It is generally accepted, among organic gardeners at least, that the ground should be covered at all times, to discourage the spread of weeds and to protect the soil surface from the elements, be it from baking to dust during hot summers or from erosion. A popular way of achieving this soil cover, particularly on vegetable plots, is the use of green manures.
Green Manures are a crop that is grown specifically to add organic matter and nutrients to the soil. It is probably impractical for the average gardener but is extremely beneficial to allotment gardeners. It is of greatest benefit on light sandy soils.
The range of plants suitable as green manures is quite wide some mature in 10 weeks, others will happily stand over winter. With a little care, green manures to fill the varying 'time gaps' can be chosen and incorporated into the regular planting programme to provide all year round soil cover.
The most obvious value of a green manure is providing organic matter, however the soft green material rots down very quickly, leaving a small amount of stable matter in the soil. This organic matter is invaluable and on soils that are short of organic matter, any amount has to be beneficial. This introduction of organic matter will increase the amount of biological activity.
The basic requirement of a green manure is to achieve a dense blanket of vegetation. However, as many green manures belong to the same families as normal vegetable crops, i.e. Trefoil is related to legumes and mustard to the brassica family, it is sensible to co-ordinate their planning with normal crop rotation. Try to follow regular crops with a green manure of the same family so that although a similar second crop is grown in the same year, the standard four / five bed rotation is maintained.
The greatest value is in the nutrients that it provides. Deep rooted plants like Lupins and Red Clover will send roots down over 5 feet and take up the nutrients from deep down in the soil.
When these roots are dug in again they break down and provide the nutrients back into the topsoil where they are of more use to the next crop of vegetables or flowers.
In addition, Leguminous plants such as Beans and Lupins have the ability to 'Fix' the Nitrogen in the soil through the bacteria living in the tiny nodules on their roots. They take in Nitrogen from the air and when the plant is dug into the soil, the Nitrogen becomes available to the next crop grown.
On light soils in particular nutrients are lost during the winter due to rain leaching them out. Growing a green crop over winter will prevent this action by holding the nutrients in their roots. When they are dug back into the soil the nutrients become available again.
Green manure crops also help to suppress weed growth as they usually cover the ground well, providing shade and taking up the water that will discourage all but the toughest of weeds.
There are numerous plants that can be used as green manures, but when choosing one, ensure that it fits in with your crop rotation. For example do not grow mustard, (which is a brassica, the same family as cabbages), on the site where you have grown Brassicas prior to, or intend to grow Brassicas.
Prepare the soil by raking to a fine tilth. If the soil is very poor you may need to provide a little fertiliser before you sow to ensure that your green manure crop grows. Fine seeded crops can be sown either in rows or broadcast seeded, while the bigger seeded varieties can be sown in rows.
Generally green manures should be dug in before they go to seed. Those grown for longer term may be cut down when flowering to encourage new growth. Incorporating the crop into the soil must be done in the right way if the maximum benefit is to be obtained. Do not let the crop become too 'woody' before you dig it in or the rotting process will take nitrogen from the soil. If the crop has gone woody, water the area with a liquid fertiliser or manure once it has been dug in. If the crop is fairly large it is best to chop it before digging in. This can be done with a spade, which tends to be hard work. A strimmer or rotary mower or in the case of low growing crops like mustard, a cylinder mower can be used. Provide a period of wilting before digging in and do not dig too deeply, 6" is usually sufficient.
Types of Green Manure Crops
The main distinction between crops is if whether they can fix Nitrogen or not.
Green Manure Plants that fix Nitrogen
A deep-rooted tall perennial. Pros - Provides plenty of green matter and is very deep rooting. Being a Legume it fixes nitrogen.
Cons - Needs a long time to crop.
Sow 15g (½oz) per Sq. M/Yd. in spring. Dig in, in autumn or sow late summer to overwinter and dig in, in spring.
Broad beans or Flava Beans
An excellent plant.
Pros - Produces plenty of green matter. Fixes Nitrogen. Some of the crop can be left to mature and the beans eaten.
Cons - None.
Sow autumn or early spring 4" apart in rows 12" apart.
Low growing plant with deep root system.
Pros - supplies plenty of green matter.
Cons - will not overwinter.
Sow in spring or summer at 1oz to the Sq. Yd. in rows 6" apart.
Tall deep-rooted plant of the legume family.
Pros - Adds plenty of green matter. Will fix large amounts of Nitrogen and Phosphates.
Cons - none
Sow in spring in rows 6" apart with 3" between seeds.
Tall plant with extensive root system.
Pros - grows over winter when land is vacant. Provides large amounts of green matter. Fixes Nitrogen.
Cons - none
Sow during late summer as for Lupins. Can also be sown during spring and summer if land is vacant.
Green Manure Plants that Do Not fix Nitrogen
Tall plant with extensive root system.
Pros - Makes copious amounts of green matter. Attracts Hoverflies (which eat greenfly)
Cons - does not fix Nitrogen
Sow in spring or summer in rows 6" apart or broadcast sow at 1oz per Sq. Yd.
Plant with extensive root system
Pros - plenty of green matter.
Cons - none
Sow in late summer or autumn in rows 9" apart, 1oz Sq. Yd.
Plant with medium vigour root system. Considered to be one of the best for green manure.
Pros - fast growing. If dug in while still growing will not rob the soil of nitrogen.
Cons - will not survive cold weather.
Broadcast sow in spring after the frosts have finished at 1oz per Sq. Yd.
A quick, low growing plant with shallow root system.
Pros - Good weed suppression. Very quick growing so can be used in areas where space is needed.
Cons - Member of cabbage family so could harbour club- root disease.
Broadcast sow at 1oz per Sq. Yd. or in rows 6" apart.
A fast growing and bulky plant. (Ensure that only the 'Westerwolds' variety is used and not the annual or biennial ryegrass, which will re-grow.)
Pros - germinates quickly, even in cold weather. Can be sown and dug in before ground is needed for crops.
Cons - none.
Broadcast sow at 1oz per Sq. Yd.
Key to chart
L - Legumes
P - Polygonum
B - Brassica
Y - yes
? - Possibly
X - No
|Dig in Difficulty
3 - Easy to dig in
2 - More difficult
1 - Hard work
1 - Poor weed control
2 - Good weed control
3 - Excellent weed control
|Related to||N fix||Hardy||Sow from-to||Duration||Dig Diff||Height||Note|
|Clover, Essex Red||L||Y||Y||Apr-Aug||3m-1yr||3||40|
|Lupin, Bitter||L||Y||?||Mar Jun||2-3m||3||50|
© copyright 1999, P. A. Owen