Understanding the benefits of green manure crops will help farmers select the crop and management technique that work best for them.
Many crops can be used as green manures: annual species such as pea, lentil, chickling vetch, buckwheat, mustard, oilseed radish and faba bean; winter annual species such as fall rye and winter wheat; biennial species such as sweet clover and red clover; and perennial species such as alfalfa and white clover. Sweet clover is common and alfalfa, pea, lentil and chickling vetch are also widely used.
Green manures offer many benefits when included in a crop rotation. They can increase soil organic matter and improve soil structure. Breakdown of crop residues stimulates biological activity in the soil.
Green manure crops also prevent erosion by providing a protective cover during fallow seasons. They can be essential to nutrient cycling in organic cropping systems and act as temporary nutrient storage units that limit nitrogen losses from leaching or from processes that make nitrogen less available to plants during fallows.
Green manure crops break disease cycles and compete with weeds. They may also provide habitat for beneficial insects, such as parasitic wasps, and produce chemicals that interfere with weed growth.
Legume green manure crops are an essential part of many organic crop rotations. Legumes can add nitrogen through nitrogen fixation and deep-rooted green manure crops such as sweet clover and alfalfa can retrieve nutrients like nitrogen and phosphorous from deep in the soil.
Green manure residues are incorporated near the surface, which means that when they break down, their nutrients are more available to subsequent crops.
A 2002 survey of five percent of organic farmers in Saskatchewan found fields were less likely to be deficient in soil nitrogen if green manure crops had been planted in the last five years.
Despite these advantages, producers may hesitate to use them. There are seed, fuel and machinery costs in the year the green manure is sown, but no income from the field.
Green manures can use up limited water reserves and small-seeded crops can be difficult to establish. Weed control can be difficult.
Many of these problems can be overcome with careful crop selection and management. Seed costs are often less for smaller-seeded crops and producers can reduce costs by growing their own seed. Water depletion can be reduced by terminating the crop early and by incorporating the residue so that some material is left upright for snow trapping.
Weed number and size are generally less when the green manure seeding rate is higher. Weeds also add soil nitrogen and organic matter, although they may interfere with the growth of the green manure crop. It is generally recommended that green manures be terminated before weeds set seed.
Green manure crops, with their many biological roles, are useful in low-input cropping systems. The type of green manure crop that is selected and the management practice that is chosen will depend on the producer’s priorities. Short-term difficulties such as taking a field out of production for a year can be balanced against long-term advantages such as reducing the depletion of organic matter, nitrogen and phosphorus in the soil.
This article was written with Yvonne Lawley, a MSc. student in plant sciences at the University of Saskatchewan under the supervision of Steve Shirtliffe. Lawley can be reached at Yvonnevandenbosch @usask.ca. Frick is the prairie co-ordinator for the Organic Agriculture Centre of Canada located at the University of Saskatchewan. She can be reached at 306-966-4975, at firstname.lastname@example.org, or www.organicagcentre.ca. The opinions expressed in this column are not necessarily those of The Western Producer.