Green manure

by Kate Bradbury

Green manures are the perfect organic fertiliser. They improve soil structure, suppress weeds and return nutrients to the soil.

Green manure seedlingsGreen manures are the perfect organic fertiliser. They improve soil structure, suppress weeds and return nutrients to the soil.

You never see bare earth in the wild for a reason: autumn rains in particular can leach nutrients, damage soil structure, and - in extreme situations - lead to erosion. In the wild, plants quickly colonise exposed earth, protecting its structure and absorbing the nutrients that would otherwise be washed away. A green manure does the same thing; you just have control over what grows.

Deep rooted plants such as buckwheat (Fagopyrum esculentum) bring nutrients from the subsoil, while leguminous manures like clover (Trifolium sp.) carry nodules on their roots, which fix nitrogen in the earth. In spring, a couple of weeks before you start planting, simply chop down and dig in your green manure, leaving it to break down into the soil. As the plants decompose, they return nutrients to the earth, providing it with the perfect food for your crops. For best results, you should cut down the crop before it’s flowered, but I can’t help leaving a few around the edge to provide food for wildlife.

I hate seeing bare earth in my garden. If I expose the soil I sow a few seeds of red clover on the surface, which germinate quickly and provide some ground cover before the other plants grow into the gaps. My frogs benefit, as they prefer sheltered ‘corridors’ of dense foliage to move undetected around the garden. Newly planted pots of spring bulbs get the same sprinkling of clover – I like to think its nitrogen-fixing nodules feed the bulbs, but even if they don't the pots look a lot nicer in autumn with a covering of leaves. I then let them grow right through spring – they help hide the bulb’s foliage as it dies down, and of course, the flowers are a magnet for bumblebees.

There are a few green manures to sow now, suitable for a range of soil types:

1. Grazing rye (Secale cereale) improves soil structure. Sow from August to November and dig in the following spring.

2. Winter field bean (Vicia faba) is good nitrogen fixer for heavy soils. Sow from September to November.

3. Mustard (Sinapis alba) is a brassica, so should not be followed by other brassicas in your crop rotation, as this can lead to the build up of diseases such as club root. Gardeners in the south can still sow it now, but those up north should wait until spring.

4. Red clover (Trifolium pratense) is good for loamy soils and fixes nitrogen. Its flowers are a great food source for bumblebees. Again, gardeners down south can probably get away with sowing it now, those up north should sow winter beans instead.

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Gardeners' World Web User 06/10/2010 at 19:56

This year i am going to take on a member of the families garden who has recently past away would it harm if i did not use green manurs ?

Gardeners' World Web User 07/10/2010 at 17:09

Hi Kate i've just sown some field beans as a green manure, but when should i dig them in- should i let them flower first? Jenny

Gardeners' World Web User 07/10/2010 at 21:20

Hi Kate, Our garden contains reddish dry sandy soil containing stones, and is difficult to dig, also on a hill. Next spring I'd like to grow fruit and veg on the site. Could you suggest the best way to improve the soil please? Would you use green manure in this case? Thanks Hannah

Gardeners' World Web User 08/10/2010 at 09:49

I would like to sow green manure on my allotment but as I am constantly battling with bindweed I don't see how I could dig it in in the spring. Any suggestions?

Gardeners' World Web User 08/10/2010 at 10:37

Jenny - technically you should dig in the plants before they flower, as they use energy to produce flowers, which would otherwise be dug back into the soil to feed your next crop. But I always let a few flower - especially clover. Cut the foliage down two-three weeks before you want to use the bed. I would leave the roots in the soil so their nitrogen-fixing nodules can feed your plants. Follow with a nitrogen-hungry crop, such as brassicas. Hannah - you'd probably be better off adding home-made compost and well-rotted horse manure. This will help bind the soil's particles together, making it more water retentive and rich in humus. Is it difficult to dig because it's stony/rocky, or is it compacted? If you add manure/compost now cover it with plastic sheeting to prevent the nutrients leaching from the soil. You shouldn't need to dig it in - worms will do that job for you. Regarding the stones, remove them by hand or sieving if you intend to grow root veg such as carrots and parsnips, but they shouldn't be too troublesome if you're just planting a few fruit trees. Ann - probably best to deal with the bindweed first, then sow a green manure afterwards. Now's a good time to deal with bindweed as its growth will be slowed until spring. A green manure will suppress weed growth to an extent, so if you dig the patch over and remove every tiny piece of bindweed root, and then sow a green manure over it, you might be lucky. Kate

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