Technical Guide on Green Manures in Market Gardening
This guide compiles all the knowledge and practices related to cover crops in living soil market gardening gathered by the GIEE led by MSV Normandy over the period 2021 - 2023. In total, 8 market gardeners committed to working on the theme of cover crops. As a result, several trials on the establishment of cover crops as well as on their composition were implemented. Several conclusive results as well as failures are shared here.
What is a green manure or a cover crop?
A cover crop refers to a set of spontaneous or planted plants covering the soil in a temporary manner. It can be a secondary intercrop, that is to say planted after a vegetable crop with the aim of producing biomass that will feed the soil life.
How is the organic matter produced by cover crops decomposed?
When the residues of a cover crop come into contact with a living soil, aerated, sufficiently warm and moist, a series of transformations begins. It is possible to group this transformation into two phases:
- the deconstruction period
- the reconstruction period.
The deconstruction period
After the destruction of a cover crop, the organic matter is fragmented by microorganisms (fungi, bacteria, nematodes, protozoa). The first substances decomposed are rich in sugars (young and "tender" leaves and stems). They provide energy. A significant release of heat, carbon dioxide, and water is observable during these reactions. The abundance of microorganisms is then very high.
Once the softest and most fermentable materials are decomposed, those that are less so are attacked in turn. Thus, cellulose (dry leaves & stems) then lignin (wood) are "digested", sometimes after a few months only, or several years in the case of the hardest woods.
Besides carbon dioxide and water, these transformations release many substances:
- mineral elements assimilable by plants (nitrogen salts, phosphoric acid, potash, magnesium, trace elements)
- organic molecules (enzymes, vitamins), poorly documented, some of which can be directly absorbed by plants.
Then, these molecules and mineral elements are either:
- absorbed by vegetation (if present on the soil)
- fixed in reserve on the clay-humus complex
- components of humus
- lost by leaching, lixiviation or released into the atmosphere as various gases.
This mass of organic products and various minerals resulting from the decomposition of the cover crop and that of the decomposers (mainly bacteria) alive or dead (bacterial humus) constitute what is called the "transient products". They are called so because they are in a transient form between fresh organic matter and humus: they persist for a short time (a few months at most).
The reconstruction period
It corresponds to humification, that is the appearance of humus from the elements formed during the first period. Humus is made of long molecules called "polymerized": resulting from the joining of smaller molecules from the transient products. These molecules are very diverse depending notably on the conditions in which they formed.
Let us remember especially that humus is:
- poorly soluble or insoluble in water
- capable of binding to clay to form the clay-humus complex
- capable of slowly decomposing to provide nutrients to plants
- improver of soil physical properties (better water retention, porosity)
What is the role of transient products in the soil?
Transient products represent a large and varied mass of nutrients for plants. It is also understood that if they are released while the soil is bare, part of them risks being lost by leaching into the soil and "evaporation" into the air.
Conversely, on a soil without transient products, the crops must find their nutrients in the "reserves" of the pantry, i.e. humus, or in fertilizers, mulch or other organic matter brought to them.
Moreover, the release of transient products is concomitant with a microbial proliferation, which plays a positive role in soil structure. Organic substances such as hormones, vitamins and amino acids are present in transient products. These substances help plants maintain good health, resist diseases and pests.
Maintaining a permanent stock of transient products
Nature is capable, without human intervention, of high plant production, even though uncultivated lands are often the least fertile and receive neither fertilizers nor treatments. Indeed, forests are permanently occupied by a large stock of transient products as long as climatic conditions allow. Mulching of plant debris is permanent; their decomposition continues as long as temperature and humidity conditions are suitable.
In contrast, cultivated lands produce transient products for a short time on average; in small quantities (especially if little organic matter is returned to the soil or deposited on the surface).
However, it is possible to ensure an almost continuous production of transient products. Green manures and mulches are excellent levers to maintain a permanent stock of transient products.
Concept of carbon/nitrogen ratio
Transient products are thus, roughly, the result of the first phase of decomposition of organic matter. Their existence is closely linked to the presence of decomposing organic matter. This is why it is important that the soil is constantly covered by a layer of mulch, straw, cover crop residues or compost.
