Green manures are fast-growing plants sown to cover bare soil when crops are not being grown. They are useful on the allotment as their foliage smothers weeds and their roots prevent soil erosion. When dug into the ground while still green, they will return valuable nutrients to the soil and improve soil structure.
A green manure can:
- Improve soil fertility – clover and other legumes harvest nitrogen from the air
- Keep soil fertility – mop up plant foods on empty land, so they are not washed out by the rain
- Protect soil structure – a ‘cover crop’ protects the soil from damage by heavy rain
- Keep down weeds – smother seedlings and compete for light and plant foods
- Help control pests – provide safe cover for beetles, frogs and other predators
- Stimulate soil biological activity – microbes and other soil organisms rapidly colonise green manure foliage dug into the soil. Increased biological activity makes for a more productive soil
- Loosen the soil – deep rooting green manures can help to loosen and aerate the soil deep into the ground.
- Protect soil life – a living mulch protects creatures in the soil from the extremes of weather
There are a number of commonly used green manures for allotments. The Royal Horticultural Society covers the common ones, their best uses, times to sow and any associated problems here.
Most on-line seed companies have a selection of green manures for sale. Many allotment societies purchase seeds at bulk rates for their allotment holders. If enough requests are made to your committee, then undoubtedly they will be willing to include a suitable variety in the order. As a guide to availability, range and pricing a good list can be found here.
Some green manures, such as crimson clover and phacelia, can be included and allowed to flower during spring and summer to encourage pollinating insects such as bees and hover flies.