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The term green manure can have many different meanings. Some people consider green manures to be any live vegetation that is grown to be worked into the soil. For some, this may include almost any plant. Here we describe a green manure as: a cover crop that is grown specifically to improve soil fertility. However, green manures can provide many benefits beyond improving soil fertility.
Many different types of plants can improve the availability of nutrients by exploring the soil with their roots and supporting life in the soil. Although plants need a wide range of nutrients, nitrogen is often the most limiting to crop growth and yield. So farmers often grow green manure crops to improve nitrogen supply for the next crop. Only legumes, with their special relationship with nitrogen-fixing bacteria, are able to improve soil nitrogen fertility. With this in mind, the majority of green manure crops at least include legumes (as will be discussed further). Green manure legumes are grown not for harvest, but are terminated and worked into the soil to improve fertility.
A legume is typically defined as a plant with seeds that grow in pods. Legumes are part of the Fabaceae family, and have the unique characteristic of having nodules on their roots that house nitrogen-fixing bacteria. Common legume green manures include peas, faba beans, lentils, clovers, alfalfa, vetches and medics.
A cover crop is a crop that is grown primarily for the benefits it can provide to the soil and to the environment, rather than for harvest. Cover crops are grown to reduce soil erosion, improve soils, smother weeds, capture or supply nutrients, and break pest cycles.
Green manures are primarily grown for their benefits to soil fertility (for more information, see below and refer to the COG Field Crop Handbook’s Cover Crop Chapter). But, green manures are multifunctional, and can provide many benefits to your farm. Green manures support soils that are healthy: soils with an abundance of soil life, soils that actively cycle nutrients, and soils with good structure. Green manures can provide soil cover at critical timings, like early or late in the season, and are an alternative to summerfallowing. By covering soils, green manures protect soils and can reduce soil loss due to wind and water erosion. Green manures provide excellent opportunities for weed management and can help to break weed cycles. Similarly, green manures can provide an opportunity to break pest and disease cycles in rotations otherwise dominated by cereal and oilseed crops. Green manures also bring diversity to the farm, both in the crop diversity that they bring to the rotation, as well as their support of soil life, wildlife and beneficial organisms.
While there is an economic argument that can be made against green manures, as income is lost when a field is removed from cash crop production, remember this wide range of benefits that green manures bring to the farm. Not only do green manures supply nitrogen to fuel a crop rotation, they also benefit the farm in many other ways. Consider the costs of growing a green manure as a wise investment in the health and fertility of your rotation, rather than an economic loss.
Legume green manure crops can be an important source of nitrogen to support organic grain rotations. Legumes are special in that they can form a partnership with a unique group of soil bacteria called Rhizobia. Rhizobia can pull nitrogen from the air and make it available for plant uptake. This process is known as biological nitrogen fixation. The legume and bacteria enter an ‘agreement’ with each other: the plant will supply the bacteria with energy, while the bacteria supplies the plant with usable nitrogen. As part of the bargain, the bacteria set up shop on the roots of the legume, forming nodules.
Nitrogen cycling with a legume green manure. Figure modified slightly from that kindly provided by Dr. Julie Grossman and Sharon Perrone, University of Minnesota.
Forming this relationship has a cost to the plant. Energy that would normally be used for growth is instead used to feed the bacteria. Because of this, legumes are often slower growing than other crops. Legumes will tend to team up with Rhizobia when the soil nitrogen supply is not sufficient to meet their needs. If there is plenty of nitrogen in the soil when the green manure is planted, the legume will not bargain with the soil bacteria, and will use the available nitrogen instead. So, careful placement of the legume in the rotation is important for maximizing soil fertility gains.
The nitrogen made available to the legume by the Rhizobia is integrated into the legume’s tissues. When the green manure crop is terminated and tilled into the soil, the plant tissues decompose and release their nitrogen to the soil through the activity of soil microorganisms. This is known as nitrogen mineralization.
The amount of nitrogen that a legume green manure can supply is related to the amount of biomass that it produces. In general, expect about 2.5% of the total dry biomass of a green manure to be nitrogen. For example, if a green manure produces 1000 lb/ac of biomass, you can expect it to supply 25 lb/ac of nitrogen. In general, Prairie growers can expect from 25-250 lb/ac of nitrogen to be supplied by a green manure, depending on the green manure crop, the environment and climate (please see the Green Manure Selection Tool and Green Manure Crop Profiles for more information). Of this nitrogen, 20-60% can be expected to be available to the following crop, depending on the termination date and method, climate, and green manure (please see the Green Manure Management resource for more detailed information). While some nitrogen may be lost from the system through leaching or to the atmosphere, the majority of the remaining nitrogen not available to the crop immediately following the green manure will become available at a later date, helping to feed subsequent crops.