However, it is also necessary that this soil/mulch cover is balanced with a good ratio between rapidly fermentable materials (sugars), those that decompose more slowly or very slowly (cellulose and lignin) and those rich in nitrogen. In this way, decomposers have at their disposal a food both easy to digest and sufficiently substantial which allows them to work quickly, well and regularly and continuously. This ensures, for plants, a regular and continuous supply.
To categorize rapidly fermentable organic matter and those that decompose slowly, the carbon/nitrogen ratio or C/N is used. In fresh organic matter, the C/N ratio is low (30 or 40) if the tissues are young or higher if they are old (around 100). When organic matter decomposes, its C/N decreases, because carbon leaves faster (as CO2) than nitrogen (as nitrates).
The C/N ratio of humus is close to 10. However, materials with a high C/N, like straw, must "lose" a lot of carbon and/or take up a lot of nitrogen to reach a C/N similar to humus. Is this a disadvantage? It is problematic if nitrogen is taken from the soil to the detriment of crops. It is not if nitrogen is not taken to the detriment of crops. It can become an advantage if this nitrogen is not only not taken to the detriment of crops but comes from a free source and is then stored in the soil, which is enriched at no cost.
Unfortunately, the concept of the C/N ratio is insufficient: it gives an interesting snapshot of the evolution of organic matter at a given moment but does not say much about how this organic matter decomposes. It does not sufficiently guide towards the sought free element sources, notably nitrogen.
The sugar/cellulose + lignin/nitrogen ratio
The C/N expresses a static state. However, consider the notions of "slow" and "fast" nitrogen and carbon. To do this, it is possible to express them by the following ratio: sugars/cellulose + lignin/nitrogen. The idea is to reflect the appearance and more or less important and sustained mobilization of soil elements on which the farmer can exert influence via rotation, organic matter input, soil work.
Cellulose and lignin evolve slowly, but their decomposition is accelerated by the presence of sugary and nitrogenous materials acting as a "starter" for microbial development. If these organic materials are buried, decomposer microorganisms take the nitrogen they need from the soil, which is momentarily impoverished. If they are properly mulched on the surface, aerobic microorganisms and atmospheric nitrogen fixers can act and provide an additional and free nitrogen supply. It is therefore important to avoid burying cover crop residues!
Knowing only the C/N ratio does not inform about the origin of the nitrogen that allows cellulose decomposition nor about the speed of reactions. However, if it is known that sugars are abundant (and the carbonaceous matter is mulched on the surface), one can hope that decomposition will be active and provide free nitrogen to the soil. If sugars are lacking, decomposition will be longer and more laborious; the absence of nitrogenous materials will delay it further. Indeed, "hard" carbonaceous materials (cellulose, lignin) are attacked only by bacteria and fungi. To decompose this type of organic matter, bacteria need nitrogen and cannot take this element from the air. Consequence: straw placed alone on the soil surface decomposes slowly.
"Soft" materials rich in sugars added to straw constitute a balanced food for azotobacter, which then proliferate by fixing atmospheric nitrogen. Nitrogen benefits bacteria, allowing them to multiply and greatly intensify their consumption of straw. The latter then disappears much faster. Overall, there is a real chain reaction that accelerates progressively.
Depending on the type of rotation followed, the sugar/cellulose + lignin/nitrogen ratio is good or, on the contrary, more or less unbalanced. In market gardening, a great diversity of crops is grown but vegetables are rich in sugars, need nitrogen and rotations are poor in cellulose. In this case, rye + vetch mixtures are suitable to rebalance the soils.
A crop to adapt to the living soil market gardening context
Green manure cultivation is found in market gardening as in other agricultural systems, but it should be specified that it must be adapted to the market gardening context where a rapid succession of a wide variety of vegetables almost continuously occupies the available surface. In living soil market gardening, soils are rich in organic matter and nutrients. Under these conditions, the primary role of cover crops is not to provide additional fertility but to maintain good soil structure, promote microbial communities via root exudates which can be considered as sugar for microorganisms, limit losses by leaching and cover the soil.
Moreover, MSV systems grow a diversity of vegetables that are rich in sugars but poor in cellulose. It is important to have a good sugar/cellulose + lignin/nitrogen ratio to ensure good soil fertility. To correct this lack of carbonaceous materials, cover crops must contain species providing cellulose, such as grasses.
