ON THE GARDEN PATH by Carolyn Herriot

Feed the soil and let the soil feed the plant.
– Dr. William Albrect

Green manure crops are seeded in fall. They are allowed to grow to a certain stage and then dug into the soil in spring. This decayed matter provides food for the soil web of life – worms, microorganisms and other soil-borne organisms – which breaks down nutrients and makes them available to plants. Green manures thereby increase soil fertility and improve plant health.
Roots of green manure crops improve soil structure by breaking up heavy, clay-based soils to allow better air circulation and drainage. Alternatively, the decomposed matter adds bulk to sandy soils, allowing them to hold more water. When the availability of animal manures is limited, green manures play an important role. Seeds are inexpensive and easy to grow and a small bag of seeds is a lot easier to lug around than a heavy sack of animal waste.
By putting down roots that hold soil structure together, green manure plants prevent erosion. Bare soils are vulnerable to being leached out or eroded by heavy winter rains. By storing valuable plant nutrients in their roots, green manures hold plant food in the soil. Green manures that are legumes, such as field peas and favas, have the additional benefit of having bacteria on their roots that fix nitrogen from the air and add it to the soil.
Soil covered by a green manure crop is not susceptible to weed infestation. You know how nature abhors a vacuum! Better to plant a crop that benefits the soil than allow a weed infestation, which robs soil of valuable nutrients.
In fall, winter-hardy crops – fall rye, winter wheat, winter barley or field peas (leguminous) – will germinate while the soil is still warm. Over the winter months, the soil will be covered with a low mat of green and as it warms up in spring, this will flourish. The lush top growth can then be cut back and the rest of the plant dug or tilled under. In three to four weeks, it will break down enough to begin spring planting in the enriched soil.
You can sow green manure crops in summer or winter by choosing plants that flourish in these seasons. Manures can be sown in rows (best for larger seeds such as beans and lupins) or broadcast over the ground and raked in (for phacelia, rye and clovers).
In summer, the choice could be buckwheat, alfalfa, fenugreek, phacelia or white clover, all of which grow fast in warm soils. These seeds can also be sown as an underplanting with another crop, as long as the two crops do not compete. The main crop can be harvested and the green manure dug under at the same time.

What plants work as green manures?

Alfalfa: winter-hardy, deep-rooting perennial. Foliage contains an excellent range of plant foods. Sow April to July. If grown for a year or more, cut down two to three times to encourage new growth.
Bell beans: winter-hardy, nitrogen fixers, related to broad beans. Sow September to November. Can be cut down and left to regrow once.
Buckwheat: tender annual, related to polygonum. Tolerates poor soils. Sow April to August. Dig under when the first white flowers appear.
White clover: vigorous, fast-growing annual that fixes nitrogen. Sow March-August. Prefers sandy soils.
Mustards: fast-growing, winter-hardy annuals with yellow crucifer flowers. Sow March to September. Cut back after six to eight weeks, or any time up to flowering, when mustards get tough quickly. Note: Mustards are susceptible to club root.
Phacelia: fast-growing annual. Direct sow March to September. Dig in before the pretty blue flowers open. Phacelia makes a good weed control.
Fall rye: winter-hardy annual. Sow September to November. Grows fast in early spring. One of the most effective green manures. Note: Do not leave until the plants get well established as they become well-rooted and hard to dig under.
Vetch: rapid-growing, winter-hardy annual, nitrogen-fixer. Sow March to September. Dig in after two to three months in summer or leave to over-winter. Dig in before flowering. Good for weed control.

From A Year on the Garden Path: A 52-Week Organic Gardening Guide by Carolyn Herriot. Second edition $24.95. Available from your favourite bookstore or order online at www.earthfuture.com/gardenpath.