Green Manuring

Green manuring is a technique which goes back at least 3000 years, and until relatively recently European interest in it centered on the Mediterranean. Whilst it is mainly used in vegetable or crop rotations it can also be used in initial preparation of ornamental beds, lawns etc.

Recent interest in green manuring has developed as organic gardening and farming have taken on a more 'scientific' slant centered on the need to replenish organic matter in soils impoverished by chemical fertilisation.

What is green manuring

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

• improve soil structure
• increase soil fertility
• increase soil biological activity in the soil
• feed subsequent crops
• taking advantage (through legumes) of nitrogen in the air
• reduce soil erosion
• reduce nitrate (and other nutrient) losses
• suppress weeds
• break pest and disease cycles

However, green manures can also be grown, or used for :

• reclaiming an old garden
• building fertility in a new garden
• preparing vegetable and ornamental beds
• preparing ground for a lawn
• providing compostable material
• providing material for mulching

 

What are the 'costs' of green manuring ?

• cost of seed - varies depending on the species chosen
• planning of suitable rotations
• rotations which may require areas uncropped for a period, possibly a whole season
• effort involved in sowing and incorporation

 

Types of green manure plants

Season

1. summer green manures
   - often short season between other crops
   
2. over-wintering green manures
   - to prevent soil erosion, suppress weeds and retain soil nutrients
   
3. long term green manures

 

Plant type

1. 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 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
   

2. 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 clubroot 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, particulary for smothering over-wintering weeds
  
• grow a fast-growing green manure which will smother seedlings

• to prevent soil erosion

• avoid leaving ground uncropped and exposed to rainfall
  
• grow an over-wintering green manure to protect the soil surface

• to reduce nutrient losses

• avoid leaving ground uncropped and exposed to rainfall
  
• grow 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

• as compostable material

• cut material from longer term green manures and compost
  
• 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 rather than incorporation
  
• use spare ground for growing material for composting
  
• 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 (6") of soil
  
• make sure the roots are buried, to avoid re-growth

or

• gather green manure material from elsewhere, chop it well with a spade and, over a period of about 2 weeks, fork in until it is incorporated in the top 15cm (6") of soil

 

Greenmanures suitable for Northern Ireland

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
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

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This article is based on material from a number of sources