Which green manures

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 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.

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. 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. 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. 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.

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.

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. 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 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. Faba Bean. Field Pea. Indianhead Lentil. Chickling Vetch.

Hairy Vetch Short term duration. Generates abundant biomass Large seeds, expensive Biennial Yellow Sweet Clover Provides winter and early spring ground cover. Can be difficult to terminate Perennial Alfalfa. Red Clover Provides winter and early spring ground cover. Can be difficult to terminate Benefits and drawbacks of green manures with varying lifecycles. 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. 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. 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. There are an almost infinite number of mixed crop options for green manures. All rights reserved. The University of Minnesota is an equal opportunity educator and employer. Home Yard and garden How-to Managing soil and nutrients Cover crops and green manures. Quick facts Cover crops form a living mulch in gardens because they grow thickly among each other.

Planting cover crops Some gardeners sow cover crops plants in spring, especially in new garden plots to improve the soil and choke out weeds. Nitrogen fixation Many plants in the legume family, such as peas, beans, vetch and clover, grow in cooperation with soil-dwelling bacteria.

Most commonly grown green manures in Minnesota. Open all Close all. Annual ryegrass oats A thick mat of oats or ryegrass prevents erosion and keeps weeds out. Although they will grow in cool weather, both die over winter.

Chemicals released as ryegrass decomposes may keep small seeds from germinating, such as those of carrot and lettuce. Buckwheat and rapeseed These broadleaf plants grow quickly in warm weather and effectively smother weeds. Buckwheat flowers are a favorite nectar source for bees.

If you allow either of these species to flower and set seed, hundreds of plants will come up the next year. Buckwheat has no frost tolerance. Rapeseed may survive mild winters. Clovers Clovers fix nitrogen. Many are somewhat winter-hardy and may begin growth again in spring. The giant variety of white clover known as 'Ladino' makes a particularly good cover crop. Hairy vetch and alfalfa Like clover, vetch and alfalfa fix nitrogen. They will survive winter and grow again in spring. Actively scan device characteristics for identification.

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Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. Measure content performance. Develop and improve products. List of Partners vendors. What Is Green Manure? Fun Fact Many green manures especially clover varieties will eventually flower, helping to attract bees and other pollinators to your garden.

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