Green manures and Cover crops in the Organic Garden
by Frann Leach
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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:

Crop Family/Group Sowing rate Sowing time In ground Notes
Alfalfa/Lucerne Medicago sativa Leguminosae/7 1oz per sq yd April-July 3-4 months, up to 1 year+ Legume. Very deep rooted. Helps condition the subsoil.
Annual ryegrass Lolium multiflorum Gramineae/9   March before flowering Fast growing with good amount of top growth.
Beans (broad/Fava/field)
Vicia faba minor
Leguminosae/7 6" each way Sept-Nov c. 5 months over winter Legume. Can be cut once to regrow. Turn in or allow to seed, harvest, cut and compost.
Buckwheat2 Fagopyrum esculentum Polygonaceae/9 1oz per 4 sq yd mid March-August 10-12 weeks Good for acid soil. Attracts bees & hoverflies. Good soil conditioner.
Chicory Cichorium intybus Compositae/4 1oz per sq yd April-Aug 16 weeks Deep rooting. Good soil conditioner.
Chinese radish Raphanus sativus Cruciferae/2 1oz per 7 sq yd May 10-12 weeks Brassica. Turn in. Do not use after other brassicas.
Clover, Alsike3 Trifolium hybridum Leguminosae/7 1oz per 8 sq yd April-July 3-12 months Legume. Good for nitrogen. A good clover for heavy or wet soils.
Clover, crimson1 Trifolium incarnatum Leguminosae/7 1oz per 8 sq yd March-August 2-3 months in summer Legume. For farming.
Clover, Essex red/Red Merviot Trifolium pratense Leguminosae/7 1oz per 10 sq yd April-Aug 3 months-2 years Legume. Reputed to be a mouse deterrent
Cowpea3 Vigna unguiculata Leguminosae/7 6" each way April After harvest Legume also useful for breaking up hardpans. Crop can be eaten. Harvest and dig in vines.
Fenugreek1 Trigonella foenum-graecum Leguminosae/7 1oz per 4 sq yd March-August 2-3 months in summer Not hardy. Cut and compost. Legume, but does not fix nitrogen in the UK.
Lupins Lupinus angustifolius Leguminosae/7 2" apart March-July
After 8 weeks
12-14 weeks
Legume. Good for light land and after digging in old turf. Dig in before flowering. Adds phosphates.
Millet Millium effusum Gramineae/9   March-April before flowering Good for dry or poor soils.
Mustard1 Sinapis alba Cruciferae/2 1oz per 7 sq yd (per 5 sq yd) March-mid September 3-8 weeks in summer Brassica. Very quick growing. Turn in before flowering. (Rate to clear wireworm.) Do not use after other brassicas.
Phacelia tanacetifolia1,2 Boraginaceae/9 1oz per 10 sq yd end March-mid Sept 8-16 weeks Bulks well. Turn in or cut & compost.
Rape Brassica napus Cruciferae/2 1oz per 10 sq yd July-Aug Summer 7 weeks, over winter to 6 months Brassica. Turn in Sept or cut and compost in Oct. Do not use after other brassicas.
Rye, Winter grazing Secale cereale Gramineae/9 1oz per sq yd mid August-November 24 weeks over winter Extensive roots and good top growth. Fastest growing winter hardy crop. Turn in early Spring.
Spinach Spinacea oleracea Chenopodiaceae/1   Spring   Cut & lay on soil, leave roots in ground.
Winter tare/vetch Vicia sativa Leguminosae/7 1 oz/80ft row. 1" deep, 4" rows March-mid September 2-3 months or overwinter Turn in March/April.
Trefoil Medicago lupulina Leguminosae/7 1 oz per 10 sq yd mid March-mid August 13-52 weeks Legume. Also known as Black medic and hop clover. Turn in before flowering. Suitable for undersowing.

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

©2004 Frann Leach. All rights reserved.

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