Using green manures

Information sheet

Green manures are living crops which can be sown to provide environmental benefits for the soil and garden. Although often used as part of a crop rotation plan they are not edible crops

  • School term: All year round
  • Level of experience: No experience needed
  • Subject(s): Science

Why use green manure?

Green manures improve soil fertility and increase humus content. They improve the structure of the soil and can prevent soil erosion during heavy rain or dry, hot summers. The plants cover the soil and thus suppress weed growth when the ground is not in productive use. Plants in the pea family can also fix nitrogren in the soil aiding plant growth. Green manures are grown from seed which is cheap and widely available from garden centres and online.

How to grow green manures

Prepare the soil by roughly digging beforehand and remove any weeds. Lightly tread the soil and sow the seeds by broadcast sowing (scatter evenly) for good coverage. Lightly rake in and water well, continue to water during dry periods until the seedlings have established. Green manures are rarely sown in rows as this does not give adequate cover for weed suppression.

Green manures should be cut down before flowering when the stems and leaves are soft. This will ensure quick decomposition, retain more of the plants beneficial nutrients and be easier to work into the soil. Spent green manure plants can be dug into the top 15cm of soil or left ontop as a mulch. Worms will draw rotting plant material down the soil profile as organic matter which increases soil aeration.

Types of green manure

Some types of green manures can be grown all year round. Others are more suitable for a particular season. Check the packet for details beforehand.

  • Alfa alfa is a deep rooted crop, which can help break up the soil and will fix nitrogen. It can be sown in spring or autumn
  • Buckwheat is deep rooted and will help break up heavy subsoils. It makes alot of growth which will increase humus content. Dig or rotivate in before the first frosts. It is attractive to hoverflies
  • Clover can be red or white flowered and will add humus and fix nitrogen in the soil. Crimson clover is best for sandy soils. If left to flower these plants are attractive to bees
  • Fenugreek quick to grow and can increase humus content. It does not fix nitrogen
  • Mustards are fast growing and can be dug into the soil 3-4 weeks after sowing. As brassicas they should not be used where members of the cabbage family may be grown.
  • Phacelia is quick to grow and adds humus to the soil if dug in before flowering. Attractive to insect life or left to flower.

Hardy green manures

  • Field beans are hardy and can be sown in autumn, they are deep rooting and so good to break up heavy soils whilst fixing nitrogen. Dig in before flowering or the plants will take longer to break down
  • Grazing rye is not to be confused with rye grass. It has an extensive root system and is good for improving soil structure. Dig in the spring before flowering.
  • Trefoil is a low growing winter hardy plant and fixes nitrogen
  • Winter tares are a fast growing, hardy plant which can be mixed with rye grass for good coverage. Also fixes nitrogen and can be sown in spring -dig in two months later.