Cover Crops and Green Manures

WHY USE GREEN MANURE/COVER CROPS?

Soil is precious and the need to keep it in the best possible condition is an important feature of farming. Sowing a cover crop over the period that the ground would normally be left bear can have the following benefits:-

  • Adds body and organic matter to the soil
  • Stops soil erosion
  • Suppresses weeds
  • Lifts valuable nitrogen from the soil and holds it in the plant until it is ploughed back in thereby avoiding nitrogen leaching
  • Legumes will fix atmospheric nitrogen and release it into the soil

WHICH ONE TO USE?

Choosing the right cover crop for your needs is also important as different species have different attributes.
Some crops are quick growing for short term covers, others are slower growing but more winter hardy therefore are better suited to Autumn sowing where the cover is required over winter or even longer.

After being incorporated back into the soil, different crops release the Nitrogen back into the soil at different rates this means that a cover crop can be matched accurately to the needs of the following crop.

If the soil is relatively nitrogen rich, a Nitrogen lifting crop may be more suited than a nitrogen fixing legume.

Species Fixer/Holder Duration N Release Spring/Autumn
Red Clover Fixer 1-3 Years Quick Spring/Autumn*
White Clover Fixer 2-5 Years Quick Spring/Autumn*
Yellow Trefoil Fixer 6-18 months Quick Spring
Crimson Clover Fixer 6-9 months Quick Spring
Sweet Clover Fixer 6-18 months Quick Spring
Vetch Fixer 6-10 months Quick Spring/Autumn
Lucerne Fixer 2-4 Years Quick Spring/Autumn*
Forage Rye Holder 6 months Slow Autumn
Mustard Holder 2-4 months Slow Spring/Autumn*
Buckwheat Holder 4-6 months Moderate Spring/Summer
Phacelia Holder 4-6 months Slow Spring/Summer
Italian Ryegrass Holder 6-12 months Slow Spring/Autumn
Sainfoin Fixer 2-4 years Fast  

*Should be sown by Early Autumn

MOWING

Most green manure crops should be mown to help with weed control, especially when the cover crop is young. Mowing early can make the difference between a well established cover crop and one full of weeds. Most species can tolerate being cut fairly close to the ground which should help to control the weeds. Exceptions are Sweet & Crimson Clovers which shouldn’t be cut lower than 15-20cm and Vetch which shouldn’t be cut at all. Fortunately, Vetches are very quick to grow so mowing to avoid weed ingress is not an issue as it will compete well with any soil borne weeds.

Cutting not only cuts the top off the weeds but also encourages the cover crop to grow lush green material and delays the crop growing to seed. If not topped, most crops will go stemmy and make them more difficult to incorporate into the soil.

Ideally, it is best to remove the cuttings, especially with legumes as this encourages them to fix more nitrogen. The clippings, if left on the ground will start to rot, releasing nitrogen which will suppress the growing plants to fix more atmospheric nitrogen. If removing the clippings is impractical (which it is in many cases), using a flail mower should help to distribute the cut material more thinly and evenly across the field.

INCORPORATION

This can be done either by rotavating or ploughing but before doing this, it is best to mow the crop first as this make incorporation much easier.

SOWING THE NEXT CROP

Be aware that many cover crops, when incorporated have an “allelopathic” effect on the soil which not only prevents germination of any weed seeds but also if sown too soon, the following crop. A period of 6 weeks should be left after incorporation to allow this allelopathic effect to disappear.

Red Clover

Red Clover is one of the more popular cover crops, either sown on it’s own or companion grass. Over the period, it can be grazed, cut for silage or mulched and on incorporation, will leave good amounts of residual nitrogen for the following crop. The ideal sowing time is Late March – Early May or Mid to Late August where soil temperatures and moisture should ensure a quick establishment. Sowing too far into September, with falling soil temperatures, means that a good establishment is less certain.

Suggested sowing rate 12.5 kilos/ha (5 kilos per acre)
Organic seed is normally available

White Clover

White Clover is slower to establish than Red Clover but is more persistent therefore making it suitable for long term covers. There are a range of varieties with different leaf sizes and the choice will be governed by the use of the crop whilst growing. Smaller leafed varieties, whilst producing less biomass are more persistent and longer lasting, especially when grazed. Larger leafed varieties are higher yielding and ideal for cutting regimes but are less likely to last the course if heavily grazed. Like Red Clover, the ideal sowing times are late March to Early May or Mid/Late August.

Suggested sowing rate 10 kilos/ha (4 kilos/acre)
Organic Seed is normally available

Yellow Trefoil

A short term, low growing cover crop, ideal for undersowing. Sowing times are the same as Red and White Clover. Although it is a short term crop, dying off after flowering it sets large quantities of seed very easily making it appear like a perennial crop. Late March – Early May or Mid-Late August are the best sowing times.

Suggested sowing rate 10 kilos/ha (4 kilos/acre)

Crimson Clover

An annual clover, grown to produce a rapid boost in soil fertility. It is especially useful for short breaks in intensive horticulture. Like most clovers, Late March – Early May or Mid – Late August are the best times to sow for a successful establishment.

Suggested sowing rate 15 kilos/ha (6 kilos/acre)

Sweet Clover

Sweet clover is a tall clover lasting 2 years and although initially slow to establish grows away quickly to produce a large amount of biomass. It also has a long tap root which helps to break up the soil. Being erect in growth habit coupled to the fact that it should not be close cut, it is not ideal as a weed suppressant. Sowing times are as per other clover species.

