Q What is green manure?

A Green manure is a plant that is sown on bare ground to be dug in later on to improve the soil.

Caption: Green manure is dug in during spring

Q When is green manure sown?

A It is usually sown after early crops are harvested in late July or once summer crops are lifted in September.

Caption: Sow green manure when crops are lifted

Q What benefits can green manure bring?

A There are several potential benefits:

• It mops up soluble nutrients, such as nitrates and potash, which would otherwise be leached away by rain. This is important in an organic system, as nutrients are hard to replace without using fertilisers, and organic matter breaks down over time and not necessarily when crops are present.
• It helps to prevent weeds taking over. Vigorous green manures, such as mustard, will quickly swamp weed seedlings.
• It maintains the soil structure and is especially useful on light, sandy soils, which can lump and form a hard crust over winter. The extensive root system also helps keep heavy clays open.
• Legumes such as beans, clovers and tares (a kind of vetch), fix nitrogen from the air in modules on their roots, which is released when they rot down.
• Deep-rooting plants, such as buckwheat and grazing rye, use nutrients from deeper in the soil than can be reached by many normal crops. Once again, these can be released into the surface soil as they rot.
• They add organic matter to the soil, though by the time they have decomposed this is significantly less than if you'd used well-rotted garden compost or animal manure.
• They provide cover for beneficial insects and food for earthworms. Mustard can also reduce the incidence of harmful nematodes.

Q Which green manures work best?

A  When Which? Gardening magazine trialled different types of green manure, we found that the best performers were mustard and phacelia; they established quickly and produced a good bulk by December. Both were killed by frost and were fairly easy to dig in during March, and the plots were weed-free in spring. Don't use mustard on sites where there is clubroot in the soil.

Crimson clover was slower to establish but overwintered to give good cover and bulk (2kg per sq m in March). Being prostrate, it was fairly easy to dig in and had prolific root nodules.

The runners-up were tares. They survived the winter to produce trailing, stringy stems that required some chopping before digging in.

Caption: Phacelia was one of the best-performing green manures in our trial

Q Which green manures didn't work so well?

A Buckwheat and fenugreek were killed by frost, but were easy to dig in. Field bean and fodder radish failed to establish well. Grazing rye might have done better if we'd sown it earlier than September.