It’s finally cooling down and for a lot of us, this marks the beginning of one of our favorite seasons - fireplace season. Yes, there’s not much better than a warm fire on a cool night. This is also the time of year we see some terrible advice floating around social media suggesting that gardeners should place fireplace ashes in their garden because those ashes contain potassium. As with most garden myths, there’s a sliver of truth in there, but let’s talk about this seasonal garden myth.

Potassium is one of the three primary nutrients (nitrogen, phosphorus, potassium) that we try to manage in our gardens. Nitrogen is the nutrient which fuels growth. It gets consumed as plants grow. Phosphorus and potassium don’t get consumed, but they need to be present in the appropriate quantities to help enable all those chemical transactions.

The only way to know the nutrient levels in your soil is to get a soil test. We talk about soil tests pretty often, so let’s just remind everyone that you can’t make an informed choice about which nutrients to add to your soil without getting your soil tested. We have instructions on our website (www.tulsamastergardeners.org) under Lawn and Garden Help/Soil.

So, let’s assume you have your soil tested and you do indeed need to add potassium due to a deficiency in your soil. It’s challenging to figure out the quantity of fireplace ashes you would need to correct this deficiency because the type of wood you burn in your fireplace affects how much potassium will be found in the ashes.

If you are burning hickory, those ashes contain about 3.6% potassium. Oak ashes contain about 4.5% potassium while some wood ashes can contain up to almost 9% potassium. This variability makes it difficult to determine the quantity needed to adjust levels properly. .

Then there is also a salt issue. Hickory ashes contain about 10% soluble salts with some wood ashes containing as much as 36% soluble salts and we all know salt and plants don’t play well together.

To complicate this further, potassium content in ashes varies by how hot the fire was with hotter fires producing ashes containing a higher percentage of potassium and soluble salts than those in cooler fires.

If this weren’t enough to discourage the practice of adding fireplace ashes to our gardens, there’s also the problem of pH. We measure the acidity/alkalinity of our soils by assessing the pH of the soil. As gardeners we want our soil to have a pH of about 7.0 or a little less. We care about pH because soil pH contributes to nutrient availability in your soil. You can have all the nutrients at the proper levels, but if your soil pH is too high or too low, that will have an impact on nutrient availability which can make it difficult to grow plants successfully.

The problem is that fireplace ashes typically have a pH of somewhere between 11 and 12. So adding fireplace ashes can contribute to raising your soil pH to unhealthy levels for your plants. Strike three…or maybe strike four.

The bottom line - get a soil test to know which nutrients you may or may not need to add to your soil and avoid using fireplace ashes in your garden. 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