One of my gardener friends swears by adding ashes from the fireplace to their garden soil, but I am hesitant. Should I engage in this practice or take a pass? BG

Fireplace ashes can be a natural source of potassium, however using fireplace ashes in your garden comes with several words of caution.

The big three nutrients we attempt to manage in our soil are nitrogen, phosphorus, and potassium. Nitrogen is what we consider a “mobile” nutrient in that it moves freely around in the soil plus it gets consumed by growing plants. Phosphorus and potassium are considered “non-mobile” nutrients, meaning that they don’t move around in the soil much or get depleted like nitrogen does.

Garden soil needs the appropriate levels of phosphorus and potassium to enable nutrient absorption by plants but changing phosphorus and potassium levels in your soil is more of an extended process rather than just adding the appropriate nutrients to your soil. Plus, adding nutrients to your soil without knowing the current levels of those nutrients places you in a position of not adding enough or adding too much. Odds that you guess just right are pretty slim. The only way to know whether you have a nutrient deficiency is to have a soil test done through the OSU Extension.

Assuming you get the results of your test and find that you do indeed have a potassium deficiency, using the appropriate amount of fireplace ashes would still a bit tricky. For example, hickory contains about 3.6% potassium while oak is about 4.5%. Different woods can run up to almost 9% potassium. This variability makes it difficult to try to guess the appropriate quantity of ashes to add. Plus, fireplace ashes also contain what are known as soluble salts. The soluble salt level in hickory rungs about 10% while some woods can contain as much as 36% soluble salts. And, as most of us know, salt and plants do not do well together.

In addition, potassium content in ashes also varies by how hot the fire was. The hotter the fire, the higher percentage of potassium and soluble salts that will be found in the ashes.

And if that weren’t enough to dissuade us from using fireplace ashes in our gardens, these ashes usually have a high pH. The pH level in your soil is important because it correlates to nutrient availability in your soil. As gardeners we shoot for a soil pH of between 6 and 7. Fireplace ashes tend to have a pH of between 11 and 12. Because of this, adding fireplace ashes to your soil could boost the pH level to an unhealthy level for your plants.

If you can’t tell where we are going with this yet, we do not recommend adding fireplace ashes to your soil. Your 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 (www.tulsamastergardeners.org). Good Luck.

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 Street, or by emailing us at mg@tulsamastergardeners.org. Photo: Pixabay