Why do cover crops work?

In Delta Farm Press

by Ernie Flint, Mississippi Extension Agronomist

The use of cover crops to improve soil quality and to reduce soil erosion is certainly not a new concept, but it is enjoying renewed interest. Yearbooks of Agriculture that were published by USDA include discussions about cover crops in some of the earliest issues prior to 1920.

During that earlier time agricultural workers and innovative farmers discovered the benefits of keeping the soil “alive” during the offseason. Some of the reasons for these positive results were discussed, but the main emphasis was the simple fact that it seemed to work. This same attitude exists today among most farmers even though we have a better understanding of the reasons benefits are seen.

There are two primary types of cover crops — grasses and legumes. The grasses are most commonly utilized in our region of Mississippi because of their flexibility and ease of management. Legumes are usually more expensive to establish and more challenging to manage even though the benefits can be greater than grasses like wheat.

Grasses stabilize the soil, hold nutrients in place, and improve soil organic matter, but the legumes can also fix nitrogen from the atmosphere to supply the following crop  However, legumes need to remain active later into the warm season and can be more difficult to subdue prior to planting the summer crop.

An added possibility for a crop like wheat or oats is that the grower has the option to retain the crop and harvest it for grain.

There is a third group of plants that have become fairly popular in recent years that include brassicas like radishes and turnips. These have been more popular in areas north of us, but a few growers in this area are experimenting with them. Proponents have stressed their use in penetrating compaction zones in the soil. This is also true for other crops like wheat, rye, and oats since their roots can grow through compaction zones during winter. When these crops die they leave root channels through compacted soil.

There are several “unseen” benefits that can be derived from cover crops, some of which I have mentioned in the past. Perhaps the best example is that of mycorrhizal fungi which colonize the roots of cover crops with exception of the brassicas. When these cover crops die, these beneficial fungi move over to the roots of the summer crop to provide benefits in the form of drought tolerance through increased uptake of water and nutrients.

Cover crops support high earthworm populations, especially when residue is allowed to remain on the surface where earthworms prefer to feed. Their activities improve water infiltration and the conversion of plant debris into available nutrients and soil organic matter.

An unexpected benefit is that weed emergence is suppressed by the mulch effect of cover crops since sunlight is required for seed germination. This aspect is especially valuable for problem species like pigweed.

The only significant disadvantage for cover crops is that they insulate the soil in early spring, preventing it from warming up as rapidly as tilled or bare soil. Those of us who have worked with reduced tillage and cover crops have noted that crops planted into these soils a few days later will usually catch up and sometimes pass conventionally farmed fields in maturity since these fields avoid most of the stresses that accompany conventional practices.

When I decided to write this one I thought I could briefly cover most aspects of cover cropping but was I ever wrong. At least it’s a beginning.

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