Using Green Manures

Green manuring goes back at least 3000 years on the continent, and has been enthusiastically adopted by modern organic gardeners around the World. Whilst it is mainly used in vegetable rotations it can also be used in initial preparation of ornamental beds, lawns etc.

Green manures and green cover crops revolve around three basic concepts: the need to replenish organic matter in soils, the need for a biologically active, healthy soil, and the avoidance of bare ground

What is green manuring?

At its simplest green manuring is the sustainable enrichment of soil by incorporating fresh (or wilted), green plant material, aiming to :

	Improve soil structure
	Increase soil biological activity through adding organic matter to the soil
	Add soluble nitrate to the soil (through legumes) from fixing atmospheric nitrogen
	Increase overall soil fertility and provide nutrients for subsequent crops
	Break pest and disease cycles within a rotation

What is a green cover crop?

A green cover crop is grown :

	So that the soil is not left bare and exposed to heavy rain so as to reduce soil erosion
	So as to reduce nitrate & other nutrient losses
	To suppress weed seedling growth

A green cover crop can be a green manure and vice versa, preferably both.

Green manures can also be grown, or used for :

	Reclaiming an old garden and raising the soils fertility
	Helping to prepare vegetable and ornamental beds
	Providing material for composting
	Providing material for mulching

What are the 'costs' of green manuring ?

	Cost of seed - this varies depending on the species chosen
	Time spent in planning suitable rotations containing green manures
	Rotations which may require areas uncropped for a period, possibly a whole season
	Time and effort involved in sowing and incorporation

Types of green manures

	Summer green manures - for a few weeks between other crops
	Over-wintering green manures - sown late summer to prevent soil erosion, suppress weeds and retain soil nutrients
	Long term green manures - sown spring or late summer when reclaiming land as a means of smothering fresh weed growth and growing organic matter for improving soil fertility and structure

Plant type

Legumes (pea and bean family)

As well as adding organic matter, legumes have the ability to take nitrogen gas from the air and turn it into nitrogen 'fertiliser' when the green manure decomposes after incorporation it releases this nitrogen (along with the all other nutrients it contains) to feed the next crop

Legume green manures, properly included in a rotation, can provide much of the nitrogen needed by following crops

Non-legumes

Non-legumes add organic matter to the soil, but little nitrogen. Cabbage family species should not be used close in a rotation with cabbage crops to avoid increasing club root infestation in the soil.

How can I use green manures ?

	To increase soil fertility - grow a longer term green manure and incorporate into the soil prior to making the bed
	There may also be an opportunity to cut some material for composting, mulching or using elsewhere in the garden
	To improve soil structure - grow any bulky green manure and incorporate at the end of the winter
	To suppress weeds grow a longer term green manure, particularly for smothering over-wintering weeds - grow a fast-growing short-term green manure which will smother seedlings
	To prevent soil erosion avoid leaving ground uncropped and exposed to rainfall by growing an over-wintering green manure to protect the soil surface
	To reduce nutrient losses avoid leaving ground uncropped and exposed to rainfall by growing an over-wintering green manure to protect the soil surface
	To break pest and disease cycles grow a green manure unrelated to crops either side of it in a rotation
	Consider having longer term green manure 'fallow' breaks in a rotation, particularly legumes which add nitrogen
	As compostable material cut material from longer term green manures and compost it grow a bulky green manure for composting rather than incorporation
	Use spare ground for growing material for composting
	As a mulching material cut material from longer term green manures and use as a mulch
	Grow a bulky green manure for composting and then mulching
	Use spare ground for growing material for composting and then mulching
	Use grass clippings as a mulch

How do I incorporate green manures ?

	Do not just bury a green manure by digging it in
	Cut a green manure close to the soil, chop it well with a spade and, over a period of about 2 weeks, fork in until it is incorporated in the top 15cm (6inches) of soil.
	Allow the cut material to wilt before starting to incorporate it.
	Make sure the roots are buried, to avoid re-growth
	Gather green manure material from elsewhere, allow it to wilt and then chop it well with a spade and, over a period of about 2 weeks, fork in until it is incorporated in the top 15cm (6inches) of soil

Green manures suitable for the UK

Always take your climate and soil into account when choosing appropriate green manuring techniques. Cooler summers, mild wet winters and heavy soils will dictate possible options.

Species	Sow	Growing period	Soil type	Nitrogen fixer	Other information
Winter tares(vetches)	Jul – Sep	up to Oct, or over winter	heavy, not too acid	yes	hardy
Mustard	Mar – Sep	2 - 8 weeks	moist, fertile	no	can carry clubroot
Hungarian grazing rye	Aug – Nov	autumn – spring	most	no	hardy
Alsike clover	Apr – Aug	few months to 2 years	damp, acid OK	yes	short-term perennial
Field beans	Sep – Nov	over winter	damp, heavy	yes	cut before flowering don't stop weeds
Buckwheat	Apr – Aug	2 - 3 months	tolerates poor	no	not hardy at all
Phacelia	August	autumn – spring	Not too heavy	No	Not always hardy
Red clover	Early August	Any time, useful long term	Avoid acid, add lime	Yes	Sow thickly, cut and mulch regularly

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Using Green Manures

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