What is ‘green manure’?
By ‘green manure’ we don’t mean manure from a very poorly horse… we mean that we sow a cover crop in a bed (or beds) with the sole purpose of letting it grow before we cut it down and turn or rotovate it into the soil (without harvesting any produce from it). Think of it as a sacrificial crop used to improve your soil – it sounds a bit bonkers but there are many reasons why we do it. An Autumn sowing of green manure will see plants grow throughout the winter before they are cut back and turned into the soil the following spring.
Green Manure Advantages:
- Some green manure crops can fix nitrogen and key soil nutrients into the soil so that they don’t leach out of the soil over the cold rainy autumn and winter months. Empty beds will leach nutrients.
- Other green manure crops can bring up nitrogen and minerals from deep in the soil into the plant foliage, which when cut back and rotovated, adds them into the soil for Spring planting.
- Green manure crops restrict weed growth, empty beds are likely to be overtaken by unwanted weeds which will draw nutrients from the soil and be a nuisance to remove the following year.
- Many green manure crops can help to break up heavy clay soil, those crops with a long tap root help to split large clumps of clay, making the soil easier to manage and cultivate.
Green Manure Disadvantages:
- A green manure crop may provide an environment for slugs and snails to breed in, which could increase their numbers (but then again so could weeds!).
- You have to allow up to four weeks after cutting back and rotovating the green manure crops before sowing a new crop. This is because some crops are allelopathic which means they naturally leave toxic substances in the soil to restrict the germination of other new crops. (NOTE. Coffee grounds also do this because of the caffeine!).
- Some varieties of mustard are commonly sold as green manure cover crops. Mustard is a brassica so if you are planning to grow cabbages, broccoli or cauliflowers in a bed it should not have been recently used to grow a mustard cover crop.
Personally I only ever sow green manure crops in mid Autumn to provide over-wintering benefits but you can sow many of them all year round. Since I typically need my allotment beds throughout spring and summer that rules out a green manure solution for me in peak growing season. Here below I have listed some common ‘Autumn sowing’ green manure crops but there are spring and summer sowing crops available with a bit of research. You can get away with sowing winter green manure crops through until around mid October, I once tried a November sowing of rye but that didn’t germinate very well:
- Italian / Hungarian Grazing Rye – a nitrogen and mineral LIFTER that also works well as a soil structure improver. Can be left to grow for up to two years before cutting (if you feel like a year off at the allotment!).
- Red Clover / Trifolium incarnatum – a nitrogen FIXER that also works well as a soil structure improver and is an excellent weed suppressant.
- Forage Pea – a legume which is again a nitrogen FIXER and improves soil structure with its deep root system.
- White Mustard (Sinapsis alba) – a fast growing brassica which covers the ground in just over one month. Mustard plants can ‘cleanse’ the soil of pathogens because they contain high levels of glucosinolates.
- Winter Field Beans – a nitrogen FIXER, very hardy and excellent for breaking up heavy soils
(Author –BY Matt Peskett)