As well as suppressing weed growth and improving soil structure some green manures have the ability to fix nitrogen. Nitrogen is vital for plant growth and soils deficient in nitrogen will produce poor crops at best.
Nitrogen can be added as an artificial fertiliser or via organic fertiliser like dried blood. It is also in manures and composts. Over time it decreases in the soil, winter rains in particular wash nutrients from the soil.
Nitrogen fixing green manures will, in conjunction with bacteria, absorb nitrogen from the air and fix nitrogen in the soil. When the plant is dug in, that nitrogen is available to the following crops. Free fertiliser!
Alfalfa (Lucerne) Green Manure By Elekes Andor – CC BY-SA 4.0
Alfalfa Green Manure
Alfalfa (Lucerne) is a very deep rooting green manure with vigorous roots that can draw up sub-surface minerals and break up soil pans.
It is can overwinter once well established and will grow for a year or more. Alfalfa Green Manure dislikes acid or wet soils but does well in dry conditions
Sow: April, May, June, July
Crimson Clover Green Manure
Crimson Clover Green Manure
Crimson Clover Green Manure is a good choice for smothering weeds, fixing nitrogen from the air and is a fast growing bulky green manure. It has deep roots that penetrate the soil and bulky furry foliage that once dug in helps to improve soil structure.
Sow: April, May, June, July, August, September
Fenugreek Green Manure
Fenugreek Green Manure
Fenugreek Green Manure is a quick growing annual that can be ready to dig in 10 weeks from sowing. It can be sown from March to August and performs best on well-drained but slightly heavy soils. It will tolerate lighter ones if they are fairly moist.
Sow: March, April, May, June, July, August
Lupins Green Manure
Lupins Green Manure
Lupins are a hardy annual green manure that can be planted from March to July and are part of the legume family. They suit light sandy acid soils and are a hardworking leguminous plant that has very long tap roots that dredge up minerals.
Sow: March, April, May, June, July, August
Red Clover Green Manure
Red Clover Green Manure
Red Clover is a fast growing perennial green manure. It is the best variety of clover for fixing nitrogen from the air, weed suppression & improving soil structure. Its roots penetrate deep into the soil and help to improve its structure by binding soils together (especially light ones)
Sow: April, May, June, July, August, September.
Sweet Clover Green Manure
Sweet Clover Green Manure
Sweet Clover ‘Yellow Blossom’ is an aggressive green manure that grows rapidly and has very strong roots that penetrate hard compacted soils that helps to improve drainage. It also produces lots of leafy foliage that adds good organic matter to the soil when dug in which improves soil structure.
Sow: March, April, May, August, September.
White Clover Green Manure
White Clover Green Manure
White Clover ‘Barblanca’ makes as excellent long-term green manure. It has good frost tolerance and once established can last from 2-5 years. White Clover makes a good weed suppressant as it produces a fair amount of foliage and is able to fix nitrogen.
Sow: March, April, May, June, July, August
Winter Tares Green Manure
Winter Tares Green Manure Seeds
Winter Tares ‘Vicia sativa’, also known as ‘Vetches’ is a hardy annual that can be sown March-May or July-September for over-wintering. It is good at nitrogen fixing and for weed suppression on heavier soils but avoid acid or dry soils and dig in before flowering.
Sow: March, April, May, July, August, September.
Yellow Trefoil Green Manure
Yellow Trefoil Green Manure
Yellow Trefoil ‘Black Medick’ is a low growing green manure that has excellent weed suppressing properties. It is good at fixing nitrogen and if allowed to flower encourages bees & beneficial insects to the area which in turn pollinate veg crops. Yellow Trefoil is great for interplanting taller crops
Sow: March, April, May, June, July, August
Field Bean Green Manure
Field Beans Green Manure
Field Beans are an overwintering green manure and is generally winter hardy. It germinates well in colder weather from September-November. Field Beans are particularly good on heavy soils and their deep roots are not only able to penetrate & break up the soil but fix nitrogen too.
Sow: September, October, November
Forage Pea Flower Green Manure
Forage Pea Green Manure
Forage Pea Green Manure is a member of the legume family that is excellent at fixing nitrogen and has deep penetrative roots that help to break up the soil. It can grow up to 1 metre tall. Forage Pea is excellent for over wintering and can be sown from September.
Sow: September, October, November