In addition, the beds cultivated in living soil market gardening are often covered with mulch and/or crop residues which make uniform emergence of cover crops difficult. It is therefore necessary to carry out several actions to maximize the chances of obtaining good emergence, even if it is not a cash crop. In practice, this is not so obvious since it requires mastering an additional crop that must be judiciously integrated into planning, hence the interest of this guide.
When intercrops do not allow it, and if space is sufficient, it may be interesting to integrate the green manure into the crop succession of cultivated plots in the manner of temporary grasslands.
Which species to choose according to sowing date?
It is important to choose the composition of the species mixture to sow according to the sowing date. Indeed, cover crops are grouped into three categories.
Winter cover crops
Sown between September and October to occupy the soil during winter and create a strong biomass the following spring. The species are said to be frost-resistant.
| Species | Family | Root type | Interests | Disadvantages |
|---|---|---|---|---|
| Rye | Grasses | Dense fibrous fasciculated | - High biomass production
- Significant carbon input if well developed in spring - Strong competition against spontaneous flora |
May regrow if not properly destroyed |
| Winter fava bean | Legumes | Taproot | - Root with drainage effect
- Nitrogen input |
- Low competition against spontaneous flora
- Expensive seeds (10 €/kg) |
| Common winter vetch | Legumes | Fasciculated | ||
| Hairy winter vetch | Legumes | Fasciculated | ||
| Winter forage pea | Legumes | - Germinates well through mulch
- Nitrogen fixation - Associates well with rye |
- Sensitive to excess water
- Low biomass - Low competition | |
| Forage radish | Brassicaceae | Powerful taproot | - Very good emergence when sown broadcast on mulch
- Rapid growth - Strong competition against spontaneous flora - Good biomass - Improves deep structure |
- Sensitive to severe frost (-8°C) |
| Phacelia | Hydrophyllaceae | Extensive and branched root system | - Honey plant
- Rapid growth - Improves surface structure |
- Easily regrows the following year
- Can become invasive - Seeds are expensive |
| Fenugreek | Legumes | Taproot | - Rapid growth
- Nitrogen fixation |
- Can become invasive |
| Oats | Grasses | Fasciculated roots | - Hardiness
- Covering |
- Moderate aerial biomass |
| Perennial ryegrass | Grasses | Dense fibrous fasciculated | - Excellent forage
- Crumbly soil structure at surface |
- Moderate aerial biomass
- Slow growth |
Summer cover crops
They need warmth to develop, sowing dates start in May, after frost episodes, and can be done until July.
| Species | Family | Root type | Interests | Disadvantages |
|---|---|---|---|---|
| Millet | Grasses | Dense fibrous fasciculated | - F | - Low competition against spontaneous flora |
| Foxtail millet | Grasses | Deep roots (60 cm) | - Forage
- Grows well under drought conditions - Weed suppressor |
|
| Sorghum | Grasses | Developed, deep (1.5 m) and structuring root system | - Very high biomass production under greenhouse and irrigated conditions
- Rapid ground cover - Possibility of several cuttings |
- Needs warmth
- Water requirement |
| Sunflower | Asteraceae | Taproot | - High cellulose production
- Seeds easy to harvest for replication |
- High pressure from pigeons, slugs and rodents |
| Amaranth | ||||
| Maize | Poaceae | |||
| Brazilian oats | Grasses | Dense fibrous fasciculated | - Strong coverage
- Rapid growth |
Autumn cover crops
Sown late summer, between August and September, to create biomass before winter. The species are generally frost-sensitive.
