You can’t eat green manures, but if you know which crops to grow, where, when and how to cultivate them you will improve your soil for the benefit of future edible crops.
Crops of green manures can be cultivated throughout the growing season from early spring onwards on any piece of empty ground, but most plots are packed with edibles then.
Late summer/early autumn is a good time to sow green manures into ground which would otherwise be empty over winter.
The technique of growing green manures is gaining more converts now that organic gardening is the accepted norm. Most seed companies now devote a separate section in their catalogs to green manure seeds.
What are The Benefits of Growing Green Manures?
The short answer is to improve the soil. The traditional autumn clear up of everything except Brussel sprouts, kale, leeks and parsnips followed by digging, exposes the soil to the damaging effects on its structure through the pounding it receives from heavy winter rains.
A blanket of green manure softens the blows considerably. The loss of nitrates through leaching is greatly reduced with a covering crop.
Some green manures including clovers, lupins and field beans absorb nitrogen from the air and fix it on their root nodules. When you dig these green manures into the soil in spring the nitrogen becomes available to next year’s crop.
Deep rooted green manures extract minerals from deep in the soil and draw them to near the surface where subsequent shallow rooted plants can make use of them.
Those green manures with deep roots loosen and aerate the soil. A good smothering of green manure plants will act as an effective weed control whilst at the same time providing cover for beetles, frogs and other predators. Green manures are an excellent source of humus when they are dug into the soil.
Some of the Best Green Manures to Grow, When and How to Sow Them
Grazing rye, which should not be confused with ryegrass, has an extensive fine root system which improves heavy soil by opening up the structure; conversely on lighter soils the roots of this plant bind with soil particles, helping them hold water. It should be broadcast sown from late summer to late autumn.
Winter tares is a winter hardy annual, fast-growing vetch which fixes nitrogen as does crimson clover. Both can be broadcast sown from early spring to mid-summer and also in late summer/early autumn.
Field beans are also good fixers of nitrogen, but they are best sown in rows 20cm apart and 10cm between the seeds. As they are winter hardy sowings can be made from early to late autumn making them a useful green manure crop to follow brassicas, or main crop potatoes.
Mustard is one of the fastest -growing green manures which can be sown broadcast from early spring to late summer/early autumn. It is, however, susceptible to club root disease so don’t use it if your soil is infected with this dastardly disease.
Phacelia tanacetifolia is an erect hairy annual with finely cut leaves. It is very good for adding humus to the soil. Sow it broadcast from early spring to early autumn.
When and What to do With Your Green Manures
- Chop off the green manure plants with a spade at ground level.
- This should be done before growth begins to toughen up and certainly before flower buds appear.
- Dig them into the top 15-20cm of soil.
Soil microbes and other organisms rapidly break the green manure down increasing the humus content of the soil. The more biological activity there is in the soil the more productive it is.
So Maximize the Potential of Your Plot with Green Manures
Don’t let empty space stay idle for long. If you do then heavy rains will damage the soil structure before weeds come in and rob the earth of nutrients.
It’s far better to grow your own relatively inexpensive green manures and allow the soil to reap the rewards prior to getting the physical benefits yourself from digging.