Green manure - it can work wonders for the soilAfter harvesting summer vegetables there are bound to be some empty spaces. So why not fill them with green manure to get set for growing good crops next spring.Green manures can be any plant grown as ground cover to improve fertility and soil structure or to compost elsewhere.  Crops such as mustard are sown across a bed, allowed to grow for a few months then dug into the soil before flowering. The ground will have been fed and be much more workable, so that new crops can be sown easily after a short break to make sure there is no residue of the manure crop.Some green manures need to be sown through the summer. But others over-winter well, preparing the ground for spring, and there is still time to sow one or two cultivars, such as grazing rye (Secale cereale) or winter field bean (Vicia faba). These crops are sown specifically to improve fertility. They are not grown for food. Don't let them flower. Strictly speaking, any plant can be grown as a green manure although some are much better than others and it is these plants, often used in agriculture, that are used in this way. They are quick growing and soon will cover the ground. Their foliage will smother weeds and their roots will stop any erosion of the soil by rain. Dug into the soil while still green, they will return valuable nutrients to the soil and improve its structure. If sown in the autumn they can be dug in during the following spring, but some can be left for a year or two. Whenever they are dug in, they release their nutrients into the soil. Winter grazing rye has an extensive root system that is really good for improving soil structure and mopping up food plants in the autumn. It can be sown during October and is not closely related to any vegetable crop so will not interfere with crop rotation. It will keep on growing even in low temperatures, making a thick mass of foliage. It is fine to sow it ahead of spring sowings of potatoes or beans. Just don't sow small seeded crops such as carrots next spring where this rye has been growing, as the foliage of grazing rye produces toxins which will inhibit the growth of small seeds for a few weeks. Winter field bean is an annual legume that is sown between September and November. It's good for heavy soils. A member of the pea and bean family, it can follow peas in a crop rotation scheme. When dug into the soil the following spring, they quickly rot down to release a good supply of nitrogen. Then plant some cabbages on the site to make the best use of this natural fertiliser. At other times of the year sow other green manures. Fast growing mustard can be an in-between crop to replenish the soil, but should not be followed by other brassicas as it could encourage clubroot disease. Another to try is Phacelia tanacetifolia. It is a decorative plant with feathery leaves. It grows quickly and can be sown after an early crop is harvested. Unrelated to any other vegetable, it can be fitted in without spoiling your crop rotation plan. Although green manure crops will give safe cover for the predators you want in the garden, such as beetles and frogs, look out for slugs and snails in wet weather. Allow at least two weeks between digging the green manure into the soil and planting or sowing a new crop, giving the green manure time to decay or it will affect the new growth. Good reasons for growing green manure crops
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