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The easiest way to add organic material to soil is to grow a green manure and then dig it in - no need to cart stuff to and from the compost heap! Green manures are an inexpensive, convenient source of organic matter, increasing water retention so you don't need to water so often and helping plants to get more out of the soil, so you save on fertiliser. They are often used as cover crops to stop erosion of valuable topsoil by wind and water in bad weather conditions.

You can also dig in grass clippings, which greatly improve the physical condition of heavy textured soils and provide much needed humus and nitrogen, before planting a second crop in a vacant plot. On very acid soil, add a little lime at the same time as you dig in the grass clippings. Give the area a week or two to get back to normal, then plant as usual. Many times, the second crop surpasses the first.

Types of green manure

Green manures basically break down into legumes and non-legumes. Legumes are valuable because they help fix large quantities of nitrogen from the air, making it available to plant roots. They also store moisture and send down deep roots that tap soil minerals, assist soil aeration and drainage and are attractive to bees. Some leguminous crops add anything up to 150 pounds of nitrogen per acre to garden soil, equivalent to approximately five tons of manure. Winter-hardy legumes are also often used in orchards to maintain soil fertility.

Non-legumes are good for short term production of bulk organic matter for the garden. They are usually sown in late summer or autumn and dug in the following spring, so as to clear the land for the growing season.

Mulching with green manures

Although all varieties of green manure can be dug straight into the soil, many can also be used as organic mulching material. Mulches such as rye grass offer nutrition for the soil, attract earthworms and keep the soil moist. And as the mulch decomposes, soil nutrients are released, so you get two effects for the price of one.

Soil improvement

Even poor soil can be improved with cover crops. Sandy soils gain moisture from the use of green manures; clay soils are helped by crops with long roots which soften the ground, such as alfalfa; and weedy soils can be cleaned up with fast-growing grasses to choke out unwanted weeds at the same time as adding nutrients. Green manures, treated carefully, will enable the soil to hold its nutrients until they can be used by the crops which follow. Cover crops are also beneficial to soils suffering from erosion or water build-up by providing erosion barriers to hold on to topsoil and absorb excess moisture.

Improving aeration and drainage

Green manures dug into the ground will greatly improve both drainage and air exchange in soils of good structure. The best way to maintain good soil porosity is to add organic matter to the soil by digging in green and farm manures, or adding compost to your soil.

Green manure crops

There are many green manures to choose from. Here are some of the more common ones:

¹ May be hit by a hard frost. ² Attracts hoverflies. ³ Inoculate seed