Green Manures

Growing green manures is an age old practice: the Greeks planted fava beans in their fields, the Romans grew beans and lupine to enrich the soil, and the Chinese knew the fertilizing value of grass and weeds. [1] Green manures are an effective method for improving soil fertility, and intensive use of green manures can even eliminate the need to bring in outside sources of fertilizer.

See also: Vegan Organic Network’s guide to green manures.

Very simply, green manures are plants that are grown in order to be cut down and buried. This may seem strange at first, though green manures offer many advantages:

  • Avoid bare soils: Green manures can cover the soil when it would otherwise be bare, reducing nutrient leakage, erosion, and soil compaction.
  • Stimulate soil life: soil organisms are stimulated around the roots of the developing green manure plants, and the plant residues feed the earthworms and microorganisms when the green manure is cut down.
  • Nutrients and biomass: Depending on the variety, green manures can do the following: fix nitrogen, making nitrogen from the air accessible to the plants and soil; create significant biomass, leading to a stable humus; and make nutrients available by tapping deeply and breaking down the underlying rock structure.
  • Reduce weeds: a dense sowing of fast-growing plants can smother or prevent weed development; certain plants (ie. rye) produce substances in their roots that reduce the germination of other seeds.
  • Improve soil structure: roots and microorganisms build aggregates (crumbly structure), holding soils particles together and improving aeration.
  • Improve drainage: the green manure roots and earthworm activity improve soil drainage.
  • Improve water retention: green manures improve the soil structure and increase organic matter, which in turn leads to better water retention.
  • Enhance biological diversity:by increasing plant diversity through green manures, this will attract and host a wider diversity of fauna and beneficial insects.
  • Increase yields: with the improved soil structure and increased organic matter that green manures bring, there can be better yields with fewer inputs and lower expenses.

Which plants?

Any plant can serve as a green manure. Older or unwanted seeds can be sown as green manures, and the resulting plants can be cut down and incorporated with the soil. Vegetable plants that are in overabundance in the garden can be used, and even weeds will have a beneficial impact on the soil if we cut them down before they flower. Nevertheless, certain seeds that we can find through seed merchants, garden centers and farmers’ coops will have an especially positive impact when used as a green manure:

Note: dates for sowing the seeds will depend on your bioregion. Look for local resources to find appropriate sowing dates. The following dates are based on the climate in Quebec, Canada.

Gramineae (grasses) (rye, oat, perennial ryegrass, Japanese millet, sorghum, buckwheat) establish themselves rapidly and vigorously. Their dense root system (with a multitude of small roots in the first few centimeters of soil) quickly absorbs nutrients and promotes the formation of a crumbly soil structure. Grasses have the advantage that we can sow them later in the fall (up to the time of the first frost).

Legumes (clover, alfalfa, melilot, horse bean, trefoil) can fix nitrogen up to 150 kg/ha thanks to the bacteria (rhizobia) that associates with their roots. A leguminous green manure producing 3000 kg/ha of organic material contains about 90 kg of nitrogen, up to 90% of which is taken from the air via nitrogen fixation. (CPVQ 2000). The rhizobial bacteria have a symbiotic relationship with leguminous plants, inhabiting the roots of the plants and taking a small amount of the plants’ sugars, and in turn supplying the plants with a usable form of nitrogen. The rhizobial bacteria live naturally in healthy soil and will form a relationship with legumes when they are planted, though in certain cases it is more effective to inoculate the seeds with the bacteria beforehand. Could plant until August.

Cruciferea (cabbage family) (white mustard, hairy vetch, oilseed radish, rapeseed) are robust and produce substantial biomass. They grow for a longer period in the fall, but must be sown a few weeks before the first frost.

Phacelia Among the other popular species, phacelia is a melliferous plant, producing flowers that are prized by pollinators. It also brings a diversification of crop families to the rotation (Boraginaceae family).

Perennials (red clover, melilot, winter rye) have the advantage of surviving through winter. They’ll regrow with a head start in spring and will have time to develop a deeper root system capable of bringing nutrients to the surface. Such root systems also help with soil decompaction and keeping the soil fauna and flora active all year long.

Mixes of different species can bring the benefits of several types of plants. In a grass-legume mix, the grasses will quickly take up the soil’s nitrogen, forcing the legume to fix more nitrogen from the air to meet its own needs. Mixes also allow the prolongation of the growing season, like an end of summer sowing of buckwheat associated with a cruciferea :the cruciferea will continue growing after the buckwheat is killed by the first frost.

