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The benefits of wood ash in the garden

By Barbara Damrosch,March 28, 2012

If you have a wood stove or a fireplace, one of spring’s rituals is shoveling out the ashes. And if you’re in tune with the philosophy of recycling plant residues back into the natural cycle of soil fertility, you’ll want a better destination for those ashes than the trash bag.

Unlike the decomposed remains of leaves, stems and other green plant parts, burned wood doesn’t contain nitrogen. But it does provide phosphorous, potassium, calcium, boron and other elements that growing plants need. It’s also very alkaline and useful for raising the pH in gardens. You’ll need about twice as much of it as lime, but it will supply nutrients at the same time, and if you’re a wood-burner it’s free.

Before you go scattering the ashes about, get a soil test done, so you know whether it will benefit you. If your yard or garden soil has a pH of 7 or higher, give the ashes to a friend with a more acidic soil. Don’t use it around acid-loving plants such as blueberries and azaleas, or on potatoes, which get scab disease if the pH is too high.

Use only wood ashes, not ash from coal, charcoal briquettes or fake logs. Keep it away from green foliage, especially that of young seedlings, which it can burn. Ashes are caustic, hence their traditional use in making lye soap or in turning corn kernels into hominy. I also keep them out of the compost pile, where they can become overly concentrated.

Wood ashes are a great homemade source of potassium, whose name comes from the word “potash,” literally the substance made from soaking ashes in a pot. Ever wonder why potassium is represented by the letter K in the periodic table of elements? Phosphorous got there first and claimed the P? Actually, K stands for the Latin word for potassium, kalium, derived from the Arabic al qali (alkali).

Here are three intelligent things you can do with that full ash bucket.

1. Sprinkle some ash on the lawn. Applied lightly and followed by a good watering, the ash will benefit the grass and also foster the growth of clover in the lawn, an attractive soil-improver that provides nectar for the bees.

2. Use the ashes to make tea for tomatoes. Put five pounds of ashes in a permeable cloth or burlap bag, tie it shut and lower it into a 50-gallon garbage can filled with water, as if it were a giant tea bag. Let it sit for about four days, then dip the tea out with a watering can and pour a cupful around your tomato plants once a week, as soon as the plants begin to flower. Most crops can use a potassium boost, but especially tomatoes.

3. Spread ashes around the base of hardwood trees, returning this valuable product to its source. Apples in particular love this treatment. Not everyone has an orchard, a woodlot, or a major wood-burning habit. But even a bit of ash from the occasional fire on cold nights makes a good end-of-winter gift for a favorite tree.

Damrosch is a freelance writer and the author of “The Garden Primer.”