For more information on the soil fertility impacts of green manures, see the
COG Field Crop Handbook.
is nitrogen that is bound into the tissues of organisms, where nitrogen is used to form proteins and DNA. Organic nitrogen is strongly bound into these structures and is not readily available for plants to use.
is also referred to as plant-available nitrogen. Inorganic nitrogen is not bound in tissues, but is rather bound to hydrogen or oxygen to form NH4+ or NO3–. Inorganic nitrogen can be taken up by plants for growth, and can also be leached or released to the atmosphere.
is the conversion of atmospheric nitrogen into plant-available nitrogen Nitrogen fixation occurs naturally when lightning strikes and through the action of Rhizobia. The Haber-Bosch industrial process uses fossil fuel energy to fix nitrogen to make synthetic nitrogen fertilizers.
is the process of converting organic nitrogen into plant-available, inorganic nitrogen. This process occurs during decomposition with the activity of soil microorganisms.
Healthy soils are the foundation of organic agriculture. Soil health includes aspects of soil biology, soil chemistry and the physical characteristics of soils.
Like all organisms, soil organisms need food. Decomposing plant material is the foundation food of the whole soil food web. A healthy soil is full of life, ranging from microscopic bacteria and fungi, to earthworms, insects and small mammals. Together, they improve soil structure and nutrient cycling, and may suppress soil diseases.
Research has demonstrated that green manures can increase the numbers of soil bacteria and fungi, improve the overall diversity of soil microorganisms, and increase the activities of these soil organisms when added to typical Prairie grain rotations
The soil food web emphasizes the importance of soil biology in cropping systems.Image source: Soil and Water Conservation Society (SWCS). 2000. Soil Biology Primer. Rev. ed. Ankeny, IA: Soil and Water Conservation Society
The most obvious change in soil chemistry happens when green manures are tilled into the soil. This is when the nitrogen that was in the green manure tissues is mineralized, making it available to the subsequent crop. Green manures can also improve levels of active soil organic matter.
Green manures can also increase the availability of other nutrients, such as phosphorus and sulfur, by exploring deep into the soil with their roots and making these nutrients part of the green manure plant tissue. When the green manures decompose, these nutrients are made readily available to the next crop. Deep-rooted green manures like alfalfa can reach nutrients deeper in the soil than more shallow-rooted annual crops. Buckwheat is a non-legume that is sometimes considered a green manure due to its ability to make otherwise inaccessible soil phosphorus available to following crops.
Green manures can also help to build soil organic matter. Soil organic matter is important for many reasons, including improving nutrient cycling, supporting soil biology, and improving soil structure. Building organic matter in soils with low natural levels, particularly the Gray and Dark Gray soil zones, can help to reduce surface crusting. Traditional wisdom has told us that incorporating young, succulent plant tissues, as most legumes are when incorporated for maximizing nitrogen, contributes to the shorter-lived, active fraction of soil organic matter but not to the buildup of longer lasting stable organic matter. To build soil organic matter, guidance suggested that we instead incorporate older, tougher plant tissues with more carbon into the soil, as these tissues take longer to break down. Recent research has, however, suggested that there is more to this story. Rather than plant tissues contributing soil organic matter, this new work suggests that it is instead the spent bodies of soil microorganisms that build soil organic matter. In this case, using any legume green manure that supports a healthy, diverse and active soil community should help to build stable soil organic matter.
Green manure roots can create spaces and channels as they grow. This improves soil aeration and can help to break up compacted soils. This is especially true for green manures with deep taproots, like alfalfa.
The soil organisms that are associated with green manures can also affect soil structure. Threadlike fungi, known as mycorrhizae, produce sticky compounds that bind soil particles into aggregates. You may be familiar with these aggregates as soil crumbs. This crumbly structure also creates pores in the soil, making the soil better able to absorb and hold water. Soils with aggregates and pores are more resistant to erosion and till more easily.
Green manures can cover the soil at times when the ground might otherwise be bare. Covered soil is less likely to be eroded by wind or water. Plants on the soil surface reduce the impact of raindrops, slow water flow and reduce the impact of wind. Meanwhile, the roots of the green manure stabilize the soil.
A soil aggregate. Aggregates are important for the physical health of soils and are formed in biologically active soils.Image source: Soil and Water Conservation Society (SWCS). 2000. Soil Biology Primer. Rev. ed. Ankeny, IA: Soil and Water Conservation Society
There are a number of ways that green manures can be used to manage weeds in Prairie organic grain rotations. Green manures can disrupt weed cycles, may have a competitive advantage over weeds, and may even be allelopathic. A brief overview is provided here, with more detailed information provided in the Green Manures and Weed Management module.
