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Wood ashes make an excellent fertilizer for the vegetable garden

Being warmed by a Franklin stove this winter can be a double pleasure if you're a gardener. You not only can save on fuel costs, but you can dream of the fine crops you'll have next summer, thanks to the wood ashes collecting under the grate.

Ashes are a rich fertilizer, free of charge. They have been used in agriculture since primeval times, but if you're an amateur gardener, as most of us are, you learn about such things bit by bit, a hint here and a hint there. For years I fretted about my stunted beets, until I chanced on a line in a gardening manual that said: ''Use wood ashes where you grow beets.'' In one season I graduated to plump, tender beets.

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This was years ago, when my only ashes came from an occasional fire in the fireplace. But with the oil crisis of the 1970s, I put a wood stove in my den to keep one room cozy in the evening when we turned down the thermostat. That meant ashes in plentiful supply, not only for the beets, but for other crops as well as flowers.

The United States Forest Service estimates that fireplaces, stoves, and furnaces across the US are burning more wood than at any time in a half century - as much as 42 million cords a year. In Maine, New Hampshire, and Vermont, for example, state officials figure that firewood is burning in more than half the homes.

That translates into a lot of potash, the chemical that got its name from an old-time method of processing wood ashes in iron pots. Your gardening ashes, of course, need no further processing. You spread them where needed just as they are, or, if lumpy, through a filtering screen.

Potash, or potassium carbonate, is one of the ''big three'' of fertilizers, along with nitrogen and phosphorus. It is a somewhat mysterious element of plant growth, and a deficiency of it causes stunting, poor root systems, and browning and curling leaves.

In addition to potash, wood ashes also contain calcium carbonate, high in the alkalinity needed to lime, or sweeten, the soil. And ashes spread around plants are also good fighters of such pests as root maggots, beetles, slugs, borers, and cutworms.

Part of the pleasure of gardening with ashes, of course, is that they're free. Whether you buy firewood or cut your own, the ashes are a bonanza.

I store ashes outdoors in plastic garbage cans until spring, taking care to keep the covers on tight to prevent water from getting inside and making the ashes lumpy and unmanageable. Wait until planting time to spread ashes on the soil. Otherwise, they will leach out of the soil, and you won't get the full benefit of them.

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In areas where the soil tends to be acidic, it's a good idea to give a light application of ashes to most of the garden just before seeding, intensifying for onions, beets, carrots, and other root crops. If you're a shallot fancier, by the way, ashes are great. For such crops as beets and Swiss chard, I apply additional ashes along the rows as the season moves on and the plants develop.

While helpful for most vegetables, the high alkalinity of ashes makes them unsuitable for watermelons and such acid-loving plants as blueberries, azaleas, and rhododendrons. Potatoes, too, are out.

Coal ashes, incidentally, will lighten heavy soils but are practically worthless as fertilizer. Environmentalists also regard coal ashes warily, since they may contain sulfur and iron in dangerous quantities.

Besides potash and calcium carbonate, wood ashes contain magnesium oxide, phosphorus pentoxide, and trace quantities of copper, zinc, manganese, iron, sodium, and boron. Yet there may be danger in too large a dose of these things. Scientists at the University of Rhode Island suggest a safe application would be a 5-gallon pail of ashes per 1,000 square feet of garden.

Experts also warn against burning colored paper or wood that has been painted or otherwise chemically treated, if you're going to use the ashes in the garden.

Gardening with ashes will waft you back to colonial times. The Puritans arriving in America found the Indians farming with the slash-and-burn technique known as swidden agriculture. In the burned-over, ash-enriched ground, the Indians poked holes to plant vegetables.

With abundant woodland, the colonists soon were farming with ashes as well. By 1632, potash was being produced in New Hampshire and exported to England, a business that peaked in 1825. Other sources of potassium, such as lake brine and soluble salt deposits, gradually took over, becoming big business.

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