Wood ashes; into the garden, with caution

9 mins read

Wood ashes. The ghosts of winter fires past. Those who heat with wood probably have a galvanized garbage can of them saved by now, planning to scatter them in the garden as soon as the snow goes off. Unless, like the Opinionated Gardener, you used most, if not all, keeping the icy path to the car walkable this winter.

A good source of potassium-containing potash, especially needed by carrots, cabbages, celery, corn, tomatoes and potatoes, wood ash, given its high calcium content, is also a fine soil sweetener or liming material. (Wood ash usually contains 3 to 8 percent potash, 1 to 2 ½ percent phosphate, 20 to 25 percent calcium and about 2 percent magnesium)

Herein lies the potential for more than one problem, says Bruce Hoskins, Coordinator of Soil Testing at the University of Maine.

Wood ash will raise the pH of the soil while supplying it with potassium (See footnote 1) and that’s okay, says Hoskins, if you have a new garden where the soil is very “sour” and also in great need of potassium (See footnote 2). But if the garden is an established one and the soil is already “sweet” enough, then adding wood ashes as a source of potassium could move your pH into the “too sweet” range and throw your soil out of whack.

The danger, says Hodgkins, is that too high a pH can interfere with the uptake of important nutrients by the plant; it will also play havoc with the microbial activity in the soil.

Sweet and Sour Soil? pH?

Chemically speaking (skip this if you remember your high school chemistry), a molecule of water contains positively charged hydrogen and negatively charged hydroxyl ions. A soil is said to be “sour” or acidic if the concentration of the hydrogen ions in the soil is greater than the concentration of the hydroxyl ions; it is said to be “sweet” or alkaline if the concentration of hydroxyl ions is greater.

Soil scientists measure the concentrations in terms of pH, using an acidity-alkalinity scale that runs from 0 to 14. At pH 7, the two ions are in balance and the pH of the soil is said to be neutral; below pH 7 the soil is acidic; above pH 7 the soil is alkaline.

Keeping the pH in the slightly acidic range of 6 to 7 enables plants to easily uptake the nutrients it needs from the soil. As soil acidity decreases and the pH rises above 7, nutrients such as potassium and phosphorous and manganese become “less available.” While most, but not all, vegetables do well in a soil that is slightly acidic many fruits such as blueberries, raspberries, rhubarb and watermelon “prefer” more acidic soils (See footnote 3).

But don’t let the face value of the numbers fool you! A pH of 6 is ten times more acidic than a pH of 7 and 100 times more acidic than a pH of 8. All of which is to say a slight change in the pH of the soil can cause a great change in the acidity or alkalinity of the soil. To lower acidity (or raise the pH) we lime; to raise acidity (or lower the pH) we add agricultural sulphur or peat moss and pine needles.

You are playing Russian Roulette with your garden soil if you use wood ashes regularly and indiscriminately. A soil test every threee years should be your guide. Contact the Cooperative Extension in Farmington at 778-4650 or go on line at http://anlab.umesci.maine.edu

Alternative Sources of Potassium

So, if the soil in your garden is in need of potassium and its pH is within the so-called “optimum” range of 6.0 and 7.0 what is a gardener to do? That was the question facing the ash-addicted Opinionated Gardener eight years ago when the folks at the soil testing laboratory at Orono reported that the pH of her soil was at pH 6.7 and the potassium level of her soil was less than “optimum.”

At the time, they recommended the OG discontinue using wood ashes (0-1-5 in the world of NPK) as a source of potassium so as not to raise the pH any higher. And given her organic gardening tendencies they recommended she apply 350 pounds of Greensand per 1,000 square feet of garden. Greensand (0-1-7) is a slow releasing, organically approved source of potassium from 70-80 million year-old marine deposits. At 350 pounds/1,000 square feet, however; the OG confesses she never followed through on the recommendation and hoped that by relying on the reported high organic matter content of her soil she would meet her garden’s need for potassium.

Hoskins admits Greensand is too slow-releasing or weak to be a realistic source of potassium and says Orono stopped recommending it for organic gardeners several years ago. Today, he would recommend applying 9 pounds of Potassium Sulphate (K2SO4) /1,000 square feet of garden. Also known as Sulfate of Potash (0-0-50), it’s an organically approved quick-release source of potassium. But before purchasing any (See footnote 4) the OG has decided to have another soil test. After eight years, it’s time.


Footnote #1: Potassium (K) is one of three primary plant nutrients along with nitrogen (N) and phosphorous (P). It is regarded as the “Great Regulator,” activate in the numerous enzyme systems which control many plant processes like photosynthesis, protein and starch formation and efficient water use.

Footnote #2: If you are starting a new garden, Hoskins suggests a one-time application of about 70 to-90 pounds of wood ash per 1,000 square feet to meet the liming and potassium needs of the soil. Hoskins figures a cord of hardwood produces between 40 to 50 pounds of ash. That’s a lot of winter fires.

Footnote #3: Potatoes are heavy feeders of potassium. But don’t use wood ash to provide them with potassium as potatoes do best in an acidic (pH 5) environment. Soils with a higher pH harbor bacteria like organisms that produce brown corky areas on the surface of the potato known as “scab.” Instead feed the soil potassium or muriate of potash.

Footnote #4: While Sulphate of Potash (0-0-50) is recommended for organic gardeners, the Opinionated Gardener has found it is not that easy to obtain in small amounts. Espoma, a New Jersey distributor of the soil amendment, says it stopped selling it two years ago due to the lack of demand. However it still sells the non-organically approved Muriate of Potash (0-0-60) aka Potassium Chloride (KCl) for $12 a 5 pound bag. However, the Vermont-based North Country Organics does sell Sulphate of Potash but only in 50 pound bags – enough to last a lifetime. But at $45 a 50 pound bag, it would make a great cooperative buy. It is available through the Paris Farmers Union. For organic gardeners seeking a supply of magnesium and sulphur as well as potassium, there is the fast releasing Sul-Po-Mag aka Potassium-magnesium sulphate (0-0-22) which is readily available through Fedco and most garden centers.


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