| Species | Family | Root type | Interests | Disadvantages |
|---|---|---|---|---|
| White mustard | Brassicaceae | Taproot | - Flowering 2 months after sowing
- Nitrate scavengers - Does not regrow after mowing |
- No mycorrhization |
| Forage rape | Brassicaceae | Taproot | - Seed hard to find | |
| Buckwheat | Polygonaceae | - Rapid growth
- Allelopathic - Germinates through mulch - Makes phosphorus available for following crops |
||
| Alexandrian clover | Legumes | Fasciculated | - Rapid growth
- Nitrogen fixer |
- Poor competitor against spontaneous flora
- Does not like acidic soils |
| Crimson clover | Legumes | - Very rapid growth
- Nitrogen fixer - Honey plant - Forage |
||
| Borage | Boraginaceae | Strong taproot system | - Improves deep soil structure
- Honey plant - Rich in potassium - Diversifies botanical families |
- Can become troublesome as it reseeds easily |
| Vetch | Legumes | - Tolerates sandy or calcareous soils
- Excellent forage |
||
| Nyjer | Asteraceae | Taproot | - No regrowth | - Poor emergence when sown broadcast
- Sensitive to competition |
Mixing Several Species
It is preferable to combine several species fulfilling various functions and having different germination conditions to ensure a homogeneous cover density. It is interesting to mix species with varied root systems to maintain good soil structure across multiple horizons. Combining botanical families provides a diversity of digestible organic matter for soil life.
It is possible to make your own mixes rather than buying premade ones to meet your needs. "You need to find a balance between excessive routine (adopting once and for all a mix 'that works' and no longer questioning it) and constant trial and error (always searching for the 'super green manure' and ending up with nothing stable or satisfactory). This depends both on each person's temperament and the technical conditions to which one is subjected." Joseph Pousset.
Understanding the pursued objective is necessary to sort through species and how to associate them. Thus, seeking a good sugar/cellulose + lignin/nitrogen ratio leads to carefully studying the crop rotation, the history of the land, as well as the types of organic matter added to the bed to determine which "side" it leans toward. Then one must choose a mix that corrects it. You can use the bioindicator plants guide to assess your sugar/cellulose + lignin/nitrogen ratio.
For example, associating oats and phacelia improves both surface and deep structure. When nitrogen is needed for demanding crops (cabbages, squashes), associating legumes and grasses allows benefiting from:
- The nitrogen-fixing effect of legumes.
- The "recovery" capacity of grasses.
- The complementarity of their root systems.
Many other criteria (forage production, control of spontaneous flora, improved fallow, etc.) can constitute objectives, more or less prioritized depending on the case, leading to the development of one mix or another.
In most situations, two to four species are sufficient to compose the mix meeting the desired objectives. It is advised to thoroughly mix the seeds with compost or sand to ensure homogeneous seed distribution during broadcast sowing.
What seed rate should I plan?
An idea of the seeding densities to plan is given in the cover sheets of Arvalis.
How to achieve good emergence under living soil conditions?
How to destroy the green manure?
Proper management of the green manure crop is crucial to guarantee its positive effect on the following crop, especially regarding its destruction. Attention should primarily be paid to the timing and the technique used to destroy the green manure.
Timing
The destruction date of the cover is chosen to align the period of strong mineralization of green manure residues with the absorption period of the following crop. Indeed, not all crops appreciate a litter of too fresh organic matter, so residues must be allowed to decompose. Generally, three weeks to one month of covering is sufficient to accelerate decomposition and avoid any risk of regrowth of the green manure as well as spontaneous flora.
However, it is important to let green manures grow as much as possible to optimize their ecosystem services (soil porosity, production of root exudates, carbon aerial biomass). Destroying a young green manure will considerably reduce the benefits and will not allow rebalancing the sugar/cellulose + lignin/nitrogen ratio. Therefore, a compromise must be found between maximizing biomass production and destroying the cover at the right time to avoid delaying the establishment of the following vegetable crop.
For a winter cover of rye, vetch, and faba bean, the ideal is to destroy the green manure at early flowering/milk grain stage since this is often when the plant is most balanced in nutrients (C/N), allowing good decomposition and limiting nitrogen hunger. This stage occurs around April-May depending on the pedoclimatic context.
