I’ve seen several posts online about how ashes from the fireplace can keep insects away from my plants. If this is true, how much should I use? TD

The internet can be a wonderful source of information, unfortunately some of that information is bad information. While it’s true that fireplace ashes have the potential to be useful in the garden, that upside comes with a pretty significant downside.

As gardeners, the three primary nutrients we are concerned with in our soil are nitrogen, phosphorus, and potassium. Nitrogen is what we call a “mobile” nutrient which means that it moves freely in our soil. Phosphorus and potassium are considered “non-mobile nutrients” meaning that they don’t move around in the soil as easily as nitrogen does or get depleted as quickly.

The only way to know the nutrient levels in your soil is to get a soil test. At the Tulsa County Extension Office, we offer soils tests that will provide you with the nutrient levels of nitrogen, phosphorus, and potassium, as well as your soil pH. Without knowing these levels, we are really just guessing at how to supplement our soil. If we add nutrients, we could be helping or hurting, depending on our current nutrient levels. We have information on our website about how to get a soil test, but back to fireplace ashes.

It’s possible for fireplace ashes to be considered as an additive to your garden, but only if you have a potassium deficiency and guessing the right amount of ashes to add can be problematic. Here’s why. Ashes from hickory contain about 3.6% potassium, while oak ashes are about 4.5%. Various ashes can run up to almost 9% potassium. This variability makes it difficult to determine what quantity of ashes to add to the soil. In addition, fireplace ashes contain what we call soluble salts. Hickory ashes contains about 10% soluble salts and some woods contain as much as 36% soluble salts. Most of us know that salts and plants don’t do well together.

To complicate the issue further, potassium content in ashes can vary depending on how hot the fire was. Hotter fires produce ashes that will have higher percentages of potassium and soluble salts.

If you’re still not convinced to avoid putting fireplace ashes in your garden, consider that fireplace ashes typically have a high pH. As gardeners, we need our vegetable gardens to have a pH somewhere around 7 which is considered a neutral pH. This is the pH range where most of the nutrients will be available for your plant. Fireplace ashes tend to have a pH of between 11 and 12. Adding fireplace ashes can raise your soil pH to the point your plants will have difficulty growing in your garden. If you’ve ever had a burn pile at home, you know that nothing grows in the burn pile.

Because of this, any potential benefits you might gain in insect repellant, can be more than countered by the potential downside of degrading your soil nutrient levels and availability. Our recommendation? Just put the fireplace ashes in the trash. 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