Green manures are a wonderful way of covering bare soil between crops and adding nutrients to your soil. Here, Ruth – our resident RHS trained specialist – gives us the basics.
Green manures are plants that are sown specifically to improve soil fertility. They are not harvested for food and should not be allowed to flower.
Why Grow Green Manures?
Improve Soil fertility
- Help lock in nitrogen from the air (Legumes*) and/ or provide nitrogen through digging in the crop.
- Maintain soil fertility by mopping up plant foods on empty land, so they are not washed out by the rain.
Improve Soil Structure
- A ‘cover crop’ protects the soil from damage by heavy rain.
- Loosen the soil – deep rooting green manures can help to loosen and aerate the soil deep into the ground.
- Smother seedlings and compete for light and plant foods.
- Protect soil life – a living mulch protects creatures in the soil from the extremes of weather.
Attract Beneficial Insects and other Predators
- 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.
*Legumes are members of the pea and bean family which commonly carry nodules on their roots that contain symbiotic bacteria. These bacteria are able to ‘fix’ nitrogen from the atmosphere and make it available to the plant. Nitrogen is one of the 3 key foods needed by plants to thrive and is fundamental to green, leafy growth. The other 2 foods are Potassium (also known as potash) and Phosphorous, responsible for supporting stems and roots, fruits & flowers respectively.
How Do I Grow Green Manures?
Green manures should be sown in situ using organic seed – follow the directions on the seed packet, but usually you would sow evenly across the area and firm in or cover lightly with soil. Plants are then hoed off just before they flower and allowed to break down on the soil surface or cover with a mulch to lock in the nutrients and avoid disturbing the soil.
What green manure should I choose?
Any plant can be grown as a green manure although some are much better than others and are available as seed especially for the purpose. Most green manures will be grown specifically to dig in before they flower to add to their nutrients to the soil, such as buckwheat, mustard or phacelia. Some are also legumes and can raise nitrogen levels in the immediate soil, such as clover and winter tares. Click here to access a table showing some of the most commonly used green manures.