October 20, 2012
Cover crops are plantings, annual or perennial, grown in soil areas between harvested crops as part of a crop rotation program or in orchard/vineyard situations on an ongoing basis. They should be part of the garden seasonal planting program as well.
To reduce erosion and compaction, cover crops can be used in the winter or the summer rather than letting a plot of soil go fallow. Rain and wind can cause huge losses of soil; by growing a cover crop during the winter, the rain hits the leaves rather than the soil, the water is directed down the stems and into the soil near the roots, and the soil does not get impacted directly by the raindrops, which can cause a compaction layer at the surface. This also reduces or eliminates sheet erosion, which can lead to the water being channeled and soil loss. In orchards, vineyards and row cropping systems, cover crops can reduce compaction from equipment as well.
Weeds have needs, too, usually in the form of a bare patch of disturbed soil that receives sunlight and a bit of moisture. Cover crops can block that sunlight while also providing intense competition for some weeds, thus drastically reduce their populations. Repeated use of cover crops is an important part of an integrated pest management program to reduce or even eliminate certain weeds without the use of herbicides.
For soils to improve in tilth and fertility, the soil structure must be improved. This is done through the aggregation of soil particles and the related increase in soil pore space. Soil aggregation occurs when humus and soil organisms are active and soils are not disturbed or subjected to compaction or inundation. The basic building block of humus is organic matter.
Growing cover crops and either turning them directly into the soil or adding their composted remains to the soil increases the organic matter content of the soil and the eventual humus content. Some cover crops can also bring micronutrients up from deeper soils and make them available to shallow rooted plants.
Rhizobia are bacteria that trick cells of leguminous and some other plants into forming galls or "nodules" on the roots through the secretion of hormones. Inside these tightly packed cells, the bacteria can carry on an anaerobic process to "fix" atmospheric nitrogen into plant-available nitrogen, the plant macronutrient most likely deficient in our soils.
These leguminous plants encourage this behavior by supplying the bacteria with the sugars and proteins they need. Farmers have taken advantage of this symbiotic relationship to build the nitrogen content of their soils before growing heavy nitrogen users such as corn. These cover crops are sometimes referred to as "green manure crops" and can be optimized by inoculating the seeds with rhizobia bacteria.
While some cover crops can be perennial, others should be turned in, mowed or crushed before they go to seed. The nutrients they fix or bring up from deeper soils and the organic matter they provide can only be made available after they decompose. In some home garden situations it is best to remove and compost the plant material in a compost system and then return the decomposed material back to the planting bed. This system can keep planting beds productive and filled with growing plants while maximizing the number and diversity of soil organisms.
When soil is left fallow or barren, the biological activity in the soil slows down dramatically. While some organisms can easily go into a dormant state, some populations are lost.
It is best to keep a succession of living plants in the soil; they encourage populations of beneficial soil microorganisms by secreting sugars, amino acids, and other compounds that help them to establish and thrive. These organisms sequester carbon, retain and cycle important soluble nutrients, keep pathogenic organism populations low, release minerals, and break down organic matter, all of which helps the plant. Soils with high populations of diverse beneficial soil organisms will grow healthier plants with fewer diseases or nutrient deficiencies.
Most seed companies carry a range of cover crops and cover crop blends; some common ones are winter rye, vetch, clovers, buckwheat and beans. Fava beans work great and offer a tasty treat. Depending on the desired plants being grown or to be grown next in the rotation program, the cover crop can be fine tuned to provide the proper nutrients or other benefits to maximize soil and plant health.
Vetch, for example, has been shown to increase the availability of phosphorus more than clover or rye. Other cover crop blends can minimize soil erosion, maximize weed control or provide nectar sources for beneficial insects.
Some plants can be used as cover crops for other crops if planted in succession, grown concurrently or both — this is known as intercropping. Companion planting is a related practice where one plant can either attract pollinators to another plant or reduce harmful pest populations by either deterring them directly or attracting their natural predators. Crimson clover is an example of a cover crop that adds organic matter, fixes nitrogen, attracts beneficial insects, provides nectar for bees and is beautiful.