Pick Green Manures for Energy Efficient Nitrogen

by Brenda Frick, Ph.D., P.Ag.

Green manures allow producers to improve their energy efficiency in a relatively simple and inexpensive fashion. Nitrogen fertilizers made from natural gas account for up to 30% 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 green manuring? Martin Entz from the University of Manitoba recently told producers at a ProCert Organics meeting in Regina that a good ball park figure is that about 3% of the biomass of the green manure is nitrogen. For instance, if the green manure produced about 3000 lb/ac of dry green material, it would yield about 3000 x 0.03 or 90 lb/ac of nitrogen.

Green manures break down slowly, releasing about 60% of their nitrogen in the first year, and 20% in the second year. An additional 20% of nitrogen goes into what Entz refers to as the nitrogen bank, for long term soil building. This means that the 3000 lb/ac green manure in the example above would provide about 54 lb/ac in the year after green manure, about 18 lb/ac in the year after that and about 18 lb/ac for long term soil building. The green manure in Entz’ example would provide enough nitrogen for a 30 bu/ac wheat crop, followed by a 15 bu/ac 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 were not removed, would allow a greater build up of nitrogen and thus allow for a greater number of feeding crops to follow.

In areas with moisture limitations, farmers may feel that they can not include a green manure crop. They may feel that 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 suggested that although green manures need to be carefully managed to avoid depleting soil moisture reserves, they still have 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. It also maximizes moisture use. In a dry year, earlier termination, at 10% bloom or even before, 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 trap. 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.

 

Brenda Frick, Ph.D., P.Ag., is the Prairie Coordinator for OACC (the Organic Agriculture Centre of Canada) at the College of Agriculture, University of Saskatchewan. She welcomes your comments at 306-966-4975 or via email at brenda.frick@usask.ca.

 

Posted April 2007