Weeds are very adaptable – that is one of the characteristics that makes a plant a weed. So, if field operations take place at the same time each year, weeds will find and exploit the opportunities that they have to grow. These windows of opportunity are often the same year after year in annual grain rotations, so weed populations adapted to these windows build. Diversifying the rotation with green manures that have different management schedules (such as planting and tillage) and lifecycles can disrupt the weeds that dominate annual rotations
Biennial and perennial green manures can provide longer-term cover and competition while also building soil fertility. These green manures provide ground cover when weeds might otherwise have an excellent opportunity to grow, namely the late fall and early spring.
Annual green manures, when used wisely, can also disrupt weed lifecycles. Green manures are typically ploughed under before they set seed, giving them a shorter growth period. This allows more flexibility in the planting date, which in turn allows weed control operations to take place at different timings than are used for annual cash crops in the rotation. Similarly, there can be flexibility in when green manures are terminated. The early termination of a weedy green manure can help to reduce the weed population and the weed seed bank. The time between the termination of the green manure and the planting of the next cash crop can also be used for targeted weed management operations.
Green manure mulches can also reduce weed pressures. If thick enough, the residue of green manures terminated by rolling or mowing can be left on the soil surface to smother weeds in the fall and early spring.
For more in-depth information, please see the Green Manures and Weed Management module.
Green manures that grow quickly or abundantly can gain a competitive advantage over weeds. Legumes are often slow growing, and often are not very competitive in their early stages. Intercropping legumes with quick growing cereals or grasses can provide early season weed suppression. Once established, many legume green manures produce a lot of plant material that can outcompete weeds. Established peas, hairy vetch, alfalfa and sweetclover are generally found to be the most competitive in Prairie grain rotations. Slower and lower growing green manures, like lentil, may not be as competitive against weeds.
Seeding rates can also be increased in weedy fields to provide better ground coverage, giving the green manure a competitive advantage.
A well established, competitive stand of yellow sweetclover. Not only does the sweetclover provide nitrogen to the system, it is also an excellent competitor against weeds once established. Photo by Joanne Thiessen Martens.
Some green manures are able to produce chemical compounds that suppress the germination, growth or development of other plants. This is known as allelopathy. In allelopathy, plants release compounds into the soil environment that have a harmful effect on neighbouring or following crops.
Of the legume green manures commonly in use on the Prairies, sweetclover has been shown to be allelopathic. Sweetclover varieties with high levels of the compound coumarin, such as Yukon, have been found to be the most allelopathic, and have been shown to reduce weed communities.
Alfalfa may also have some allelopathic properties. Buckwheat, which may be used as a green manure for its suggested ability to release soil phosphorus, is also allelopathic.
Be mindful that allelopathic crops may also negatively affect the growth of the following crop in the rotation.
In the same way that green manures can interrupt weeds, they can also interrupt the life cycles of other pests. Adding legume green manures creates a space in the rotation where suitable hosts for cereal or oilseed insect pests or diseases are not available. Diseases that are specific to cereal crops, like blotch diseases, rusts, and leaf spots may be reduced in the rotation by providing a break from cereals with a green manure legume
On the other hand, when pulses are part of the rotation, a legume green manure may serve as an alternate host to a pest or disease. Blights and stem rot may be common to pulse cash crops in the rotation, such peas or lentils, and to a legume green manure. If pulse crops are part of your rotation, ensure that they are not immediately proceeded or followed by a legume green manure if these diseases are an issue in your rotation.
For additional information, see the Prairie Organic Grain Initiative Crop Rotation Factsheet.
Green manures can also provide habitat and food for beneficial organisms. They can support insects, birds and mammals that eat or parasitize insect pests. Green manures also support healthy soils, which can in turn contribute to plant health. Healthy plants are more resilient to attacks by insects and disease, while healthy soils may have the potential to suppress soil-borne diseases.
Some green manures may even deter migratory insect pests. Work on the Canadian Prairies suggests that a pea green manure can deter and even harm the grasshopper.
For more detailed information, please see Managing Green Manures.
Diverse agroecosystems are at the heart of organic production. Green manures can add to the diversity of crops being grown, while supporting the diversity of other flora and fauna that make their home on the farm. The biodiversity that green manures bring to the farm should be valued when weighing the benefits of adding green manures to the crop rotation.
Green manures support biodiversity. For example, this red clover green manure is providing food for this pollinator bee. Photo credit: Andy Hammermeister.
It is very important to consider the soil, climate and water availability on your farm before choosing a green manure. On the Canadian Prairies, the soil zone is a good, but general, indicator of soil moisture. The moisture availability for a crop is lowest in the Brown soil zone, and increases as you move through the Dark Brown, Black, Dark Gray and Gray soil zones.
