience problems with heavy residues, especially com stalks. Issues include wet soils that delay spring planting and difficulty achieving adequate seed placement and slot closure. Some techniques to manage heavy corn stalks include:

  • Let corn stalks stand after harvest.
  • Apply manure to help soil microbes speed up residue decomposition.
  • Plant cover crops into the residue; the drill cuts up the crop residue in smaller pieces and the increased biological activity and microclimate created by the cover crop favors residue decomposition.
  • Use planter attachments to move residue from the row.

Typical C-N ratios of organic materialsIrrespective of the management, having a healthy biologically active soil is key to the breakdown of residues, especially those with a high carbon to nitrogen (C:N) ratio and high lignin content (See the table to the left). Fungi specialize in breaking down lignin so high-fungal populations found in long-term no-till soils will be beneficial. Long-term no-till is beneficial for earthworms that assist in the breakdown of crop residues. A readily available source of nitrogen to complete decomposition is important. A soil with high organic matter content will have more nitrogen available for microbes to decompose high C:N residue.

On the other hand, residues higher in nitrogen, for example those from legume crops like soybeans, break down rapidly and leave very low levels of soil armor. Cover crops with higher C:N ratio should be used to improve soil armor. By knowing the carbon to nitrogen ratio of crop residues and cover crops it is possible to maintain adequate and manageable levels of soil armor.

Fifty percent soil armor is considered acceptable, but higher levels provide more soil erosion protection, help conserve soil moisture and reduce soil temperature during the summer months, and help protect and feed soil organisms. This is very different from the older recommendations when 30 percent residue was considered adequate.

Managing soil armor in forage cropping systems is challenging since minimal amounts of residues remain after harvest. This is especially true in pure stands of legumes such as alfalfa. One way to address this is to grow perennial grass/alfalfa mixtures instead of pure alfalfa. If this is not desirable, it is possible to interseed annuals, such as small grains, into pure alfalfa in early to mid-August following a forage harvest. To add more diversity, other low-growing species may be added. This provides active soil armor, increases forage production, and usually improves forage quality as well. It is also a good practice to leave the last cutting of the year standing to feed the microbes and protect the soil. However, if the last cutting is to be removed, then interseeding a winter small grain in August following a harvest will assure that some additional growing cover will remain as soil armor during the winter months and into the early spring.