How to improve the fertility of your soil is a question that all good gardeners take seriously. One of the most under-used methods of soil improvement is the use of green manures (often called ‘cover crops’ in America), plants grown specifically to be dug back into the soil to improve it. In principle this sounds pretty easy – just sprinkle some seed on the ground after the main crop has been harvested and then dig the plants in after a few weeks. But in practice there’s a lot more to it, so I thought I would do a little experimenting to find the perfect green manure.
As regular readers will know, I live in an area with very sandy soil: great for digging and growing carrots but not at all good at retaining nutrients which are easily washed away. I’d love to say I can produce enough compost for my garden but despite my best attempts at bins of rotting organic material and leafmold there just never seems to be enough. The charity Garden Organic recently found that growing green manure can reduce the loss of the key nutrient nitrogen in the soil by up to 97 percent compared to soil left bare. So green manures seem to be the perfect solution.
Green manures work by drawing goodness out of the soil and storing it in the plant’s cells and root nodules. When the plants are then dug back into the soil they rot down and gradually release these nutrients to the next crop in a more readily-available form. Regular use of green manures improves the soil structure, breaking down hard soils and adding organic matter to light soils like mine. Green manures can have other benefits as well. Many of them provide good soil cover, suppressing weed growth and preventing erosion. Others attract beneficial insects to the garden such as bees and hoverflies which prey on pests like aphids.
So how do you choose a green manure to sow? The following types are readily available:
- Legumes, such as winter field beans (like fava beans), lupins and fenugreek which fix nitrogen into the roots (as long as they are dug in before flowering when the nitrogen is lost). Other peas and beans, such as sweet peas, can also be used. I have used winter field beans very successfully when planting a late green manure since they will even grow when temperatures are starting to take a dive during mid-autumn.
- Clovers, red or crimson clover being the best as it dies down, also in the legume family.
- Winter tares, also known as vetches, are also winter-hardy but like rye they can be difficult to dig in. Again, part of the legume family so they fix nitrogen into the soil.
- Rye, such as Hungarian grazing rye, will grow well at low temperatures but can be difficult to dig in and get rid of.
- Mustards, can be very effective but, as they are part of the brassica family, they can interfere with your crop rotation.
- Buckwheat and Phacelia are both excellent at attracting beneficial insects and are easily dug in.
- Winter-hardy salad crops, such as corn salad and miner’s salad (Claytonia) are easily dug in once used and can provide some extra salad leaves while growing.
- Others which are not normally regarded as green manures can also do a great job. Poached-egg plant (Limnanthes douglassii) is a great example – bright flowers, grows well over winter and digs in easily. I regularly plant this in my garden and leave a few to flower to attract hoverflies.
[The above list includes most of the available green manures in the UK. For a US-list, see the article on the National Garden Association website]
Whilst this looks like a wide variety of options, there are some important factors to consider. Firstly, many green manures are great for farmers with machinery to dig in the plants but are not half as easy for gardeners who have to do it by hand. Well-known author Bob Flowerdew recommends that you avoid ryes, tares and vetches, fodder radish, and many clovers for exactly this reason. Secondly, not all green manures grow well on all soils. Tares don’t do well on dry or acid soils, clovers prefer light soils and beans prefer heavier ground.
This year I set apart an area where I could grow three of the best as a trial: fenugreek, phacelia and buckwheat. For me, both the buckwheat and the fenugreek struggled to provide much ground cover and were relatively poor at germinating. Phacelia was the complete opposite. It required little weeding, quickly producing lush growth up to about 16 inches (40cm) high and even attracted a range of bees and insects if left to flower. I shall be sowing more of it before the end of the season and plan to incorporate it in several places across my garden next year, particularly because it’s from the waterleaf family of plants and doesn’t interfere with crop rotation.
So if I was asked to name my top three green manures they would be phacelia, poached-egg plant and winter field beans. I’m still on the lookout for other good green manures though, so please do share your experiences below.