Heading into cooler weather and fires in the fireplace, I’ve been asked before if those wood ashes are of benefit in the lawn or garden. The short answer is yes, but let’s take a deeper look and, lastly, discuss when it would not be a good idea.
Wood ash from a fireplace or brush pile will act similar to a liming agent to reduce acidity in the soil. Unlike lime, wood ashes don’t “behave” the same. Lime will only go so far. Think of the Texas hill country full of limestone. Crops, pastures, landscapes and more grow just fine. But too much ash and the soil pH may swing too far away from being acidic, making the ground too alkaline.
Wood ashes do add nutrients to the soil. One old publication that I found lists the nutritive content of wood ash at 0-2-6 (N-P-K). But the same fact sheet lists a warning that it is quite alkaline and should not be used on high pH soils.
With a 0-2-6 designation, there would not be any nitrogen. Nitrogen is a gas that would likely be completely released during burning. The 2% phosphorous and 6% potassium will be found with other secondary and micronutrients that would benefit the soil. While some nutrients can be over applied, the benefits in wood ash are certainly present.
If tilled into a garden soil, the ashes may help aerate the soil with air pockets around the ashes. Aerated soils are far superior to compacted soils that don’t have enough oxygen for microbial activity. On clay soils, you may find a more workable (more friable) soil that clumps less.
Placing dry wood ashes around slug and snail-sensitive plants may deter those pests. Though this only works if the ashes are dry, and it may provide limited protection.
Feel free to add ashes to your compost pile. Keep ashes to no more than 5% of the pile. Those nutrients should be utilized by plants in the final product. Know that hardwood trees such as oak are expected to contain more nutrients that from softwoods. Nonetheless, ash from any tree will have nutrients in it.
Lastly, it is not a good idea to use wood ashes from wood that was painted or treated with a preservative in your vegetable garden as they may contain contaminants.
Perhaps the first question is whether your garden or landscape is acidic and even needs the pH raised. I guarantee no one will know by looking at it. Testing your soil for pH would be the first step before adding any ashes. But if you are looking for a rule of thumb, 20 lbs. per 1,000 square feet would be a good maximum.
