Mark Schonbeck and Ron Morse uncover some facts about cover crops.
In no-till cover crop systems, the known benefits of cover crops are maximised by allowing them to grow until shortly before planting the vegetable or other cash crop, and by managing the cover crop without tillage. The best cover crops for this purpose have the following characteristics:
- They produce a lot of biomass, at least 3 tons above-ground dry matter per acre.
- They are readily killed by mowing, rolling or other mechanical means, forming a mulch or they are reliably winter-killed, leaving a mulch for spring no-till planting, or they die down naturally in time to plant summer vegetables.
- Their residues are sufficient to provide effective weed control in the subsequent vegetable crop.
- They provide habitat for natural enemies of vegetable crop pests.
- They have favourable (or at least neutral) effect on levels of available soil N, P and K.
- They do not suppress the vegetable through chemical (allelopathic) or microbial effects.
- They do not present serious weed, pest, disease or other management problems.
Often, a combination of a grass and a legume is used, since this enhances biomass production and therefore mulch thickness, weed suppression and organic matter inputs. The combination also offers a balanced carbon to nitrogen (C:N) ratio, which gives a gradual release of plant available N, in contrast to the N-immobilisation (tie-up) by an all-grass cover, or the rapid N release and potential leaching losses from an all-legume cover.
The higher diversity of a two-species cover crop can also enhance allelopathy (suppression of weeds by natural chemical substances from the mulch), diversity of beneficial soil microbes, and nutrient effects. For instance, legumes tend to enhance availability of phosphorus (P), while grasses, especially rye, enhance availability of potassium (K).
The most widely-known and extensively researched organic no-till systems are those based on hardy winter annual cover crops, mostly combinations of cereal grain rye (or winter rye), hairy vetch, crimson clover and Austrian winter peas. These crops are planted in early fall, and mowed or rolled after they flower the following spring, usually in May. Summer vegetables like squash, cucumber, pepper, tomato, eggplant, okra, sweet corn, beans, or (in cooler regions) mid-season brassicas, are then transplanted or direct-seeded no-till into the cover crop mulch.
Greater stability in more diversity
In recent years, growers and researchers have begun experimenting with a much wider range of annual cover crop species for no-till vegetables planted at other seasons. Other cool-season annuals like oats and fava beans can be planted in early spring, then killed in mid summer for late plantings of cucumber, bean or summer squash.
Summer annual (frost-tender) cover crops like millets, cowpeas or soybeans can be planted after the spring frost date, then knocked down at the end of summer to plant fall brassicas or other fall crops. Finally, cover crops that are not winter-hardy in a given location can be planted in mid to late summer and allowed to winterkill, forming a mulch for no-till spring vegetables.
One of the basic tenets of sustainable agriculture is that greater diversity yields greater agro-ecosystem stability, more beneficial organisms, fewer pests and diseases, more sustained crop yields, and more opportunities for farmer innovation. This is true also of cover crops.
Source: Rodale Insitute
Read More: Organic producers give back soil what they borrow