Soil zones of the Canadian Prairies
Green manures, like all crops, draw water from the soil as they grow. In areas where moisture can be limiting, this water use must be carefully considered. Enough water must remain after the green manure is grown and terminated to support the healthy growth of the next cash crop. This is particularly important if you are in the dry Brown soil zone where water, rather than nitrogen, can be the main limit on crop yields. The use of green manures in dry areas or dry years can be a delicate balancing act between supplying nitrogen and conserving water.
Perennial green manure crops use a lot of water and can deplete soil moisture. Alfalfa is particularly noted for its water uptake, and should be avoided or approached with caution in drier regions, especially the Brown soil zone. But, alfalfa or red clover can be an excellent choice in areas with higher soil water and low soil organic matter, like the Gray and Dark Gray soil zones. Biennial sweetclover can also use quite a lot of water, and should be used with caution and careful management in the drier soil zones. Studies suggest that sweetclover green manures should be terminated early in their second year of growth even in the Dark Brown soil zone to conserve water.
Annual green manures often withdraw less water from the soil and may be the best option for drier regions. Estimates from studies in the Brown and Dark Brown soil zones suggest that annual green manure water usage can be similar to that of a wheat crop. There can also be variation in the water use and water use efficiency (how much biomass can be produced with a set amount of water) among annual green manure types. Indianhead lentil and chickling vetch are generally good choices in dry regions.
In dry areas, summerfallow is often used as a period for soil water to recharge. Tillage during the summerfallow year can deplete soil organic matter and make the soil susceptible to erosion. Soil moisture can also be lost through evaporation from uncovered soils. Prairie studies have shown that green manures can be grown in place of summerfallow without depleting soil water. When managed carefully, water use by the green manure and water loss through transpiration from plant tissues may be no more than water losses seen in a traditional summerfallow. To do this, annual legumes with low water use should be used and terminated early (at the bud stage rather than at flowering). This is a judgement call that will have to be made on a case-by-case basis with a watchful eye on the green manure stand, soil moisture and weather conditions.
Adjusting the timing of termination for your green manure crop can be important in dry conditions. Prairie studies show that early termination can reduce the drain on soil water, while also allowing time for soil water to replenish before the next crop. This does come with a tradeoff: less nitrogen will be fixed by the green manure crop if it is terminated before the bloom stage. Your choice of how to terminate the green manure stand can also impact soil moisture. Leaving green manure residues on the soil has been found to improve soil moisture; the residue traps snow and reduces evaporation losses from the soil surface.
For more information on crop choice for each Prairie soil zone, please see the Green Manure Selection Tools and the Green Manure Profiles.
Rhizobia bacteria are responsible for nitrogen fixation, and take up residence in nodules on the legume root. Photo by Brenda Frick.
Just as moisture varies by soil zone, so too does soil organic matter. Organic matter in virgin soils is highest in the Black soil zone, followed by the Dark Brown and Brown and Dark Gray soil zones, and is lowest in the Gray soil zone.
Soil organic matter is an important source of fertility. As organic matter is broken down and decomposed, nitrogen is released. In general, every 1% of soil organic matter can be expected to contribute about 10 lb of nitrogen per acre each year. Soils that are naturally high in soil organic matter, or where soil organic matter has been built through careful management over the years, can provide at least some plant-available nitrogen each year. However, the release of this nitrogen necessitates the breakdown and mineralization of soil organic matter, depleting the organic matter pool. Relying on soil organic matter for fertility is not a sustainable long-term strategy, especially when considering the additional benefits that soil organic matter brings to the system (such as the support of soil biology, water holding capacity, and good soil structure).
Entering into a relationship with nitrogen fixing Rhizobia is costly for a legume. It must divert some of its energy to support the Rhizobia, rather than using it to grow. When nitrogen is readily available, legumes will forgo their association with Rhizobia, and will use the nitrogen already available in the soil rather than fixing their own. So, in highly fertile soils, growers may not see the same nitrogen supply advantage from using leguminous green manures that may be apparent to others with less fertile, lower organic matter soils.
This would suggest that high organic matter soils, such as those in the Black soil zone may not respond as strongly to green manures. The fertility benefits of legume green manures may be at least partially masked by the high inherent fertility of these soils. It is likely, however, that even these high organic matter soils will gain from green manuring, both in fertility and through the many other benefits that green manures bring about.
Growers in soil zones with lower soil organic matter may be able to doubly benefit from green manures that build organic matter while also providing nitrogen benefits. In these areas, growers may lean toward choosing longer lasting perennial and biennial green manures. These longer lasting green manures have older and tougher tissues at termination that are thought to decompose more slowly than younger tissues, thereby helping to build soil organic matter. Recent research suggests that even shorter-lived annual green manures can contribute to building soil organic matter in these depleted soils, through their ability to support soil biology. Growers with soils low in organic matter should note that they may not see immediate fertility responses from their first few green manure crops, as soil organic matter is initially built.
