Green manures are plants that are grown specifically to benefit the soil, replacing nutrients, improving soil structure and increasing organic matter content. All soil fertility cannot be derived from plant-based compost. Shortages of raw materials and the problem of removing crops from the garden, combined with losses due to leaching and oxidation means that there will always be a shortage of compost available . To maintain organic matter levels in the soil, gardeners must also rely on extensive use of green manures, particularly legumes, for nitrogen and deep-rooting green manures for the recovery of phosphate and potash from the subsoil.
Fertility building with a ley
Summer sown green manure between rows of onion. Photo: Stephane Groleau
A ley is an area of your plot taken out of cropping production and replaced with growing green manures for fertility building. Some green manures are from the legume family and have the ability to take up nitrogen from the air, tapping a free source of soil fertility. Red clover and lucerne are the usual nitrogen-fixing green manures chosen for the ley, although mixtures including these and grasses are also widely used.
The green manure ley may grow for several years and has the benefit of improving soil structure, as the deep roots of a green manure like clover penetrate and break up the soil and the subsoil and the root channels remain long after decomposition. A grower cannot create such a complex and intricate network of tiny air pores and drainage channels with a fork. A subsequent crop will be able to take advantage of this improved soil structure.
A simplified rotation that you might like to try on your patch may be:
Plot 1 – Fertility building ley (lucerne or clover sown early spring)
Plot 2 – Potatoes (followed by an overwinter green manure, e.g. cereal rye)
Plot 3 – Brassicas (undersown with phacelia)
Plot 4 – Legumes / Alliums (followed by an overwinter green manure of vetch)
Plot 5 – Roots and salads
How does nitrogen fixation work?
Nitrogen-fixation is essential to the cycling of nitrogen out of the atmosphere and into the environment occupied by living organisms. There are a group of nitrogen-fixing bacteria called rhizobia that have a special intimate relationship with leguminous host plants: peas, beans, pulses, peanuts, vetches, lupins, lucerne and clover.
The rhizobia live in a free state in the soil and exist quite happily in this way until a legume is planted into the ground close to where they are living. As the legume seedlings develop, their roots start to secrete substances into the soil, which attract the rhizobia nearby. The bacteria eventually enter the roots and stimulate the formation of swellings, called nodules, inside which the microbes multiply. At the same time, the bacteria take on different shapes to such a degree that they no longer look much like the soil rhizobia from which they came. For this reason, in the roots they are called ‘bacteroids’ and these now have the ability to fix nitrogen from the air.
In exchange for a share of the legume’s sugars manufactured by the leaves and stems of the plants, the bacteroids pass on nitrogen in a usable form to the host plants and to adjacent plants and leave a surplus in the soil to be taken up by subsequent vegetables via rotation.
Carbon-rich green manures for building humus
Digging in young lush green manures will add immediate nitrogen and stimulate activity in the soil, but will not generally boost the organic matter levels. On the other hand, mature, dry and carbon-rich residues like cereals and straw will take longer to break down but will boost the humus reserves, releasing nitrogen over a longer period of time. Carbon-rich green manures will decompose faster if they are chopped, shredded and kept moist before digging in.
Choosing a green manure
Green manures increase fertility and get life back into the soil. Like any organic crops, green manures should not be grown in endless monoculture, as they have their advantages and disadvantages for following crops in rotation.
Different green manures
Ley for longer-term fertility building before heavy feeding crops (e.g. potatoes) in rotation red clover, lucerne (pure stand). Grass mixes are not recommended prior to potatoes.
Catch crop for maximising nitrogen fixation crimson red clover, vetch.
Resistance to foot traffic soil damage in crops white clover, trefoil.
Paths white clover various types.
Undersowing outdoor crops red clover, lucerne, vetch, cereals.
Undersowing greenhouse crops Kent wild white clover, bird’s-foot trefoil.
Overwinter green manures that are winter killed phacelia, buckwheat, mustard.
Late autumn sowings cereals in general, especially rye.
Summer weed suppression phacelia, rye and buckwheat.
Reducing wireworm populations mustard.
Recommended nitrogen fixers by growers
Lucerne good perennial ley up to 5 years that is drought resistant. Needs a high pH, well-drained soil and inoculum to establish. Can be grown as a pure stand or with non-aggressive grasses. Apr-Jul. 2g/sq m.
Red clover good perennial ley up to 3 years that can tolerate wetter conditions. Roots have many branches and a taproot, high yielding in terms of green material, rapid recovery after mowing. Ensure eelworm-free. Can be grown as a pure stand or with more aggressive grasses, e.g. ryegrass. Apr-early Sep. 1-2g/sq m.
