16:35 - 10 August 2011
A green manure is a type of crop that you grow en masse on bare patches of ground, but in particular vegetable patches and fallow ground, to feed and condition the soil. It is not grown for an edible crop, but its biomass is used to add organic matter to the soil and to replenish lost nutrients. You normally grow a green manure from seed, often in late summer or autumn so that it covers the ground and stops other weed seeds growing in that space.
Sometimes green manures are grown to mop up surplus plant fertiliser that may have been applied to the area to stop it being leached out by the winter rain.
Many green manures are leguminous plants (pea and bean family) such as clover and vetches, which fix nitrogen in root nodules. When these plants are dug back into the soil this important plant nutrient is then available for the follow-on crop.
Green manures add vital organic matter to the soil when they are dug in and start to decompose, additionally their root system can be used to hold the soil together to prevent soil erosion.
Many gardeners use green manures for all of these reasons and more. It is a more natural way of feeding the soil and mimics the cycle of life seen in nature where a plant grows, flowers, fruits and dies and the decomposes back into the soil, thus returning lost nutrients to the soil.
Since most vegetable gardeners either harvest and remove the whole crop, e.g. leafy vegetables that are harvested and eaten, or they compost plants that have finished cropping, each subsequent year more and more goodness is removed from the soil.
It can be replenished by making garden compost that is dug in as a soil conditioner, or by adding farmyard manure, but green manuring is a way of making the garden keep working to feed itself. It’s a self-sufficient way of feeding the soil with many additional benefits. When the plants are dug into the soil and start to break down they hugely increase the microbial activity in the soil as they work on the plant matter, this also encourages healthy beneficial soil organisms such as worms. This all adds up to healthier soil, which every good gardener recognises as the secret to success in the garden. There are few drawbacks to using green manures: some of them can self seed around the garden if allowed to flower and set seed and the ground cover effect that they create can also be a magnet for slugs, which may shelter beneath the leafy cover, but otherwise, green manures are a win win choice and a cost effective way to improve the soil especially over the winter.
Flowering green manures provide pollen and nectar for foraging insects, sometimes at a time when these are in short supply, for example in early spring or late summer/autumn. This can be especially important if you are trying to encourage pollinating insects into an area or indeed if you keep bees.
Good green manure plants to sow in late summer include: Phacelia, winter field bean, winter tares, mustard and red clover (Trifolium pratense).
How to grow a green manure
All you do is sow them over the area to be covered and let them grow. When you want to plant that area, cut down the leaves and stems and leave on the soil surface to wilt and then dig them into the top layer of the soil. Allow about 2-3 weeks for the decaying plants to break down and then sow or plant your follow on crop.