Green manures allow producers to improve their energy efficiency simply and inexpensively.
Nitrogen fertilizers made from natural gas account for up to 30 percent of the energy use in modern agriculture. Replacing such fertilizers with green manures improves the energy efficiency of the farming system.
How much nitrogen can be gained by using green manure? Martin Entz from the University of Manitoba recently told producers at a ProCert Organics meeting in Regina that about three percent of the biomass in green manure is nitrogen.
If the green manure produced 3,000 pounds per acre of dry green material, it would yield 3,000 x 0.03, or 90 lb. per acre of nitrogen.
Read Also
VIDEO: Green Lightning and Nytro Ag win sustainability innovation award
Nytro Ag Corp and Green Lightning recieved an innovation award at Ag in Motion 2025 for the Green Lightning Nitrogen Machine, which converts atmospheric nitrogen into a plant-usable form.
Green manures break down slowly, releasing 60 percent of their nitrogen in the first year, and 20 percent in the second year. An additional 20 percent of nitrogen goes into what Entz refers to as the nitrogen bank for long-term soil building.
This means that 3,000 lb. per acre of green manure would provide 54 lb. per acre of nitrogen in the year after green manure, 18 lb. per acre in the year after that and about 18 lb. per acre for long-term soil building.
The green manure in Entz’ example would provide enough nitrogen for a 30 bushel per acre wheat crop, followed by a 15 bu. per acre flax crop, without generating a nitrogen deficit.
In areas with sufficient moisture, forage or grazing crops might be included for green manure. Several years of alfalfa, if the hay was not removed, would allow a greater build up of nitrogen and allow for a greater number of feeding crops to follow.
In areas with moisture limitations, farmers may feel they cannot include a green manure crop because they think the moisture of two years is required for a single crop. The recently published Canadian Organic Standard recommends green manures. Is this a mistake for drier regions?
Studies at Swift Current, Sask., suggested that although green manures need to be carefully managed to avoid depleting soil moisture reserves, they still play an important role in cropping systems for the semiarid prairie.
Wheat crops that followed an annual legume green manure yielded better than wheat crops following black fallow in wet years. In dry years, leaving the green manure until full bloom depleted moisture for the following crop.
Timing the green manure plow down can be a balancing act. Allowing the green manure to reach full bloom maximizes the amount of nitrogen it will produce as well as moisture use. In a dry year, earlier termination at 10 percent bloom or earlier will reduce moisture loss. Nitrogen will also be reduced, but in a dry year this is less important.
In dry cycles, green manure crops can be included in a successful rotation by paying careful attention to techniques that enhance water retention. Tillage for incorporation can dry the soil and reduce the residues that trap snow. Alternative methods of termination may be possible.
Undercutting with a Nobel blade or other wide blade cultivator can allow standing stubble for snow traps. Crop strips can be used to trap snow. Non tillage techniques such as mowing, rolling or crimping may be possible for the termination of annual legumes.
The benefits of green manures may be greatest where moisture is abundant and thus nitrogen is most likely to limit crop growth. Substantial benefits can also be achieved in dry areas if care is taken to retain and build moisture reserves.
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 brenda.frick@usask.ca, or www.organicagcentre.ca.
