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.