When choosing a green manure, you should also think about your climate: the length of your growing season, frost dates, and hardiness zone.
Many green manure legumes are quite cold hardy at planting, but germination can be slow in cold soils.
While the perennials and biennials discussed here are hardy to Prairie winters, consider your hardiness zone when planting winter annuals like hairy vetch. Hairy vetch has the potential to overwinter in years with good snow cover, especially if it is planted later in the season and has not flowered prior to the winter. If your plan is to have the hairy vetch winterkill, be sure to plant early enough to allow time for it to reach flower (70-90 days), or you will need to have a backup plan to deal with actively growing hairy vetch in the spring.
Think about the length of your growing season and the ability of the legume green manure to reach the desired growth stage for termination in your region and hardiness zone. This can be particularly important when double cropping, or planting a green manure after a winter cereal harvest. Consider whether or not your chosen green manure will be frost-killed, and whether or not this is desirable for your operation. Most annual green manures will winterkill under Prairie climates.
For more information on the water and growing season requirements for green manure crops, please see the Green Manure Selection Tools and the Green Manure Profiles.
Green manures can be an essential part of the organic field crop rotation. With the value they can provide to the farm, green manure crops should be treated with care equal to that given to cash crops in the rotation. The establishment of a good stand can optimize the benefits provided by the green manure, both in terms of nitrogen fixation as well as secondary benefits.
There are a number of options that can allow green manure inclusion in most rotations. Options for full or partial-season green manure crops, annual or longer lasting crops, and pure or mixed stands can provide flexibility and allow most goals to be met.
There are annual (peas, lentils, beans, vetches), biennial (sweetclover) and perennial (alfalfa, clovers) legume options for green manures. With this variation, there is a suitable green manure for most any rotation on the Prairies. Annual green manures can occupy a full season in the rotation, or can even be planted after the harvest of a winter grain where the climate allows. Biennial green manures are often underseeded into grain crops, so although they grow for two years, only one year of the rotation is dedicated solely to the green manure. Perennial green manures are also generally underseeded into a grain crop, and can then be grown on their own for one or more years. Each green manure lifecycle brings its own unique advantages and challenges.
Common green manure species in use on the Canadian Prairies
For a summary of the characteristics of key green manure species for Prairie organic grain rotations, please see the Green Manure Selection Tools.
With the many species and lifecycles to choose from, green manures can be selected that fit most any rotation. Green manures can be chosen that will occupy a short window in the rotation, a full year, or multiple seasons. This allows the benefits of green manures to be obtained in many ways, depending on the rotation and needs of the system.
Options for the integration of green manures into the crop rotation. Image by Natural Systems Agriculture Lab
Annual green manures can grow quite quickly, and many flower in just over two months. In areas where the climate allows, annual green manures could be planted after the harvest of a cash crop, also known as double cropping. This can be an attractive option if you do not want to dedicate a full year of your rotation to a green manure. In double cropping, the green manure is often killed by frost, which reduces the management required for the crop. The green manure residues are likely to break down much more slowly in cold weather, so the nitrogen released as they decompose is less likely to be lost to leaching.
Double cropping green manures also comes with challenges. Establishment of the green manure can be difficult, especially if the preceding cash crop is late-maturing, or if the environment is cold or dry. Another challenge can be slow crop growth and the potential for the crop to be frost-killed before producing optimal biomass and reaching peak nitrogen content. The use of soil water should also be considered carefully, especially in the drier Brown and Dark Brown soil zones.
Researchers have successfully used double crops in the Black soil zone in south-central Manitoba, and with some success in the Dark Brown soil zone around Lethbridge, Alberta. Studies of climate patterns suggest that there is potential for double cropping in southwestern Manitoba and southern Saskatchewan, so long as moisture levels permit. Growing season length and temperatures likely would not support a successful double crop in areas further to the north and west. Even in those areas where the growing season could support double cropping, variability in late season moisture means that decisions to double crop should be based on the on-the-ground conditions at the time.
See Managing Green Manures for more detailed information.
Many growers dedicate a full year in the rotation to a green manure. A common argument against green manures is the loss of income that results from taking a field out of cash crop production for a full year. When facing this concern, remember all of the benefits that green manures bring: nitrogen to fuel the next stages of your rotation, healthy soils, biodiversity, and weed and pest management opportunities.
Annual green manure species are well suited to occupying a full season in the rotation. Many of the annual green manures suited to the Prairies do not take a full growing season to reach flowering, when they should be terminated for peak nitrogen (see the Green Manure profiles for information on when the various green manures flower). So, dedicating a full season to a green manure also provides opportunities for other activities. This could include: additional weed management operations before planting or after termination, the planting of a winter cereal after the green manure is terminated, or even the planting of a cover crop to further boost soil organic matter. This flexibility can be of great value in the rotation.
