How to Use Firewood Ash as Fertilizer

Firewood ash is an excellent organic fertilizer when applied correctly to the proper plants. It can bring 13 natural nutrients to your soil and deter insects as well. However, not all firewood ash can be used this way. Learn more about what you can use for this purpose and when so that your garden thrives.

Tip: Having a soil test preformed yearly is recommended. The results allow you to accurately apply the correct amounts of fertilizer and amendment to your soil.

Step 1 - Collect Firewood Ash

Leftovers from trash fires, cardboard, and pressure-treated, painted, or stained wood will contain chemicals and substances that harm plants, so first make sure you collect only materials that has come from burning natural, untreated, and unpainted wood.

Allow ashes to cool overnight or longer before scooping and storing them in a metal can with a tight lid to keep air out. The can will help keep embers from reigniting while they sit a little longer. Afterward, use a medium mesh screen to sift out any chunks of wood. You can crush these pieces and sift them back into the can or dispose of them, whichever you choose. Ash is caustic because it is alkaline, like bleach, so protect your hands, eyes, and lungs properly while you work.

Step 2 - Apply to Plants

Spread the ash in the spring when the soil is dry, on a dry day with no or low wind. Lack of moisture is very important because if the ashes get wet before application, the water will leach some of its nutrients.

You can scoop it out and sprinkle it by hand around plant stems or at their base as long as you're wearing your gloves. Hand application is the most effective way to ensure that your fertilizer goes where you want it and in the right quantities. You can distribute larger amounts with a trowel or shovel or other broadcast applicator.

Be sure to spread your ash in moderation. Adding this to the garden or lawn will quickly raise the pH—it is not slow release— and lead to a salt buildup in the soil that will harm, more than help, your plants. You can safely spread up to 20 pounds (about a five-gallon bucket of ash) per 1,000 square feet of soil, or five pounds per 100 square feet per year. Limestone does the same thing as ash, so make sure your total applications of both fertilizers don't exceed 20 pounds.

Tip: Wood ash also adds potassium to the soil.

Step 3 - Store or Dispose of Unused Ash

An annual application of ash is plenty, so the best thing to do when you're done is to store it away or get rid of it. You can return the unused material to your metal can, sprinkle it on your compost pile, or otherwise dispose of it. Ash will help neutralize compost, making an ideal pH level for decomposition. Do not, however, allow it to pile up in hills or clumps, as it will leach salts into the soil.

When to Use Ash

As mentioned previously, there are many cases where a firewood ash fertilizer is not beneficial. Plants like potatoes, blueberries, cranberries, rhododendrons, azaleas, magnolias, pine trees, and oak trees can all be negatively affected. Instead, stick to using it on things like lawns, tomatoes, and grass.

Warning: Don't mix firewood ash with nitrogen fertilizers! Dangerous ammonia gas is created when high nitrogen fertilizers are mixed with alkaline materials like wood ash.