White clover shallow-rooted, low-growing clover suitable for paths for up to 9 years. Need strong-growing varieties to recover from mowing. Best established in spring. Apr-early Sep. 1-2g/sq m.
Crimson red clover annual, best for N fixing between crops and is usually only grown for 2 – 3 months. Jul-early Sep. 1-2g/sq m.
Vetch deep-rooted, quickly produces a large weight of green material especially in early spring. Suitable for undersowing when it is to be incorporated the following spring. Does not recover from constant mowing and should only be lightly topped once to control the first flush of weeds. Apr-early Sep. 8-15g/sq m.
Kent W W Clover low-growing clover suitable for undersowing greenhouse crops. Trim with shears. Apr-early Sep. 1-2g/sq m.
Bird’s-foot trefoil low-growing suitable for undersowing greenhouse crops and tolerant of shade. Trim with shears. Seeds can be expensive. Apr-early Sep. 1-2g/sq m.
Recommended ‘lifters’ by growers
As well as the nitrogen fixing green manures there are the non-legume green manures that are known as ‘lifters’. They mop up available nitrogen directly from the soil, which is later released back into the soil after the green manure is dug in.
Cocksfoot grass and chicory strong taprooted species for improving soil structure and building humus. Non-aggressive species that can be grown in a ley with red clover or lucerne. Apr-late Aug. 3g/sq m.
Ryegrass aggressive quick growing grass should be mulched back or dug in before seed heads appear. Good for foot traffic. Often included in ley with red clover. Sep-Nov. 2g/sq m.
Cereal rye the most winter hardy of cereals, which will germinate at 3ยบC. Best root system of annual cereals, can reduce N leaching by two-thirds. Incorporate in April when the seed head can be felt at the base of the stem. Sep-Nov. 23g/sq m.
Barley less hardy than cereal rye or winter wheat. Likes cool and dry conditions. Produces more biomass than other cereals and seeds are inexpensive. Sep-Nov. 15-30g/sq m.
Oats more sensitive than barley, but can tolerate wider pH, good on all soil types, fibrous roots. Sep-Nov. 15-30g/sq m.
Buckwheat good for summer use and grows quickly, incorporate before it goes to seed. Will grow on infertile soil, frost-sensitive. Apr-early Sep. 6g/sq m.
Rape superior at mopping up nutrients, frost-sensitive, brassica family and can carry club root. Mar-Sep. 2g/sq m.
Mustard frost-sensitive but provides large quantities of green material in 6 – 8 weeks. Brassica family and can carry club root, can be used to suppress wireworm populations in appropriate rotations, dig in before flowering. Mar-Sep. 2g/sq m.
Phacelia Fern-like leaf for weed suppression. Flowers attractive to beneficial insects especially bees. Incorporate after 2 months. April – August 1g/sq m.
Sowing a green manure by hand – timings
1) At the beginning of the sowing period (e.g. early May) generate a stale seedbed prior to sowing and broadcast the seed at the higher seed rate. The stale seedbed technique (also known as false seedbed) exhausts the weed ‘seed bank’ at the surface. The first flush of weeds is scratched out of the surface by a shallow cultivation. This will give the green manure more than a fighting chance against the weeds.
– Prepare a seedbed two weeks ahead of sowing the green manure.
– Once the fast emerging weeds appear (at about 10 days), carefully cultivate the area on a dry day by scratching it to a depth of 1cm using a metal rake, taking care to disturb only the very surface layer of the soil.
– Allow the weeds to wilt and die and then sow the green manure seeds immediately.
– Repeat and prepare a stale seedbed for a second time if there has been a prolonged wet period.
2) At the height of the sowing period (summer), e.g. June and July, broadcast the seed at the lower seed rate.
3) At the end of the sowing period (late summer/early autumn), e.g. August and early September, broadcast the seed at the higher seed rate.
With all three timings:
– Rake the seed gently into the soil.
– Pat the soil down with a roller, your feet or the end of the rake.
Overwinter green manures
Wind and water erosion may be prevented by using green manures, as ‘cover crops’. Since adverse weather conditions tend to be in winter, bare soil at this time is bad practice. The overwinter green manure roots hold the soil, and the top growth prevents most damage from splashing and surface run-off.
It must be remembered that the greatest loss of nutrients is due to leaching and not removing crops from your vegetable patch. Vegetable growing makes heavy demands on the soil and there is no point in building fertility and then allowing it to wash away with the winter rains. Green manures will ‘fix nutrients in carbon’ in the aerial parts of the plants and, even if the green manure dies over the winter, the nutrients are stored until the soil micro-organisms break them down and are unlikely to be leached.
