Chapter 6

Cover Crops for Organic Farms

Buliding Complimentary Cover Crop Mixtures

Every farm is different. Even within one farm, management objectives for a given field and crop will vary based on weather, site location and history, crop rotations, and many other factors. The design of a cover crop mixture must, therefore, take into account the current and future management objectives for each field. Whether the goal is breaking up a plow pan, overcoming low fertility, knocking out a pernicious weed or a combination of cover crop benefits, different cover crop mixtures and specific management approaches will be needed. The starting point for developing mixtures is understanding, the strengths and weaknesses of individual cover crop species. For instance, legume cover crops with greater biomass and nitrogen content will supply more nitrogen and a greater flower density in a cover crop stand and will attract more pollinators. However, more is not always better. In some cases, excessive biomass production by species in a mixture can lead to challenges for cover crop termination and incorporation and for planting the following crop, as well as reduce the efficacy of other species in the mix. Some examples of cover crop mixtures and their benefits are described in Table 6.5.

Complementary Growth Periods

Cover crops can have different growth periods. For instance, when planting a wintering cover crop consider a mixture that includes both winter-killed and winter-hardy species. Rapid growth of the winter-killed species in the fall will increase nitrogen uptake, weed suppression, and erosion control in the fall.

Complementary Growth Forms

Cover crops can compete with each other for space and light, reducing the benefits provided by the less competitive species. Selecting species with complementary growth forms helps alleviate competition between species. Cover crop growth forms can be divided into several categories, including tall, open canopies; short, dense canopies; and vining. Species with similar growth forms are likely to compete with each other, while scover crops with differing growth forms are more likely to be complementary.

Complementary Nitrogen Acquisition Strategies

For soil with low nitrogen levels, planting legume cover crops will be the most effective. For soils with excessive nitrogen levels, non-legumes such as grasses and brassicas are more suitable given that they can scavenge nitrogen. A legume cover crop planted in a soil with high nitrogen levels will grow as well while planting a non-legume into a soil with low nitrogen levels will result in suboptimal biomass production due to nitrogen deficiency.

Complimentary Weed Management Strategies

The use of complex cover crop mixes may take advantage of the allelopathic suppression of weeds. Allelopathy has been shown to be species-specific phenomenon therefore a broader spectrum of weed control may be obtained by growing mixtures. Allelopathy needs to be taken into consideration and managed when direct seeding subsequent crops.

Complementary Insectary Mixture Strategies

The use of diverse cover crop mixes can be used to enhance pollinator habitat or refuges for beneficial insects. Insect pests usually exhibit higher abundance in monocultures than in polycultures. This is partly due to more stable populations of beneficial insects (predatory and parasitic) in polycultures from continuous availability and diversity of food sources and micro habitats. Differences in flower morphology such as shape, size, and color will influence the types of beneficial insects that are attracted to a particular cover crop. Therefore, to promote a diversity of beneficial insects, cover crops mixtures containing a variety of flower morphologies may be required.

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