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Master Gardener: Fireplace ashes aren't the answer to soil problems
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Master Gardener: Fireplace ashes aren't the answer to soil problems

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I read in an online gardening group that I should spread my fireplace ashes around our garden. Is that a good idea? — L.T.

Utilizing fireplace ashes in your garden can be problematic. Here’s why.

Fireplace ashes contain potassium, which is one of the big three nutrients we need in our soil. The other two are nitrogen and phosphorus. Nitrogen is what we consider a “mobile” nutrient, which means it moves around in the soil fairly easily and then gets consumed by growing plants. This consumption is why we add nitrogen to our gardens on a regular basis.

Potassium and phosphorus are considered “non-mobile” nutrients, meaning they do not move around in the soil very much. Soils need appropriate levels of phosphorus and potassium to enable nutrient transfer from the soil to your plants. But over time, phosphorus and potassium levels don’t change that much. Once the levels of these nutrients are appropriate, there’s no reason to continue to add them to your soil. Too much can actually harm your plants.

Using fireplace ashes as a soil supplement can be problematic in that their potassium content varies depending on the type of wood and the type of fire. Hickory typically contains about 3.6% potassium while oak is about 4.5%. Some wood ashes can run up to almost 9% potassium. This variability makes it hard to know the quantity of ashes that would be appropriate to add to your garden to boost potassium levels.

In addition, fireplace ashes also contain what are called soluble salts. In hickory ashes, soluble salt content comes in at about 10%, while oak ashes can have as much as 36%. As a rule, salt and plants don’t play well together.

As I mentioned, the potassium content in fireplace ashes also varies by the type of fire, meaning that it depends on whether the wood was burned in a “hot fire” or “cold fire.” Ashes from “hot fires” tend to have a higher percentage of potassium and soluble salts than do ashes from “cold” fires.

And if that weren’t enough, fireplace ashes usually have a high pH. Most gardens (with the exception of azaleas and blueberries) prefer what we call a neutral pH of about 7. The pH of fireplace ashes tends to run between 11 and 12. Because of this, adding fireplace ashes could contribute to raising your soil pH to an unhealthy level.

Getting a soil test is the only way to know your soil nutrient levels. Even if your soil was found to be low in potassium and pH, you could only guess at the quantity of fireplace ashes to add to your soil, and then there’s still the salt problem.

Because of all these things, we do not recommend adding fireplace ashes to your garden. The best strategy is to get a soil test to determine your soil nutrient levels and make appropriate adjustments from there. We have information on how to get a soil test on our website (tulsamastergardeners.org). Good luck.

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You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th St., or by emailing us at mg@tulsamastergardeners.org.

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