Full season annual green manures also open an opportunity for a period of reduced tillage in the rotation. A green manure can be terminated with a roller-crimper to form a thick mulch that can be no-till planted the following spring. The green manure mulch provides soil cover, weed control and the benefits of reducing the number of tillage operations. Research has demonstrated successful integration of no-till green manure systems into organic Prairie grain systems, with spring-planted hairy vetch showing the most promise. For more information, see the Managing Green Manures resource.
Even longer-lived biennial or perennial green manures can be planted in a way so that there is really only one full season of the rotation dedicated solely to a green manure. While biennials or perennial legumes can be planted and terminated in the same season, they are slow to establish and will not likely generate a lot of biomass or nitrogen. But, the biennial and perennial green manures are well suited to being planted into an established cash crop. This is known as underseeding or relay cropping. With this practice, a cash crop is still harvested in the first year of the biennial or perennial green manure. The small-seeded, slow-growing perennial and biennial green manures also benefit from the weed competition provided by the grain crop. The green manure can be terminated after the cereal is harvested, or, in most cases, is left to grow on its own for at least another full season.
Relay cropping can be an attractive option, but there are points to consider before adopting this practice. The ability of a green manure crop to establish and grow well may be hindered by the existing grain canopy. Likewise, the green manure may compete with the grain crop and result in lower yields. The green manure may also interfere with the grain crop harvest. Relay cropping is best suited to areas with high moisture availability, and should be approached with extreme caution in drier areas.
For more information, see the Managing Green Manures module.
Multiple-season green manures of perennial or biennial legumes can also be included in the rotation. While this means fields are taken out of cash crop production for multiple years, there are many benefits. These long-lived stands provide not only nitrogen, but can also boost soil organic matter. Perennial green manures tend to have extensive rooting systems and taproots that can bring up nutrients from deep in the soil, improve soil structure and reduce compaction. Multiple-season green manures provide continuous ground cover, protecting soils from erosion and outcompeting weeds.
The costs of including longer-lived green manure stands must be carefully weighed against the benefits. For this reason, perennial legumes are often favoured on farms that include livestock. Here, the perennial green manure can be grazed or harvested as hay. This allows income from the field from sales of milk or meat while the green manure is in place. While many of the benefits of green manures are maintained when grazed or hayed, haying does reduce the overall green manure biomass and can impact the ability of the green manure to supply nitrogen to the subsequent crop, and can also remove other important soil nutrients. For more information, see the Farm Type Consideration section.
Perennial green manures also tend to require more water than shorter-lived species, and should be considered with extreme caution in drier areas, such as the Brown soil zone.
Planting mixes of green manure legumes with other cover crops can be beneficial. Common green manure mixes include peas and oats, vetch and barley, or sweetclover and red clover.
Green manure mixes.
Mixes that include more than one legume, like sweetclover and red clover can be a form of crop insurance, as each performs best in different conditions. For instance, sweetclover will perform well under drier conditions and on hilltops or ridges in fields, while red clover will perform best in wetter years, or in areas of the field that tend to be wetter.
Mixes of legumes and cereals have multiple benefits. The cereal emerges quickly and vigourously, providing early-season ground cover and weed control for the slower growing legume. The cereal will also take up any nitrogen in the soil, nudging the green manure toward forming an association with Rhizobia for nitrogen fixation. As the crop grows, the cereal acts like scaffolding, giving vining legumes a structure to climb. This improves light penetration and airflow, and can reduce disease. The cereal also provides additional biomass, and also contributes carbon to the system that slows decomposition and spreads out the mineralization of nitrogen to align more closely with crop demand.
When considering mixed green manure crops, be careful that the cereal does not come to dominate the stand. Plant the green manure at its full recommended rate while reducing the seeding rate of the cereal to 25-50% of recommended.
There are an almost infinite number of mixed crop options for green manures. Try out combinations that you think will work on your farm in small areas. For more information and guidance, see the Prairie Organic Grain Initiative Intercropping Factsheet.
Your choice of green manure crops may depend on whether your farm is dedicated only to cash crop production, or whether your farm also includes livestock. While all options could be viable regardless of farm type, some strategies may lend themselves more readily to farms that also include livestock.
A healthy rye crop grown after a green manure. Photo credit: Brenda Frick
On farms dedicated to cash crop production without livestock, green manures are a very important source of nitrogen. This is especially true where livestock manures or other fertility inputs are not readily or affordably available.
Cash crop farmers have many options for green manures. The needs of livestock need not be considered, so the door is open to most green manures. To minimize the interruption in cash flow, cash crop farmers often favour shorter green manure periods in the rotation. Depending on soil zone and climate, relay cropping, double cropping, or full-season green manures can be good options. Green manure mixtures may also perform well on cash crop farms.
