Green manures

Green manures are crops grown specifically for building and maintaining soil fertility and structure, though they may also have other functions. They are normally incorporated back into the soil, either directly, or after removal and composting. Green manures have generally been little used in the United Kingdom by conventional (nonorganic) producers, but have been enthusiastically adopted by organic producers.

Why use green manures?

Green manures are crops grown within a rotation for the purposes of:

  • building soil organic matter and soil structure
  • supplying nitrogen and other nutrients for a following crop
  • preventing leaching of soluble nutrients from the soil
  • providing ground cover to prevent damage to soil structure
  • bringing crop nutrients up from lower soil profiles
  • smothering weeds and preventing weed seedling growth

Green manures for building soil fertility during conversion

It is quite common to grow green manures, particularly legumes, during the land conversion period to build soil fertility and structure.

On arableaid registered land this is sometimes done on land under a 'set-aside' requirement, and some concessions are available for organic producers as to how legumes can be utilised as set-aside crops.

Green manures in field vegetables

Where a farm-type rotation including vegetables is in operation, the incorporation of grassclover leys and grazing livestock probably reduces the need and opportunity for using green manures. In this situation, field vegetables are likely to be grown as a rotational break that also allows reseeding of the leys.

Green manures might, however be grown say between two vegetable crops, or a vegetable crop and one or two cereal crops, if an extended break is desired.

Green manures in market gardens

In a market garden, the situation may be very different, particularly if acceptable manures free from Genetically Modified Organisms (GMOs) are not readily available.

Because of restrictions on sources of brought-in manure, concerned with maintaining freedom from GMOs, it is imperative that market-garden vegetable growers make maximum use of green manures for providing organic matter and nutrients, and as a major part of the weed control programme.

In practice this means that within the cropping rotation green manures should effectively account for:

  • at least 1/4 of the rotation when supplemented with manures
  • 1/3 to 1/2 of the rotation when manures are not available

Moving green manures

Green manures do not necessarily have to be incorporated into the ground on which they were grown. It is feasible to cut a green manure and incorporate it into another field or area either fresh, wilted or after composting.

However, the rotation should be designed that no area is over-exploited for growing green manures for use on another part of the farm.

Composting green manures

In some circumstances it is feasible to cut and compost a green manure crop for application to another part of the farm.

An example of this would be taking and composting a cut from a pure red-clover or redclover/Italian ryegrass green manure crop, perhaps with the addition of straw to the mix. Further growth would not be cut, but would be regularly topped and mulched back onto the soil, taking care not to smother the clover and ryegrass plants.

Feeding green manures

Green manures are not generally fed, but there are possibilities for utilising organic poultry or other manures to boost fertility without the manure being used at the same time as a crop for human consumption is being grown.
A light dressing is all that should be used with legume green manures, but a higher rate could be applied for feeding a non-legume such as fodder radish.

Green manures for weed control

Green manure crops are an effective tool for controlling weeds.

A fast growing, short-term green manure between crops will smother weed seedlings, and the cultivations necessary to incorporate the green manure will further reduce the weed burden.

This can be taken a stage further by undersowing suitable crops with a green manure such as the small-leaved Kent Wild White clover, which will smother weeds, fix nitrogen for following crops and protect the soil surface.

A well-established over-wintering green manure will smother weed seedlings and some, notably grazing rye, will prevent weed seed germination as they decompose after incorporation into the soil.

Green manures as over winter cover crops

After a crop has been removed, bare soil is vulnerable to damage to its surface and structure by rainfall, particularly during the winter. A green manure sown in the autumn will protect the soil surface.

In addition, nutrients still available for plant growth can be leached out of the soil and lost into watercourses where they can cause pollution. As well as acting as a cover crop a hardy green manure crop will mop up soluble nutrients, and retain them for release when the green manure is incorporated in the spring.

Cropping and grazing green manures

In some instances it is possible to take a crop from a green manure or to graze it with livestock for a while. Examples include harvesting field beans, saving a hay crop, grazing red clover or making silage from a cereal/legume mixture.

Whilst this may initially seem attractive, and provide an output from the green manure, it should be remembered that cropping and grazing removes nutrients and organic matter from the field which are not necessarily returned to the soil until later on.

The benefit of the green manure is therefore only partial and the consequences for the following crop and the rotation as a whole should be considered very carefully.

Types of green manure crop

Green manures are essentially of two types:

Legumes (clover family)

Legumes develop on their roots (in association with special bacteria) nodules that have the ability to take nitrogen from the air and convert (fix) it into a form that the plant can use. This can then be utilised by crops grown after the legume has been ploughed and incorporated into the soil.

Non-legumes

Non-legumes do not fix nitrogen, but can provide useful amounts of organic matter and retain nutrients that might otherwise be leached.
Some non-legume green manures are very quick growing and can be incorporated within gaps in production during the growing season.

Green manures suitable for Northern Ireland, and their uses
A wide range of green manures is available, but some are less likely to be successful in Northern Ireland due to soil and climatic factors. 

Legume-based
Crop or mixture Typical issues
Red clover Medium term in vegetable rotation
Red clover / Italian ryegrass 2-year in vegetable rotation or during set-aside or for composting
Alsike clover Medium term - tolerates acidic, damp soils
White clover Undersowing in vegetables and polytunnels -
for example, variety Kent Wild Wh
Field beans Autumn sown - overwintered - spring incorporated
Vetches (tares) Medium or long term (overwintered) - smothers weeds
Peas / oats / vetch Medium term - bulky - can be composted
Lupins Late spring sown – incorporated prior to autumn planting - tolerates acid soils – produces a lot of nitrogen
Non-legume-based
Crop or mixture Typical issues
Grazing rye Grazing rye Autumn sown - bulky - builds soil structure
Mustard * Late spring sown - short term catch crop
Fodder radish * Autumn sown - overwintered
Sunflower Late spring sown - incorporated prior to autumn planting
Buckwheat Short term during summer

* Beware of clubroot transmission

Brassicas as green manures

Because they are also susceptible to clubroot and can increase the level of infection, brassica (cabbage family) green manures should not be sown in close sequence with brassica crops. Crop rotations should be carefully planned to avoid this.

Incorporating green manures

In general, all green manures should be incorporated whilst they are still relatively soft and green, and before they have chance to set seed.

However, in some cases such as longterm vetches, tougher organic matter will also be present in the long stems.

Traditional deep ploughing may not be the best way to incorporate a green manure as it may simply bury the green manure in a layer beneath the furrow where it will only slowly decompose.

Ideally, green manures should be incorporated throughout the surface few centimetres of the soil where most rooting of the following crop will occur.

This is also the place where decomposing organisms are most active.

Lush growth should be cut or mulched and allowed to wilt. Several passes of a rotavator, or an alternative cultivator, at intervals of a few days will then incorporate the green manure throughout the soil profile.

It is important to make sure that incorporation is complete so that the plants do not re-root and become weeds in their own right.

After incorporating a green manure, allow two to three weeks before planting the next crop, particularly if it is to be sown directly into the soil.

Mulching green manures

During the growth of a medium to long-term green manure it may be desirable to mulch the crop occasionally by 'topping' it with a mower. The material removed will then decompose and be recycled to the growing green manure crop.

It is important that mowing and mulching are carried out whist the crop is relatively short and that it is not cut so low so that regrowth is prevented, or the plants smothered by mulched material. 

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