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- Green manuring is the practice of growing a crop that will be worked into the soil. This helps improve soil health, fertility and increases the yields of crops grown on that land in following years.
- Green manure crops have been used since the Greeks turned beans under in 300 B.C. They were commonly used in rotations in North America until the widespread availability of commercial fertilizers.
- Many types of crops can be used as green manures. Legumes are the most commonly seeded green manure crops in Western Canada. Decomposing legume residue adds nitrogen to the soil and reduces the amount of commercial fertilizer needed for the following crop.
- The ideal legume green manure crop has a high rate of nitrogen fixation, is high yielding, provides a significant amount of nitrogen for the following crop, protects the soil and helps preserve soil moisture. It should also be inexpensive to seed and easy to establish.
- The two most common legume green manures are sweet clover and red clover.
- Legume green manures are best suited to the black and grey wooded soil zones, in addition to the wetter parts of the brown and dark brown soil zones.
- Legume green manures are usually established with a cereal cover crop. The cereal crop is seeded at a reduced rate and harvested early as greenfeed or silage. The legume green manure is then worked under during the summer of its second year.
- The best time to incorporate a sweet clover green manure is at the bud to 10% bloom, usually towards the start of July. Red clover can be incorporated any time between July and August.
- The most common way to work the green manure under is with a tandem or offset disk, or a plow. These methods work the greatest amount of legume residue into the soil. Legume green manures can also be mowed or sprayed out, and the residue left on the soil surface in zero-till operations.
- It has been estimated that 40-60% of the nitrogen from the decomposing legume residue will be available to the following crop. This amount varies with the type of legume, method of incorporation, environment and soil conditions.
- Research studying different termination methods of sweet clover green manures in Alberta has found that in some cases, the amount of available nitrogen in the soil the spring after termination is the same whether the green manure was mowed and the residue left on the soil surface or disked to a depth of 10 cm.
- Treatments where the green manure was terminated with a moldboard plow had the highest amount of available nitrogen in the soil.
- Legume green manures also add other nutrients, such as phosphorous, potassium, sulphur, calcium and magnesium, to the soil when they decompose.
- Legume green manures improve soil structure and tilth. Their deep and widely branching roots can increase soil drainage, water infiltration and soil aeration.
- Legume green manures provide ground cover that helps prevent soil losses due to wind and water erosion.
- This ground cover also helps control weeds, while the green manure crop is growing and when residue is left on the soil surface after it has been terminated.
- Legume green manure crops can act as a break in the disease cycle when included in rotation with cereals and oilseeds.
- Drawbacks to using a legume green manure include: no cash flow in the year it is grown and incorporated, extra establishment and management costs and the potential loss of soil moisture in drier areas (brown soil zone).
Cover and Green Manure Crop Benefits to Soil Quality - available in PDF format only
Grazed Green Manures
Growing Forage Legumes in Rotation with Annual Crops
Improving Soil Fertility with Green Manure Legume Crops - Frequently Asked Questions
Legume Cover Crops as a Fallow Alternative
Legume Green Manuring
Management of Sweetclover for Nitrogen Supply, Soil Conservation and Weed Control
Reduced Till Termination of Sweet Clover
Selecting Green Manure Crops for Soil Fertility
The Basics of Green Manuring
Effects of Sweetclover Cultivars and Management Practices on Following Weed Infestations and Wheat Yields
Integrated Approaches to Managing Weeds in Spring-Sown Crops in Western Canada
Integrating green manure and grazing systems: A review - available in PDF format only
Long-Term Assessment of Management of an Annual Legume Green Manure Crop for Fallow Replacement in the Brown Soil Zone
Productivity, Water Use and Nitrogen Fixation of Annual-Legume Green-Manure Crops in the Dark Brown Soil Zone of Saskatchewan
Suitability of Undersown Sweetclover as a Fallow Replacement in Semiarid Cropping Systems
Sweet Clover Termination Effects on Weeds, Soil Water, Soil Nitrogen and Succeeding Wheat Yield
The Green Manure Value of Seven Clover Species Grown as Annual Crops on Low and High Fertility Temperate Soils
The Nitrogen Dynamics of 1-, 2- and 3-Year Stands of Alfalfa in a Cropping System
The Potential Role of Annual Forage Legumes in Canada: A Review
The Use of Annual Legume Green Manure Crops as a Substitute for Summerfallow in the Peace River Region
Yellow Sweetclover, Green Manure and Its Residues Effectively Suppress Weeds During Fallow
Yield of Dry Matter and Nitrogen from Tops and Roots of Sweetclover, Alfalfa and Red Clover at Five Stages of Growth