Cross Canada Green Manure Use on Organic Vegetable Farms

Organic Agriculture Centre of Canada

The Canada Organic Standard requires organic matter be generated on farm before off-farm nutrient sources can be used. We were curious about the impact of this standard on Canadian organic vegetable producers. A handful of vegetable producers from coast to coast were interviewed about their green manure use, as green manures are one way to satisfy this standard.

Farmers listed several benefits they realized from using green manures: improved weed control, added erosion protection, improved soil health (adds organic matter, stimulates soil organisms, improves tilth and drainage), better nutrient retention (instead of letting them wash away), decreased pests (i.e. Rhizoctonia, nematodes, wire worms) and added fertility (both directly, e.g. legumes for nitrogen, and by making nutrients more available, e.g. improved phosphorus uptake by buckwheat). Most of the farmers mentioned several of these benefits to using green manures in their rotations.

All veggie producers interviewed use green manures extensively. They selected different crops depending on their needs:

• To cover the soil & capture nutrients: rye, oats, barley, buckwheat, spring wheat.
• For nitrogen fixing: peas, vetch, lentils, faba beans, clover, alfalfa. 
• For disease control: brown mustard greens, oilseed radish, buckwheat.
• To break up clay: alfalfa & oil seed radish.

In BC, a mixed vegetable farmer uses four annual green manure crops in succession on ten percent of his land annually. Fall planted rye is disked in mid May, followed by inoculated field peas which are tilled in when they reach 30 inches and are in flower. Oats, barley or buckwheat is seeded and disked when flowering, towards the end of August, and then the same field is seeded back to fall rye for the winter. Vegetable crops are planted the following spring. Pigs are pastured on a portion of the green manured area.

A Quebec vegetable farmer seeds red clover or alfalfa each year on a third of his land and these are grown for two years before tilling in. On the balance of the land with heavier soils he seeds either oat/pea, or oat/vetch combinations in late summer when there is no chance of any seed set. Both combinations winter kill. He uses oat/pea in fields scheduled for early spring vegetable crops and oats/vetch in areas where he is planning later spring plantings. This allows sufficient time for the greater biomass produced by the oat/vetch combination to breakdown. On sandy fields instead of an oat/legume combo he prefers to use rye to build up organic matter. Green manure crops are always seeded after the cash crop is tilled in to reduce the chance of any weed development. One worker focuses exclusively on the seeding and management of green manures. 

For nematode control, an Ontario garlic farmer uses mustard as a green manure crop. He mows it twice, and then tills it in. If weeds are the problem he uses buckwheat instead of mustard. He seeds fall rye after either green manure and then garlic is planted the next fall. The third year other veggies are planted. In the fourth year, he goes back to garlic. When he uses clover as a green manure, he throws in some corn seed to generate long lasting biomass.

Challenges and Tips:

• Finding organic seed or any seed of the type you want. Suggestion: buy bulk from a good grower, but be sure to get clean seed.
• Need equipment to manage the crop, especially rye which many vegetable farmers find challenging versus buckwheat. Or use green crops that winterkill (i.e. oats & peas). Remember to seed late enough that the green manure crop won’t set seed.
• Plant two crops together, like oats with either vetch or pea. The cereal helps keep the legume upright so it is easier to handle.
• Mowing before tilling can help with incorporation.
• Perennial crops can be very effective in controlling weeds but can increase pests (wireworm & cutworm) and can be difficult to incorporate.

• Too much or too little moisture is problematic at seeding.
• Seed heavily for best weed control.
• Seed after working in a cash crop.
• Have seeds on hand when you need them. Don’t leave an opportunity for weeds while you are waiting to get seed.
• Mow green manure crops before they set seed.

This article was written by Rochelle Eisen, member of the O-Teamon behalf of the OACC with funding provided by Canada’s Organic Science Cluster (a part of the Canadian Agri-Science Clusters Initiative of Agriculture and Agri-Food Canada's Growing Forward Policy Framework).  The Organic Science Cluster is a collaborative effort led jointly by the OACC, the Organic Federation of Canada and industry partners. For more information: oacc@dal.ca or 902-893-7256.


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Posted August 2011