Destruction techniques
The technique used to destroy green manure depends on the cover, the weather, the soil bearing capacity but above all the equipment available on the farm. That is why in low-mechanized systems, mulching with a tarp is mainly favored, especially since in green manure mixes, different species do not mature simultaneously. Prior shredding or rolling of the crop may be necessary.
| Type of destruction | Advantages | Disadvantages | Cost |
|---|---|---|---|
| Shredding | - High throughput operation
- Rapid degradation of small size residues - Reduces pressure from spontaneous flora |
- Low mulch thickness and coverage rate < 100% on covers with low biomass
- Little or no effect on grasses or crucifers - High fuel consumption - Possible destruction of wildlife (shred from the center of the plot outward) - Equipment availability |
50 - 55 € |
| Rolling with tractor & FACA roller | - Speed
- High soil coverage rate - Effective when done at the right stage |
- Slower degradation than with a shredder
- Requires a roller FACA - Less effective than a shredder if biomass is low (< 50 cm) because stems are not pinched properly |
20 - 25 € |
| Manual rolling | - No cost
- No tractor needed - Very good coverage rate |
- Moderately effective destruction
- Time-consuming on large areas (> 300 m²) |
0 € |
| Frost | - No cost
- Maintains residues on surface - No compaction on sensitive soils - Preserves structural improvements |
- Requires significant frosts in the region (-6°C)
- Limited choice of covers - Possible early destruction |
0 € |
| Rolling + Frost | - Accelerates cover degradation
- No soil structure degradation if soil is frozen - Low cost - Fast technique |
- Requires significant frosts and well-developed covers (effect of stem pinching)
- Penalizes loamy hydromorphic soils where cover matting can slow and limit soil drying in spring - Equipment availability |
20 - 25 € |
Points of caution
Winter covers grow exponentially during spring. This rapid growth is accompanied by a high water demand. Therefore, in case of lack of precipitation during spring, the winter cover may dry out the soil's useful water reserve. To remedy this, it will be necessary to irrigate the bed before planting the vegetable crop and until emergence to secure production.
If there is insufficient water and the weather forecast does not predict rain in the coming days, it is possible to advance the destruction date of the cover to limit profile drying.
If the cover is not sufficiently mulched, there is a risk of regrowth of the green manure during the following crop, either due to seed setting caused by too late shredding, or by suckering (phacelia) or tillering (grasses).
Results of trials conducted during the GIEE MSV Normandy
Summer cover of giant plants
Double biomass input via destruction of an autumn cover followed by a winter cover
The objective of this trial is to evaluate whether it is possible to achieve a double biomass input by successively sowing an autumn cover and a winter cover. Mid-August is the ideal sowing period for the autumn cover so it can produce significant biomass before its destruction in mid-October to sow the winter cover.
Mustard, Micheli clover, niger, and sunflower followed by rye, vetch, and faba bean
Buckwheat, niger, forage radish, white mustard, chia, sunflower, Micheli clover, phacelia followed by rye, vetch, forage pea, and faba bean
Combining green manures and crops
Meadow - vegetable crop rotation
In the case of an intensification objective for vegetable crops, without available slots to grow green manures or simply due to lack of space, it may be interesting to integrate green manure into the crop succession of plots cultivated similarly to temporary meadows.
Feedback
This section aims to gather various feedback from market gardeners who have integrated cover crops into their rotations, available on Triple Performance.
GAEC BioTaupes
You can find the farm portrait of GAEC BioTaupes here:
As well as a video by Xavier Dumas explaining how cover crops are integrated into vegetable rotations on the GAEC BioTaupes farm.
Ferme Biji Biji
You can find the farm portrait of GAEC BioTaupes here:
As well as a second video:
Alban Réveillé
You can find the feedback from Alban Réveillé, within the framework of the GIEE "Testing cover crops in organic market gardening in the Pyrenean foothills: from green manure to planting in a cover" led by CIVAM Bio Ariège-Garonne, here:
GAEC Légumes en Salat
Florian Bégard and Thomas Broué from GAEC Légumes en Salat, established in organic market gardening on a 1.6ha plot in Montsaunès (31), explain their motivations and objectives in using cover crops in open field and present their first trials:
CIPAN
Catch Intermediate Crop for Nitrate (CIPAN). These are fast-growing plants sown on a plot to limit leaching of mineral elements between two crops. Mustard is often used as a CIPAN. Note that in market gardening on living soil, leaching of elements during winter is limited because the soil, by principle, is never left bare. Moreover, no-till or superficial soil work reduces mineralization of elements. Added to this is the clay-humus complex which retains nutrients. However, it is always interesting to sow nitrate scavenger plants for their rapid growth.