When and where to add green manures

Green manures can be introduced in a growing system at different points in the season:

  • Succession: the sowing is done before or after the main crop. A crop that ends early (ex. winter wheat, summer cabbage) leaves enough time to establish a green manure before winter arrives. This is very useful for farms that don’t have the time or the land for a full year of green manure. In fall, they also serve as cover crops to prevent erosion and nutrient leakage.
  • Intercropping: the green manure is grown between rows, or directly under the main crop (undersowing). A common practice is to sow hay and a summer cereal at the same time. After the cereal is harvested, the hay will grow fully. In hoed cultures, it’s possible to sow by broadcasting the seeds during the last hoeing. A mix of ryegrass/red clover can be established in corn by the end of June or beginning of July. With vegetables (ex. cabbage, tomato, pepper), once the transplants are well installed (about 2 weeks), they’ll associate well with dwarf white clover. Alleys can also be seeded (ie. red clover between the strips on plastic mulch for onions).
  • Full season: the green manure will occupy the area for one year or more. This gives time to replenish fertility (especially in poor soils) or to get new land ready. We will cut or mow it one or two times during the season. If needed, it can serve as compost material or for mulching. Even if the green manure is harvested in this way, it will still leave a significant root system in the soil (15 to 30% of the total biomass of plants is found in the soil). Eventually, what has been taken should come back in the form of compost.

How to plant green manures

Establishing a green manure:

  • The land should be free of weeds. If there are lot of perennials (ie. couch grass), a fallow of about four weeks should be done during a hot and dry period in summer. This consists of a regular passage (each week) with a chisel plow or a heavy cultivator to expose roots to the sun and to force weed seeds to germinate so the weeds can be dealt with in advance of sowing your green manure.
  • Green manure seeds must be evenly spread to assure a complete and uniform cover.
  • If there is an addition of compost, it is applied before sowing.

Burying a green manure

  • Use a chisel plow, a heavy cultivator, rotary instruments, a harrow or even a shallow ploughing (less than 20 cm).
  • Annual green manures that are destroyed by winter can be buried the following spring.
  • In clay soil, it’s advised to bury the green manure in the fall in order to take advantage of the frost/defrost effect. Though, this should be done when the soil temperature is below 10°C.
  • When burying, one must avoid working the soil in humid conditions to avoid compaction.
  • For a faster decomposition, the green manure can be cut or mowed.
  • In a garden, using a walking tractor (tracked rototiller) facilitates the work. Though, it is possible to avoid burying the green manure by instead covering it with a mulch (cardboard, hay, plastic) from the fall until spring.

Growing on uncultivated soils

To start a new soil and deter weeds effectively, the following technique with green manures can be efficient: a fallow in June, followed with a green manure of buckwheat buried in July after 5 to 6 weeks growth, then followed by a green manure of winter rye. This method is also a good way to get to know the land by observing the plant growth: seeing which areas are more fertile, or have poorer soils, or are humid.

In the end, even if the establishment of green manures has some associated expenses, it’s a winning practice since it reduces the need for off-farm inputs, while also holding nutrients, bringing in nitrogen from the air, and making minerals soluble. There are also increased yields resulting from an improved soil life and structure. So as Iain Tolhurst says [2], «the question is not about bringing enough nutrients to the fields, but rather, not losing the nutrients that are already there.» And green manures accomplish that function marvelously.

The importance of prairies

With their biological diversity and their abundance of older roots, prairies or long term green manures (more than a year) create a thick layer of organic material in the top inches of soil. This sod holds an abundance of nutrients and fixes nitrogen (when legumes are present). Up to 203 kg/ha of nitrogen can be locked up in a mix of oat, dactylis, brome, festuca and clover. [3]

Short term green manures are decomposed very fast, which is favorable to soil life and helps bring nutrients back to the soil However, with short term green manures there isn’t a genuine accumulation of organic material in the soil. Only fibrous materials have this property (such as small branches that are decomposed by Basidiomycota fungus). So until chipped branched wood becomes common practice in agriculture, prairies represent a good alternative if we want to maintain long term soil health without wasting farming capital or importing other farmers’ capital (ex. compost produced outside the farm).

Resources

Vegan Organic Network, guide to Green Manures

CPVQ. «Engrais verts et cultures intercalaires.» in Guide des pratiques de conservation en grandes cultures. Québec, 2000.

LAFRANCE, Denis. « Les engrais verts et la gestion des mauvaises herbes en culture maraîchère » dans La culture des légumes.

[1WARMAN , P . Prof. « Principes fondamentaux de la culture d’engrais vert », Ressources du sol et de la terre, Ministère des ressources renouvelables. 1981.

[2First stockfree certified farm in United Kingdom based solely on green manures for its fertilization.

[3La France, Maynard et Laverdière Recherche sur l’intensification des engrais verts en culture maraîchère (rapport en préparation).


21 September 2010
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