The autumn-lifted crops which are not suitable for undersowing (see below) can be followed by a green manure once the soil is cleared. This will typically be potatoes and onions. Depending on the month it may be possible to sow clover before early September; or cereals from mid-September to early November.
Legumes do not fix nitrogen during the winter months. Therefore, non-legumes like cereals are more suited to the role of overwinter cover, as their early growth is vigorous and they can establish themselves quickly.
Undersowing green manures
Tomatoes undersown with trefoil at Hardwicke. Photo: Stephane Groleau
When using overwinter green manures, clovers and vetches need to be sown by August to get good establishment. There is a conflict of land use, as crops may be growing at this time. One way of getting around this problem, popularised in the UK by Iain Tolhurst and in the US by Eliot Coleman , is the technique of undersowing. Undersowing is where the green manure seed is sown underneath the growing crop. It is getting the best of both worlds – cropping and soil protection/increasing fertility. The undersown green manure provides places for foot traffic and other compaction damage when harvesting the vegetables.
Undersowing green manures will, even in a growing crop, add some nitrogen and organic matter to the soil. But, as Iain Tolhurst argues, its real value comes in ensuring that the soil is covered prior to the winter period, when so many nutrients will be lost from the soil due to leaching. Their use is also likely to favour the following crop. Undersowing usually takes place at the beginning of July.
Technique for undersowing
Eliot Coleman’s  tips for successful undersowing include:
– a clean, weed-free seedbed providing the motivation for regular weeding;
– weed at least three times using a hoe before undersowing;
– the last hoeing should be the day before undersowing;
– the crops are then undersown with the grower holding a container in their hands and scattering the seeds as evenly as possible.
Crops suitable for undersowing (adapted from Iain Tolhurst )
|Crop||Height||Preferred green manure||Dates green manure will germinate & cover||Optimum undersowing date|
|Brassicas||20cm / 8″||Red clover||Apr-early Sep||July or later|
|Leeks||When early leeks are fully grown||Cereals||Sep-Nov||Late Oct|
|Squashes & courgettes||6 leaves||Red clover||Apr-early Sep||July|
|Sweetcorn||25cm / 10″||Red clover||Apr-early Sep||July|
|Runner beans||50cm / 20″||Red clover||Apr-early Sep||July|
|Tomato/cucumber||50cm / 20″||Kent Wild White clover / birdsfoot trefoil||Apr-early Sep||July|
|Melon||6 leaves||Kent Wild White clover / birdsfoot trefoil||Apr-early Sep||July|
|Aubergine||20cm / 8″||Kent Wild White clover / birdsfoot trefoil||Apr-early Sep||July|
Crops not suitable for undersowing
Potatoes foliage too dense
Onions cannot tolerate root competition (see 3.5.7 for strip method)
Carrots root crops cannot tolerate competition
Parsnips foliage too dense
Lettuce growing period too short
Winter salads cannot tolerate root competition
Spinach, etc foliage too dense
Celery cannot tolerate root competition
Beetroot growing period too short – cannot tolerate root competition
Radish/turnips growing period too short – cannot tolerate root competition
Swede cannot tolerate root competition
Mowing the green manure
The good news is that green manures can generally out-compete the weeds, as long as they are sown evenly. It may be necessary to rogue the odd perennial weed. When managing clover or lucerne it is necessary to have a regime of mowing. The first mowing prevents the annual weeds from going to seed. Subsequent mowing depends on how quickly the plants are growing. It is important to prevent the green manure growing too long or they may be too much material for the mower to process and it might lie on the ground and be difficult to cut. Frequent mowing will ensure that the mulched material rapidly assimilates into the soil and provides the ideal conditions for earthworm breeding. However, it is a good idea to let strips of the green manure flower to encourage natural predators like hoverflies and lacewings.
The equipment needed for mowing is either a general garden mower; a strimmer; a scythe or a pair of shears.
Principles of mowing
– Make sure that the ground conditions are dry, so that the mower wheels/your feet do not compact the soil.
– Mow several times a year, making the last cut of the year in late September or early October.
– Do not allow the green manure to go to seed.
– Mow tightly – as close as possible to the base of the green manure stems to ensure that the annual weeds are also killed.
– If large quantities of material are deposited by the mower, this suggests that the green manure was too long before mowing. Ensure that in future you do not let the ley grow so long and spread the piles of material evenly with a rake so that the clumps do not kill the green manure underneath.
– Do not mow large areas at once. Insects will migrate to crops when green manure leys are cut and pests like aphids may increase because there is so much raw fertility. It is better to leave areas or strips for the insects to migrate to.
