Wood Ash and Potash
by Lee Reich
Smokey Ritual
I wonder if my neighbors suspect that I’m engaging in some sort of occult ritual as I take occasional winter rounds around the farmden followed by puffs of grey smoke. Perhaps I’m entreating tiny gnomes living within the soil to keep weeds at bay next season? Or begging garden gremlins to make my soil fertile? No, and again no! I’m merely spreading wood ashes on the vegetable gardens, the lawn, beneath fruit trees and bushes—everywhere, in fact.
I must be careful with my terminology: I’m not disposing of wood ashes; I’m enriching my soil with wood ashes. Wood ash is a rich source of potassium, a nutrient required by plants in amounts second only to nitrogen. Potassium helps build strong stems and helps plants resist disease. Potassium also regulates the opening and closing of stomates, the tiny pores in leaves through which leaves “breathe” out oxygen and “breathe” in carbon dioxide for photosynthesis.
Put the Ash in a Pot and You’ve Got…
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| Lee spreads ash in the yard and garden. Photo by Lee Reich. |
The close connection between potassium and wood ash is reflected in a traditional source of, and root of the word, potassium—”potash.” The easiest way to obtain potassium compounds used to be to leach them out from wood ashes using water in iron pots—the first step in homemade soap, among other uses. Go to a nursery and ask for a potash fertilizer, and you will be handed a bag of potassium sulfate (sulfate of potash), potassium chloride (muriate of potash), greensand (a mined mineral that offers micronutrients in addition to potassium), or—if it was a century ago— wood ash. “Potash” technically means potassium oxide but sometimes also refers to potassium carbonate or hydroxide.
Wood ash contains from one to about ten percent potash. Ashes from hardwood trees are at the top of this range and softwood ashes are at the low end. I store my ashes in a metal pail under cover of the garage to prevent rain from washing much of the nutriment out of the ashes.
Since “potash” is technically potassium oxide, which includes the weights of both potassium and oxygen, raw potassium concentration is always less than the potash concentration. Seventeen percent less, to be precise. The archaic designation equating potassium with potassium oxide remains because otherwise fertilizer buyers would feel gypped. When buying a bag of 10-10-10 fertilizer, the 3 numbers are generally taken to mean percentages of nitrogen (N), phosphorus (P), and potassium (K, from the latin, kalium). As a fertilizer salesperson, you’d have to go through the whole explanation of why your customer is getting 10-10-8.3, but it’s the same as the previous, not quite correctly stated, 10-10-10.
Exactitude Not Required
I’m not overly precise in my spreading of these ashes. I start at one end of the farmden in the beginning of the wood heating season, then work my way across the farmden, aiming to spread no more than five to ten pounds of ashes over every hundred square feet. Eventually, lawn, vegetable gardens, fruit trees and bushes get their annual dose of potash (not only from wood ash; there’s also plenty of potash in compost, and leaf and wood chip mulches).
Wood ash is good stuff, but is not to be overused. Plants need adequate, not excessive, potassium. Too much potassium upsets the balance of other nutrients in a plant. Wood ashes also make the soil more alkaline, so they never get spread in my blueberry temple. Wood ash is best kept away from rhododendrons, azaleas, pin oak, mountain laurel, and other plants that enjoy very acidic soils.
I do not dump—whoops, spread—all my wood ashes around the farmden. I save a little for use in potting soils, mixed in at the rate of a quarter of a cup per gallon of mix (except in potting mixes destined for acid-loving plants, of course).
And Something for Pests
And some I sift then save to use in the garden during the summer for pest control. Plants sprinkled with dry ashes become unpalatable to rabbits, bean beetles, and onion and cabbage maggots. A thick line of dry ashes on the soil becomes a Maginot Line against slugs, until washed away by rain. The alkalinity that washes into the soil from wood ashes spread over the ground kills cutworms.
Cucumber beetles are repelled (or killed?) by a spray made from a handful each of wood ash and hydrated lime mixed into two gallons of water, then sprayed on the leaves. Squash bugs are repelled by a sprinkling of wood ashes mixed with turpentine, at the rate of one tablespoon turpentine per gallon of wood ash. And a paste of wood ashes and water on the trunk of a peach tree will keep borers at bay.
Squash bugs have never been a serious problem in my garden and my cabbages rarely get maggots, so I can’t personally vouch for all these pest deterring and killing properties of wood ashes. But wood ashes are nontoxic and the above suggestions come from reputable sources, so the treatments are worth a try when needed. The main drawback to using wood ashes for pest control is that they usually must be applied again following rain. Also, keep wood ashes off tender seedlings, or they will be burned.
Perhaps there is some sort of gnome or gremlin in these ashes that impart them with such a myriad of uses.
Lee Reich, PhD is a garden and orchard consultant; he hosts workshops at his New Paltz farmden, which is a test site for innovative techniques in soil care, pruning, and growing fruits and vegetables. leereich.com.
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