Suggested sowing rate 12.5 kilos/ha (5 kilos/acre)

Vetch

A crop which will fix large amounts of atmospheric nitrogen, as well as being an impressive weed suppressant. March – May or September are the best times to sow with Autumn sown vetches often sown in combination with Forage Rye. Be especially careful after incorporation as vetches are renown for their allelopathic properties so make sure that any following crop is not sown earlier than 6 weeks after ploughing in.

Suggested sowing rate 75 kilos/ha (30 kilos/acre)
Organic seed is normally available

Lucerne

A Perennial crop that is normally grown on it’s own although a small amount of Timothy or meadow Fescue can be added as a companion crop. Lucerne is a high protein legume which is suited to light or free draining soils but doesn’t like heavy or waterlogged land. It establishes relatively slowly but in years 2 and 3, will produce a significant amount of green material either for hay/silage or ploughing back in. To get the best out of Lucerne, it requires topping 3-4 times a year. Lucerne can be sown Late March to Early May or August and really needs to be sown with an accompanying inoculant. These inoculants which are passed by the Organic Certifying Authorities come either injected into peat which requires mixing with the seed before sowing or in many cases nowadays, as a seed coating which makes the sowing of the seed far easier.

Suggested sowing rate 20 kilos/ha (8 kilos/acre)
Organic seed is normally available

Forage Rye

Although not a legume, because it grows in colder temperatures than most crops Rye is the ideal crop for holding Nitrogen and reducing leaching over the winter period. It is normally sown on it’s own or in combination with Vetches to give an excellent winter hardy cover. Because of it’s cold tolerance, it can be sown later than any other green manure crop, being sown in September or even October. It establishes quickly and gives excellent weed control. Because of the high sowing rate, Rye tends to be one of the more expensive of the cover crops although the later sowing time means that it has the distinct advantage that there is less of a rush to get it in after the previous crop than the majority of other cover crops.

Suggested sowing rate 180 kilos per hectare (75 kilos per acre)
Organic seed is sometimes available

Mustard

Mustard is one of the cheapest cover crops and grows rapidly lifting nitrogen from the soil and holding it until ploughed in. Because of its quick growth it is an excellent weed suppressant. Mustard can be sown any time after late March and will flower after around 6 weeks so is a very short term cover crop. Mustard doesn’t require topping during it’s growth stage but it is best to top it before incorporating.

Suggested sowing rate 20 kilos/hectare (8 kilos/acre)
Organic seed is normally available

Buckwheat

Not a Nitrogen fixer but a rapidly growing annual crop that will not only lift nitrogen from the soil but will also scavenge for phosphate which is also taken up and released for any subsequent crop on incorporation. Buckwheat produces a large amount of biomass and although it’s large leaves give generally good weed suppression, some of the ground hugging weeds may survive under the canopy. With a high seeding rate a relatively expensive seed, Buckwheat has a limited appeal for green manure use and tends to be used more in the UK for gamecover. The ideal sowing time is April or May when there is no risk of frost.

Suggested sowing rate 70 kilos/hectare (30 kilos per acre)

Phacelia

Phacelia is a rapid growing Nitrogen lifter, ideal for growing over the summer. Phacelia is known to be particularly good at attracting bees and wasps producing a vivid purple flower. It is a fine leaved plant but it’s structure is as such that it produces a dense cover ideal for weed suppression. Although Phacelia produces less biomass than many other green manures, it dense roots system is very good for breaking up the soil pan. Sown anytime after March, it doesn’t need topping during the growth stage and is generally free of pest and diseases.

Suggested sowing rate 10 kilos per hectare (4 kilos/acre)
Organic seed is sometimes available

Italian Ryegrass

One of the most rapidly growing grasses, Italian Ryegrass can be sown as a pure stand or with Red Clover or Vetches. It will go over 1 or 2 winters and has a certain amount of frost tolerance although topping prior to the winter is always an advantage. It will produce large amounts of green material but needs topping regularly during the growing season to prevent it going stemmy and producing seed heads. Italian Ryegrass can be sown from Late March to Late April or August to Late September.

Suggested sowing rate 35 kilos/hectare (14 kilos/acre)
Organic seed is normally available

Sainfoin

Sainfoin is a perennial crop with an erect growth habit, lasting anywhere from 2-4 years. Ideal for chalk and limestone areas, Sainfoin, although relatively slow to establish it will produce enough biomass to compete against weeds once established. Highly palatable when grazed Sainfoin can also be cut to produce good quality hay or silage and is best topped 2-3 times per year. Normally sown in the spring, Sainfoin is best planted in April/May and although late August would be acceptable, this is less reliable as a reasonable amount of moisture is required to get Sainfoin going.

Suggested sowing rate 70 kilos/hectare (30 kilos/acre)


INNOCULANTS

Legumes need a Rhizobium bacteria to help the plants fix atmospheric nitrogen. Most species utilize a type of Rhizobium which is readily available in the soil but some species such as Lucerne, Sweet Clover & Sainfoin require a different strain of Rhyzobium which is not readily available so it is always best to sow these species with an inoculant which contains this particular bacteria.

The inoculant is normally available as a peat based product, coming in sachets which need to be mixed with the seed just before sowing although some varieties of Lucerne are now available which have been pre-inoculated, being incorporated as a coating which is even easier to sow

Whichever type of inoculant that is used, it is only needed to be done the once at time of sowing and both types are passed by the organic certifying authorities.

For more indepth information on any particular species, contact Simon on 01529 421081 or simon@copeseeds.co.uk