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Cover crops as miracle workers.

Rich, fertile soil with lots of humus is the key to a bountiful vegetable garden. But continuously planted garden soils are often overworked and depleted of nutrients, and newly cultivated soils may lack friability and organic matter.

One way to improve the condition of the soil is with cover crops also called green manures of fast-growing plants such as fava beans, vetch, and wheat. Either shear them off for compost or till them under.

By planting a combination of cover crops including nitrogen-fixing legumes as well as grains you increase the nitrogen content of the soil and provide carbon, which helps prevent the nitrogen from leaching away. The plants' root growth improves soil structure. One rye plant can produce 3 miles of roots in a day. "Growing it in your garden is like having the roots do the digging for you," says John Jeavons of Ecology Action research farm in Willits, California. When harvested at the right time, grains and fava beans provide an edible crop. And, during the growing season, cover crops act as a living mulch, helping control erosion and choking out weeds.

Help for any size garden

Whether you have raised beds, a small vegetable plot, or a mini-farm, green manures can be used if your soil is in need of conditioning. If you don't want to give up your entire garden to cover crops, plant one section at a time and wait until the following year to do the next section. If your soil is in poor condition, don't expect instant results. Cover crops can improve it somewhat in one season, but major changes in soil structure can take several years of replanting.

Fall is prime planting time

Many of the best green manures are coolseason crops. They're planted in fall after the soil has cooled, then are harvested in spring. In mild-winter areas, October is the prime planting time (in cool, wet climates, plant before heavy rains start; in cold climates, plant two months before the first hard frost).

Jeavons has been working with cover crops at his farms in northern California for more than 17 years and has developed this mix for mild climates (per 100 square feet of soil area): .7 ounce fava beans, 1.1 ounces purple vetch, .15 ounce cereal rye, and 2 ounces wheat. In cold climates, replace purple vetch with either winter hairy vetch (hardy to 10 degrees) or woolly pod vetch (hardy to O degrees).

Both fava beans and vetch are nitrogenfixing crops they fix nitrogen in their roots and leave a dose of it in the soil. When grown together, these crops form a symbiotic relationship: the favas act as trellises for the vetch to climb on, and the vetch helps keep the fava plants from blowing over in wind.

The extensive root systems of wheat and rye benefit soil structure, and their carbon content at maturity helps keep nitrogen from leaching out of the soil.

For a neater-looking cover crop, you can use white clover instead of the fava-vetch mixture. Mustard makes a good cover crop for heavy or compacted soils in mature orchards.

Careful planting provides best coverage

Two weeks before you're ready to plant, start the wheat and rye seeds in flats (you can hand-broadcast the seeds instead, but Jeavons' tests show that yields can be 35 to 50 percent lower). Just before planting, turn the soil thoroughly and water it well. Soak the vetch seeds overnight. To double nitrogen production of fava and vetch, coat the seeds with legume inoculants before planting (dampen the seed and then lightly coat with the powder). Since inoculants are plant specific, you'll need a different one for each legume.

Broadcast the vetch seeds over the soil and gently chop them in about 1/8 inch with a rake (don't rake them).

Next, plant the fava beans about 1/4 inch deep and about 21 inches apart; offset rows so seeds are planted in a diamondpattern. (To avoid compacting the soil during planting, sit or step on a 2- by 4foot plywood planting board.)

Finally, plant the wheat and rye seedlings at 5-inch intervals, alternating five wheat to one rye and offsetting each row (adjust the planting scheme so rye plants aren't all growing in a line). Water if rains aren't adequate to keep the soil moist.

In two months (around December), cut back plants to about 6 inches tall and compost greens; repeat two months later. Harvest fava beans and vetch after they start blooming but before they set seed. (To harvest favas for food, allow the pods to develop. At this stage, most of the nitrogen has moved from the root nodules to the beans. To make sure some nitrogen is left, leave at least half the beans on the plants before composting.)

Harvest in stages or all at once

One way to harvest fava and vetch is to pull plants up by hand (if practical) so the wheat and rye plants are left to turn golden brown (for carbonaceous material) and form seed heads (for food). Place the green material in the compost pile and let the wheat and rye mature another month. If you're not growing wheat and rye for food, you can cut down the entire crop at once with shears, a sickle, or a heavy-duty mower (see photograph above). Either compost the material in a separate area and add it to the soil later in the season, or disk or chop the plants directly into the soil; don't use a rotary tiller, as plants like vetch will tangle in the blades. Wait about four weeks before planting. For a carbon supply in both cases, add straw or old leaves to the green manure.

Where to get seeds

Purchase seeds as soon as possible so they arrive in time for planting. Both of these sources sell seeds and inoculants:

Bountiful Gardens, 19550 Walker Rd., Willits, Calif. 95490. Free catalog.

Peaceful Valley Farm Supply, Box 2209, Grass Valley, Calif. 95945; (916) 272-4769. Catalog $2.
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Date:Oct 1, 1989
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