Wood ashes are very alkaline, with pH levels of around 9 to 11. But what is pH and what pH is too acid or too alkaline to safely handle or for ecosystem health? The “p” in pH stand for “power”, as in “raised to the power of” in mathematical notation, and the “H” stands for hydrogen. Technically pH is a measure of the strength of an acid solution, i.e. a solution of H ions. It is calculated as -log10H. Complicated, but here’s what it means. It has a negative sign, implying the lower the number the stronger the acidity, so water with a pH of 4 is much more acidic that water with a pH of 5. It’s also a log-scale, meaning a unit change represents a ten-fold change. So pH 4 is actually 10 times more acid than pH 5, and 100 times more acidic than pH 6. A pH of 7 is deemed “neutral”, neither acidic nor alkaline.
pH is a log scale. So pH 4 is actually 10 times more acid than pH 5, and 100 times more acidic than pH 6.
We are exposed to solutions of many different pH levels in our every day lives. Battery acid has a pH of 1 – take care! In the kitchen, vinegar, soda drinks, and lemon juice are quite acidic with pH levels of 2 to 3. Milk is almost neutral with a pH just under 7. Meat has a pH of 5 to 7. Baking soda in solution is alkaline with a pH of about 8.5. Among the most alkaline products in our home are cleaning agents such as Javex (pH 11), and TSP (pH of 12-14), and we know to take care when using them. Wood ashes are almost as alkaline when dissolved in water, with a pH varying from about 9 to 11. Thus adding wood ash to our soils has two distinct benefits. It is alkaline so it can neutralize soil acidity, and the source of the alkalinity is calcium minerals, so it can replenish the Ca lost to decades of acid rain.
If you are interested, here is an interesting interactive demonstration on pH values from the University of Colorado Boulder. Click or tap on the Macro view and select various liquids from the drop-down menu at the top. More can be added by pushing the red button on the dropper. Dragging the green ‘sensor’ to the liquid will give you the pH reading on the scale. Try adding water via the tap to see how it affects the pH value.