What are Green Manures?

As my garden moves into autumn and I harvest and clear spent crops from my beds, I am looking at an increasing number of empty spaces. Some of these gaps will be filled with autumn or overwintering crops, but others will not be replanted with a food crop until the following spring.

The Importance of Covering Bare Soil

It is important to remember that bare soil degrades relatively quickly. There are no live roots to mop up soluble nutrients or preserve the soil structure. Prolonged periods of rain (sound familiar?) will wash nutrients from the soil and cause compaction as air is driven out by water. So it's important to protect the soil in one way or another.

How Can I Protect Bare Soil?

Beds can be covered for winter with layers of compost and (in areas with very wet winters) by a further layer of breathable ground cover sheeting. This is the method I usually employ, but I think I will modify my approach after learning more about green manure crops.

Part of the reason is my gardening lapse last year when I had let some of my beds get overtaken by weeds in autumn - and had then left the weeds to cover the soil over winter. When clearing the beds in spring, I was surprised by the number of earthworms in the rootzone of the weedy beds compared to the compost-covered beds. 

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The answer is obvious I suppose: the roots of a growing plant are part of a larger nutrient exchange system that includes earthworms, fungi and microorganisms. The roots are also a habitat, so it makes sense to preserve the natural order of things where possible.

I say 'where possible' because, depending on when crops are being harvested, time may not be on your side. Green manures need time to get established and provide their benefits before the weather gets too cold.  

Cover Crops

The best way to protect soil is in the way that nature intended: to grow something on it. Cover crops are sown to cover bare soil after your main season crops have been harvested.

Their benefits include providing cover to suppress weeds, storing soluble nutrients and preventing them from being washed out by rain, and in some cases (e.g. field beans and clover) fixing atmospheric nitrogen and making it available to follow-on crops.

Green Manure Clover 'Rozeta'

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Many cover crops will need to be sown by September to build up enough plant and root mass to be beneficial, although some - such as winter field beans - can be sown in October or even later.

I know I will be mixing both methods this year: covering some beds with compost and sowing green manures in others. I may even cover green manure beds with compost in mid winter so I have beds ready to go in spring. There will be some experimentation which I will keep you posted on here. Experimentation is always fun!

Types of Cover Crop:

Legumes: e.g. clover, vetch - these are known in particular for their nitrogen fixing qualities.
Brassicas: e.g. daikon or tillage radish, mustard. Although we like to eat it, kale can be a very effective cover crop in commercial agriculture. Brassica cover crops shouldn't be used before growing other brassicas in the same rotation, to avoid buildup of clubroot.
Cereals or Grasses: e.g. rye, oats, ryegrass. 
Others: I was tempted to include buckwheat in cereals but apparently it's not strictly a cereal! The purple flowering, pollinator friendly Phacelia is another great option.

Green Manure 'Phacelia Tanacetifolia'

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Is there a Difference Between Green Manures and Cover Crops?

The terms 'cover crop' and 'green manure' are often used interchangably, including by us. To get more technical about it:

• a cover crop is grown primarily to protect bare soil and suppress weeds
• a green manure is used primarily to improve soil, e.g. by adding organic matter or fixing nitrogen

However you probably don't need to be told that many suitable crops can and will do both of these things. Another way of thinking about it is to take an example: buckwheat is a cover crop until you cut it back (while still 'green', hence the name) and leave it as a mulch on the soil surface, at which point it acts as a green manure.

A further point to make is that the term 'cover crop' is more often used in an agricultural or commercial context (where there are larger areas of bare soil to be protected), whereas home growers are more likely to recognise the concept of a 'green manure'.

What are the Benefits of Green Manures?

Weed Control

A bare patch of soil will quickly become colonised by weeds. While the weeds will cover and protect the ground, they will also produce and drop seeds, making it difficult to get rid of them when it's time to sow. A dense cover crop will out-compete the weeds, and can be easily dug when the space is needed for a new crop.

Green Manure 'Winter Vetch' (Tares)

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Protect Soil Structure

By covering the vulnerable soil surface you protect the soil structure from heavy rain. Soil is made up of different size particles which can be separated by rain washing through the soil and can result in impenetrable layers or 'pans'.

Loosen the Soil

Deep-rooting green manures can help to loosen and aerate the soil deep into the ground, improving drainage and water infiltration.

Improve Soil Fertility

Green manure crops that are also members of the legume family take atmospheric nitrogen from the air and process it into a form usable by plants. This is done by beneficial bacteria living in root nodules in the plant's roots. When the plant is cut, the roots break down in the soil and release the nitrogen.

Green manures also add organic matter as they break down, improving nutrient availability and supporting a healthy population of beneficial microbial life.

Green Manure Clover 'Kardinal'

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Lock Up Soil Fertility

Soluble nutrients are washed through bare soil, particularly over the winter when heavy rain is more likely. Cover crops of green manure need nutrients to grow which are stored in the plant's cells. These nutrients are released when crops die back or are cut and dug into the soil.

Rest Period for Soil

Soil which has been intensively worked may need time to recover its structure and fertility. Sowing green manure is an excellent way of protecting the soil over the recovery period, with all the benefits mentioned above.

Clover is a particularly good crop for giving soil a breather, as it fixes nitrogen and needs to be in the ground for a full season to be beneficial.

Green Manure Options

There are a broad range of manures you can sow depending on the time of year. As already mentioned, clover needs a long growing season so is best sown earlier in the year (after new potatoes in July is an ideal time). Crops like field beans, winter vetch and Phacelia can be sown later in the year. 

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Rye grasses are often used, but I tend to avoid them as they can be a problem getting rid of the following year. Rye also releases a chemical that inhibits seed germination. Therefore if you are growing rye, wait at least a month after digging in before sowing a new crop.