As temperatures rise (or fall), many of us head outdoors to enjoy a nice evening fire (assuming there’s no Spare the Air alerts, of course!). There’s just something primal about sitting around an open fire with family and friends, enjoying the stars, good conversation, and maybe a bottle of wine. But what about the ashes left over from the fire? Can they be used in the garden? The answer is yes, and no.
What is in your wood ash?
Before adding wood ash to the garden, it is important to know what it was before it was burned. Ashes from BBQ grills, plywood, pressure treated, stained or painted wood, cardboard and even paper bags should never be used in the garden. These materials can contain toxic chemicals that you certainly don’t want in your soil, especially if you are growing food plants.
Wood ashes vary in their nutrients, depending on the type of wood that was burned. Soft woods, such as pine, contain only one-fifth of the nutrients held in hardwood. Ashes from good quality hardwoods contain a lot of potassium, or potash. This is the “K” in the standard N-P-K found on fertilizer packaging. On average, wood ash contains 0-1-3 using this scale. Wood ash generally contains the following micronutrients as a percentage of weight:
Potassium supports root growth and cell structure. Stronger roots and cells make plants more resistant to pests, diseases, and environmental stresses. Wood ashes can also improve soil structure, but there’s a price.
Wood ashes and pH
Wood ashes are very alkaline. Using the pH scale, substances are measured on a range from 0 to 14, with 7 being neutral. Lower numbers are more acidic and higher numbers are more alkaline. Wood ash can have a pH of 9-13! Our San Jose soil tends to already be alkaline, so adding wood ash might end up being a bad idea. The only way to really know what you are working with is through a soil test from a reputable lab. Over-the-counter soil test kits are not currently reliable or accurate enough to be useful.
If your soil could benefit from applying wood ash, the best time of year to do this is in the fall and winter. Save cold wood ashes until then and apply over a period of time. Smaller plants and seedlings can be dramatically impacted, for better or worse, with sudden changes in pH.
Other garden uses for wood ash
If your soil does not need its pH altered, there are several other uses for wood ash in the garden:
If you’re feeling really adventurous, you can use those wood ashes to make soap or shine up Granny’s silver. Just be sure to wear protective clothing when working with wood ashes. Wood ash particulates can irritate your lungs and wet wood ash (lye) can dissolve your fingernails, so be careful!
You can grow a surprising amount of food in your own yard. Ask me how!
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