Green manures are plants grown specifically to improve the soil. Their use goes back far beyond the introduction of industrial agriculture, whose practice is dependent on machinery, pesticides and artificial fertilisers. All of these use fossil fuels in their production and also damage the microorganisms that help to keep soil naturally healthy and productive. Green manure plants are described as cover crops when used primarily to protect the soil.
Green manures function in a variety of ways:
• by being deep rooted;
• by covering the soil to prevent leaching of nutrients;
• by fixing nitrogen;
• or by breaking up the soil with fibrous root structures.
In all cases they are then dug or mulched back into the soil, adding organic matter which helps retain moisture and feeds the soil’s beneficial organisms. To dig in, chop up the green manure with a spade and turn it into the top few inches, 10-12 cm or so, of soil.
Mulching out is the easier option; cut down if necessary and leave the clippings on the soil then cover to exclude the light using cardboard and grass cuttings or a biodegradable ground cover.
Keep covered for at least 6 weeks, but longer if you do not need the ground, by which time most of the green manure will have broken down and been incorporated into the soil by earthworms.
Good healthy and biodiverse soil is the key principle for organic growing and green manures play a vital role in helping to cycle nutrients. Use them also to cover bare ground, particularly over winter.
This is a very important aspect of climate-friendly gardening, as covering the soil also prevents carbon in the soil’s organic matter from reacting with oxygen in the air to produce carbon dioxide, which is then lost to the atmosphere. Covering the surface also reduces soil erosion.
The seeds we offer for trying out are: Buckwheat, Crimson Clover, Phacelia, Red Clover, White Clover and Winter Tares, which are well suited to small-scale growing. For more information about their specific uses, go to Tamar Organics, the seed company we sourced our supply from.
Picture Note: The plant shown in the main picture is the very popular cover crop Phacelia, here in flower and before flowering below. It's not able to fix nitrogen but its finely divided leaves are good at covering the soil and decompose fairly rapidly after being cut down. That creates a dilemma for bee lovers as the flowers are excellent pollinator attractors. Guidelines usually suggest keeping some plants for the bees.