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All Things Organic - What to know when choosing legumes for green manure
By Ian Cushon
Soil scientists admit there is limited knowledge of the complex world
of soil organisms, yet human life is dependent upon the world beneath
our feet. Plant nutrition in organic production is dependent on biologically
active soil to provide nutrients. Legumes stimulate soil biological activity.
Legumes increase soil organic matter, which improves soil structure, act as a ground cover to help reduce soil erosion and increase soil's water-holding capacity. Over the years, we have tried several different legumes as green manure plowdowns. At first, we used biennial yellow clover. Before converting to organic farming, sweet clover was a relatively small part of our rotation. Yellow blossom sweet clover is the most common plowdown on prairie organic farms. It is a large nitrogen producer, seed cost is low and it is weed-competitive, establishing early and aggressively. Moisture use can be a concern in drier areas, where plowdown should occur by 10 percent bloom. This optimizes nitrogen fixation and moisture use. Plowing down later than 10 percent bloom will increase the risk of lost yields because of moisture deficiency. In drier areas, an annual legume such as chickling vetch provides adequate nitrogen, but does a better job preserving moisture.
Clover has some disadvantages. It can be competitive in the first year when used as an underseeded crop. Planting it a few days after the crop is seeded or during in-crop harrowing can delay emergence, which will lessen crop competition. For this reason, some producers prefer the less aggressive red clover, a short-lived perennial. It produces less biomass and fixes less nitrogen than yellow clover. It's best adapted to regions with more moisture.Yellow blossom sweet clover is prone to weevil damage. In some years, sweet clover weevils will completely defoliate clover seedlings, resulting in patchy growth and even a complete loss. Research has shown that green manured yellow blossom clover can suppress weed growth. This can be valuable during the green manure fallow year, reducing the amount of tillage. The effect can also be carried into the following crops.
Alfalfa is also used as a soil-building crop. It produces considerable biomass and fixes a lot of nitrogen. It is deep rooted, pulling nutrients and moisture from deep soil zones. To derive benefits from alfalfa, the optimum life of the stand is three to four years. Selling alfalfa hay will export significant amounts of soil nutrients. Alfalfa in the rotation works best when you have livestock. When cut for forage, alfalfa can help control Canada thistle and will suppress annual weed growth in following crops. Alfalfa needs a lot of moisture so it is best adapted to wetter regions.
Organic farmers can also choose several annual legumes for soil building. They use less moisture than forage legumes and provide more flexibility in cropping decisions, but seed costs are higher. Annual legumes do not produce as much nitrogen and biomass as forage legumes. This makes them easier to incorporate, but means they are less weed competitive and leave less residue to protect the soil. In drier regions, annual legumes should be planted early, leaving time for moisture recharge after incorporation. Snow trapping can assist in moisture recharge. The primary choices for annual legumes are chickling vetch, Indian Head lentils and peas. Chickling vetch was developed for drier regions. It is moisture efficient and fixes lots of nitrogen. In drier regions, Indian Head lentils produce less biomass and fixes less nitrogen than chickling vetch. In moister regions, Indian Head lentils produce slightly more biomass. In my experience, chickling vetch does not establish quickly and is not as weed competitive as I would like. But many farmers in the drier western areas like chickling vetch. Seed costs are high, so many farmers produce their own seed. Indian Head lentil seed costs are lower. Small seeded forage pea varieties such as Trapper and 4010 are also good options for annual legumes. They are competitive, produce lots of biomass and fix relatively large amounts of nitrogen. Forage peas and chickling vetch can be seeded with oats to increase biomass production and weed competitiveness. This also forces the legumes to fix more nitrogen.
Farmers should carefully make their own assessment of which green manure legumes are best adapted to their farm.
© 2012, Organic Agriculture Centre of Canada (OACC)