There can also be benefits to including longer-lived green manures on annual cash crop farms. They can serve to diversify the rotation, break weed and pest cycles, and improve soils. Consider creative options, such as harvesting seed, producing honey, custom grazing, or exchanging hay for manure to offset the economic cost of longer green manure stands.
Livestock mob grazing a green manure. Photo credit: Brenda Frick.
Mixed farms, with both cash crop and livestock production, open the door to including longer-growing perennial green manures that can be used as livestock feed. Selling animal products changes the economics of longer-term green manure stands. But, livestock needs and nutrient dynamics must be considered.
Longer-term perennial forage legume green manures can be grazed. Grazing has also recently been explored as an option for termination of shorter-duration annual green manure stands (see the Managing Green Manures module for further information). Like haying, grazing green manure crops can help to offset the economic losses associated with taking a field out of production through the ability to sell meat or milk. Unlike haying, grazing green manures keeps much of the nitrogen and other nutrients in the field.
When considering what green manures to use when grazing, it is important to think about their attractiveness and the palatability for livestock. Avoid green manure crops that may be problematic or toxic for livestock. Perennial green manures are good candidates for grazing. Many of the annual green manure species are also good candidates for grazing. For more information on the grazability of green manures, see the Green Manure Selection Tools.
Annual green manures are often highly nutritive for grazing. As they are generally young plants when grazed, they contain higher levels of proteins and minerals than typically grazed grasses. In some cases, the nitrogen level of annual green manures may be too high for optimal grazing, so consider including a grass in the green manure mix. This can also offset potential bloat issues associated with some green manures. Grazing the green manure at a more mature stage can reduce the risk of bloat.
Consider whether your chosen green manure will regrow after grazing, and whether this is desired. Many large-seeded annual green manures will not regrow after grazing, while smaller-seeded crops and perennials have regrowth potential. For green manures that do not regrow, think of grazing as a termination method and wait until the desired date or stage of termination before grazing.
The nutrient dynamics of a green manure stand are altered by grazing. Grazed green manures add nitrogen to the soil both at the time of stand termination and as the livestock graze (for annual green manures terminated by grazing, this occurs at the same time). Grazing livestock will remove some nutrients, but will return back the majority through excretion. These excretions contain nutrients in plant-available forms, speeding up nutrient cycling. For uniform soil fertility, encourage livestock to distribute excrement throughout the field. This can be accomplished by moving animals through smaller paddocks, and by spacing out congregation areas like feeders and water sources.
With the speedy nitrogen cycling that results from grazing, consider including a catch crop in the rotation following a grazed green manure to limit nitrogen losses from leaching. Catch crops take up available nutrients, securing them in their plant tissues until they are terminated closer to the date of cash crop planting. But, make sure that there is enough time available for the breakdown and mineralization of the catch crop before planting a cash crop.
Harvesting green manures for hay can help offset the loss of income from a green manure in the rotation. Perennial green manures will often regrow after being cut for hay. When planning to harvest a green manure for hay, it is important to think about its regrowth potential, palatability and potential toxicity. It is also important to consider the implications that haying has on soil nutrients.
Haying reduces overall green manure biomass and reduces the amount of nitrogen that the green manure can add to the soil. If the green manure is able to regrow before termination, hay can be harvested without sacrificing much soil nitrogen. However, the removal of hay also removes other important nutrients, most notably phosphorus, from the field. Returning animal manure can help to offset these losses. The ruminant gut speeds up the release of nitrogen from plant tissues, so be aware that returning manure to fields results in readily and quickly available nitrogen.
Perennial legumes, whether used strictly as a green manure or also used for hay, should be considered with caution in the drier soil zones. The high water needs of these perennial green manures can have a negative impact on yields of following cash crops.
More detailed information on planting and termination is provided in the Managing Green Manures resource. However, it is important to consider planting and termination when selecting a green manure crop.
Field peas are a fairly large-seeded green manure. Photo by Brenda Frick.
Green manure crops can be an important component of the organic grain rotation, and should be managed as carefully as you would manage a cash crop. Proper seeding and the establishment of a full and healthy green manure stand will optimize the benefits that you receive from the green manure.
The availability of seed is perhaps the first thing to consider. Green manures are becoming more prevalent, and the supply of organic or untreated seed should follow. It is important to source seed early to ensure that the green manure that you want to use is available.
The cost of seed should also be considered. Larger-seeded annual green manures require a high seeding rate to ensure good coverage, which can lead to higher seed costs. Follow seeding rate recommendations. If planting into a weedy field, increase the seeding rate (1.5-2x) to better outcompete weeds. Be sure that the seed you choose can be planted at a high enough rate to establish a good stand at a price that you can afford.