Digging in a green manure
– The green manure should be chopped and shredded at ground level several days before digging in to allow for wilting to take place. As rye can be particularly difficult to kill, the green manure can be pulled up, laid flat on the soil surface to wilt and then dug in.
– A green manure can be incorporated by inverting the soil using a ‘turfing’ technique.
– Cleanly cut the edge of the turf with a spade.
– Undercut the green manure turf at a depth of at least 10cm/4″ until it breaks.
– Turn the turf over by hand ensuring that no greenery is present on the surface.
– Leave for at least two weeks before trying to create a seedbed with a rake.
– If the green manure regenerates turn it in again.
Avoiding nitrogen lock-up
How quickly the green manure breaks down will be affected by soil temperature, moisture content and the carbon to nitrogen (C:N) ratio of the green manure. When green manures are dug into the soil, soil organic matter is one of the products of their decomposition. When a carbon-rich green manure such as a cereal is turned in, the soil micro-organisms multiply rapidly to feed on the organic matter, decomposing it but also consuming a lot of nitrogen doing so. This process leaves less available soil nitrogen (nitrogen lock-up) for subsequent crop growth, until breakdown has completed and the microbes begin to die and release their nitrogen to the soil.
A general rule of thumb is to leave the soil for at least two weeks after turning in the green manure, before attempting to create a seedbed for another crop. If you were to turn something in with an even higher C:N ratio than a green manure, for example sawdust, this might cause nitrogen lock-up for several years and should be avoided.
Weeds and green manures
If you have bare ground then weeds will generate. Therefore it is a good idea to have a green manure growing instead.
If you are reclaiming a weed-infested patch then it is a good idea to dig out all the weeds with a fork. Forking, which involves lifting, turning and breaking up the soil to remove the weeds, is the traditional way of clearing weed-infested ground. By loosening the soil and breaking it into crumbs, it is possible to remove the tiniest bits of roots. After all the weed roots are removed then sow a green manure straight afterwards (especially if digging prior to the onset of winter) and the green manure will also have a cleaning effect.
If you have a green manure ley (see above) in rotation then you will automatically be lowering your weed burden. After a ley break the weed population is likely to be at its lowest. However, establishing a ley can be very vulnerable to dock infestation, because there is the potential for huge numbers of dock seeds in the soil to germinate in spring and autumn. Fortunately, dock seedlings cannot compete well with grasses at this stage and so having a clover and grass mix will reduce the likelihood of early invasion.
Cabbage white butterflies and undersowing white clover
Stan Finch  has pioneered research into the benefits of undersowing for reducing competing insect problems. Many researchers have shown that the numbers of competing insects found on brassica crop plants are reduced considerably when the crop is:
– allowed to become weedy,
– intercropped with another plant species, or
– undersown with a living mulch such as clover.
Stan Finch carried out laboratory and field cage tests to determine how undersowing brassica plants with subterranean clover (Trifolium subterraneum) affected host plant selection by eight pest insect species of brassica crops. The pest species tested were:
Pieris rapae – small white butterfly
Pieris brassicae – large white butterfly
Delia radicum – cabbage root fly
Phaedon cochleariae – mustard beetle
Plutella xylostella – diamond back moth
Evergestis forficalis – garden pebble moth
Mamestra brassicae – cabbage moth
Brevicoryne brassicae – cabbage aphid
In all tests (except one in which the brassica plants were about three times as high as the clover background) 39-100% fewer of the competing insects (of all eight species) were found on the host plants presented in clover than those presented on bare soil. The differences were not accounted for by an increase in natural predators and therefore, lower colonisation accounted for fewer pest species.
However, undersowing with clover only reduced the small white butterfly oviposition (laying of eggs) by 40-60%, which may be insufficient to reduce the damage to acceptable levels. In these circumstances fleecing in early spring and netting in the height of summer need to be considered. Ensure the netting does not rest on the plants and also has a narrow mesh or the cabbage white butterflies will push their way through. The long-term solution includes encouraging natural predators like parasitic wasps by planting attractant flowers.
 Tolhurst I (2002) Undersowing green manures in vegetable crops. In Growing Green International magazine no.9 page 22 reprinted with kind permission of the Soil Association. www.veganorganic.net/undersowing-green-manures
 Coleman, E (1995) The new organic grower. Chelsea Green publishing.
 Finch S & Edmonds GH (1994) Undersowing cabbage crops with clover – effects on pest insects, ground beetles and crop yields. IOBC / WPRS Bulletin 17(8) 159 – 167
This article was originally VON information sheet Num 8