‘Green manures’ are plants grown especially to boost your soil by replacing nutrients, improving the structure and enhancing water and air retention, as Sebastian Larson explains.
Our soil is the most precious resource of all; without it there would be no real terrestrial life, no crops and certainly no food for humans. For many thousands of years we evolved a way of using the soil that put as much back as we took out and, only in the last century or so have we ignored these principles in the mistaken belief that all a plant needs to grow are minerals.
The best soils provide water – but not too much – and air without, being too dry. They also offer nutrients (phosphates, nitrates and potassium) and living organisms, and are alive with insects, bacteria, fungi, worms, molluscs, and crustacea (centipedes and millipedes), plus a lot of decaying and fresh plant material which ultimately provides the food on which the living creatures feast. Plants don’t grow well in sterile soil and some (brassicas especially), grow better where the soil is rich in certain beneficial fungi.
It’s important that we replace the nutrients that our crops extract, but not just from a packet or a quick spray with the nutrient-charged hose. Soil needs replenishment with organic matter, which acts like a sponge to soak up water, creates vital air pockets for healthy roots and releases inorganic nutrients in a slow and controlled manner.
Green manures are plants whose main job in the garden is to provide solely for the soil. They fall into several groups: those for composting, those for digging-in and those that fix nitrogen directly from the atmosphere.
They can also be used to suppress weeds when the soil is being left bare for a long period, perhaps over the winter. This has the dual purpose of keeping the soil nutrients locked in such a way that the rain cannot so easily leach them away, as well as out-growing weeds.
Of course, many green manures attract pollinating insects to the garden – there’s no end to their usefulness and, just in case you were wondering, you can eat some of them too!
Traditionally we dig over the plot in the autumn and lay manure on the soil, or sometimes dig it in. Green manures can be planted straight after digging so they will germinate before all the warmth has left the soil.
In the spring, a month before planting begins, the crop is simply dug in and allowed to rot, releasing the vital nutrients and improving the sponginess of the soil.
Varieties for autumn sowing include red clover, Trifolium pratense. ‘Broadcast’ the seeds, that is evenly throw them around the soil, at a rate of a handful per square metre. They will grow to a height of 40cm and can be mown and put on the compost heap. From late January to mid-February they can be dug-in, at which point they will die and rot quickly.
If your soil is heavy, try tares, which is actually a bean, Vicia sativa. This plant will grow a little higher than clover, but will also dig-in quite easily. But make sure you dig it in before it flowers, to stop the plant producing beans (although these are edible!).
Both of these plants fix nitrogen from the atmosphere.
Lupin and mustard are both good plants to grow in the spring. They come up quickly and form a ground-covering crop that can be dug-in easily. Bitter lupin, Lupinus angustifolius, should be sown like carrots, a couple of centimetres deep, and a couple of centimetres apart. Mustard, Sinapis alba, needs to be broadcast like clover and, again, dug-in before flowering.
Henry Doubleday in Victorian times started working on this plant as a crop, before which it was known as knitbone, and used to help fix fractures. However, it’s so rich in nutrients that it can be composted easily to create a nitrate-rich mixture, ideal for tomatoes and most other jobs around the garden.
Comfrey tea is a simple infusion of comfrey leaves, stuffed into an old pillowcase and allowed to soak in water. The rich, horrid-smelling, black ooze that results makes wonderful organic feed, a perfect boost all over the plot!
Mostly, comfrey comes as plants that should be planted at a distance of around a metre apart. It grows rapidly and can be harvested once the flowers start to appear. Simply cut it with shears to a height of about 20cm above the soil. It’ll keep re-growing right through to October/November, when it switches off for the winter.
Once comfrey goes dormant for its first winter, cover with a good mulch of well-rotted manure or compost, and wait for spring. In its second year it should grow furiously, providing three or four cuts during the season.
Some other useful green manure plants
Sunflower (dig in when young)
Bird’s Foot Trefoil