The availability of the proper inoculant is also important. Legume inoculants are preparations of nitrogen fixing Rhizobia bacteria. Each green manure legume is uniquely suited to a particular strain of Rhizobia, so be sure to use the proper inoculant (see the Inoculant section of Managing Green Manures for details). For organic producers, it is also wise to check with your certification body to make sure the chosen inoculant is permitted – some inoculants have been bioengineered or contain other ingredients that may not be permitted for organic production.
Choose a green manure that can be seeded at a time that meets the needs of your rotation. A perennial or biennial can be planted as a relay crop into a cash crop, or can be planted in the spring for a full season or multiple seasons of coverage [link to previous section]. Annual green manures lend themselves well to green manure mixtures, and often provide some flexibility in planting timing. They can be planted in the spring as a full-season green manure, or can be double cropped with a winter cereal cash crop in some environments. The key is to ensure that the green manure you select lends itself well to the planting timing, or flexibility in planting timing, that best suits your rotation and your needs.
If considering a mixed stand of green manure legumes and non-legume cover crops, consider the planting requirements for all crops. Can the seeds all be planted in the same operation, or will they each require their own seeding operation? If multiple seedings are required, consider perpendicular plantings to improve coverage. Consider the role that each plant in the mixture will play, and whether all crops can perform well together. Consider whether the crops you are using have shade tolerance. For more information, see the Prairie Organic Grain Initiative Intercropping Factsheet.
Consider the temperature and soil moisture that are likely to be available when you plant. Think about whether the planting date of the green manure crop will overlap other key planting or management dates for cash crops in your rotation, and whether you have the equipment and labour resources required.
For more detailed information, see the Managing Green Manures module.
Termination of a productive yellow sweetclover green manure stand with discs. Photo by Laura Telford.
Choose a green manure that lends itself well to your needs for termination, both in termination timing and method.
Ensure that the green manure you select will reach the desired level of maturity in your climate. Peak nitrogen accumulation is reached by the bloom stage. Consider whether your goal is to get as much nitrogen as possible, or if you will need to balance this with water conservation or weed management.
Think about how much biomass your chosen green manure is likely to produce – see the Green Manure Profiles and Green Manure Selection Tools for estimates. Is your equipment able to properly terminate or incorporate this biomass? With some high biomass crops, mowing before tillage may be needed. Specialized equipment is needed when terminating by roller-crimper.
Consider whether termination operations will overlap with other harvests or management requirements for other cash crops in your rotation, and whether you have the equipment and labour resources required.
Think about whether the green manure will be killed by frost, and whether or not this is desirable.
Consider whether you will incorporate the green manure immediately, leave crop residues on the soil surface, graze, or introduce a no-till phase into your rotation. Ensure that the green manure crop selected aligns well with these needs.
Think about nitrogen release from your terminated green manure. This will vary based on the environmental conditions (faster if warm and moist, slower if cold or dry) and termination method. As well, the inclusion of a cereal in the mix can increase the carbon content of the green manure residues, which can slow nitrogen mineralization.
All of the options and points to consider may make choosing a green manure seem like an overwhelming task. Taking the decision step-by-step can help you to focus on narrowing choices down to a few options that will work best on your farm.
Step 1: Consider which green manures are best suited to your soil zone and climate. Refer to the Green Manure Selection Tools, which includes an easy reference table of how each main green manure type might perform in your soil zone.
Step 2: Consider your rotation. Think about how a green manure might fit best in your rotation. Of the green manures best suited to your region, think about which might best fit your operation. There are many green manure properties detailed in the Green Manure Selection Tools that may be helpful.
Step 3: Consider your farm. Do you have livestock that you also want to support with the green manure? Would you prefer to use an annual green manure? Of the options you’ve narrowed down with steps 1 and 2, consider which would work best for your farm operation. Again, referring to the Green Manure Profiles and the Green Manure Selection Tools can help you tease out the options.
Step 4: Consider the management of the green manure. Now that you’ve narrowed down your options, consider the management requirements of your remaining choices. Consider whether you have the time and equipment needed, and whether seed is readily available and affordable.
To help you through the process, also think about what makes a good green manure. You may also find this list of questions to ask yourself when choosing a green manure helpful.
Characteristics of the main legume green manure species for the Canadian prairies (PDF Download)
Suitability of Commonly Used Green Manure Legumes to the Main Prairie Soil Zones (PDF Download)
This table provides information based on typical climate and moisture conditions in the five Prairie soil zones. In atypical years, refer to the recommendations for zones with typical conditions closest to what you are experiencing. For instance, in droughty years in the Black or Gray soil zones, refer to recommendations for the Brown or Dark Brown soil zones.
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The Green Manure Toolkit was Developed by the Organic Agriculture Centre of Canada.