The Benefits of Using Wood Ash in Your Garden

Are you wondering what to do with the wood ash from your fireplace or wood stove?  Here's an excellent use, if you're a gardener. Wood ash can be a valuable resource for your garden, providing nutrients and helping to improve soil pH. 

Wood Ash Properties

Wood ash contains various nutrients that can benefit plant growth. The most common nutrient found in wood ash is calcium, which can make up about 20% of its content. Potassium, also known as potash, is another essential component, occurring at concentrations of up to 5%. Magnesium, phosphorus, and sulfur are also typically present in wood ash in smaller amounts, ranging from 1% to 2%. Additionally, wood ash may contain trace amounts of iron, aluminum, manganese, zinc, boron, and other nutrients that plants need.

Modifying Soil pH with Wood Ash

In addition to its nutrient content, wood ash can also help modify soil pH. When wood is burned, it produces carbonates that react with and neutralize acid in the soil, resulting in an increase in soil pH. The extent to which wood ash can neutralize acid depends on the type of wood burned and the burning process. Generally, wood ash has about 50% less acid-neutralizing capacity than commercially available acid neutralizers like pelletized lime or aglime. As a rule of thumb, approximately four cups of wood ash can be substituted for one pound of aglime.

Potential Downsides and Precautions

While wood ash can be beneficial for your garden, there are a few things to keep in mind. Firstly, it's important to use wood ash from an appropriate source. Only use wood ash that comes from trees grown in natural areas. Do not use ash produced from trees grown near industrial sites or in soils that may be contaminated with toxins or heavy metals. It's also crucial to avoid using ash produced by burning treated wood, waste oil, plastics, or garbage, as these materials can introduce harmful substances into your garden.

Another consideration is the effect of wood ash on soil pH. Applying wood ash to acid-loving plants such as blueberries, azaleas, rhododendrons, birch trees, red maples, and pin oaks may not be beneficial and can contribute to problems like chlorosis.

Many vegetables and other landscape plants prefer slightly acidic soils, so wood ash should be used judiciously when growing these plants. Also, increased pH due to wood ash use may promote certain diseases in some cases, such as potato scab for potatoes. It's essential to consider the specific needs of your plants.

Finally, it's worth mentioning that wood ash may contain heavy metals such as cadmium and lead, although the levels can be minimized by carefully selecting the wood source. If you have concerns about heavy metal contamination, consider testing the ash before use. The UW Soil and Forage Lab can provide guidance on heavy metal contaminants.

Using Wood Ash in Your Garden

1. Determine Nutrient Needs

Before applying wood ash to your garden, it's important to understand the nutrient needs of your plants. Conduct a soil fertility test through a certified soil testing lab, such as the UW Soil and Forage Lab, to determine any deficiencies or imbalances in your soil.

2. Choose the Right Type of Wood Ash

Use wood ash that comes from trees grown in natural areas. Avoid ash produced from trees grown near industrial sites or in contaminated soils. Do not use ash from burning treated wood, waste oil, plastics, or garbage.

3. Sift and Remove Large Charcoal Pieces

Before applying wood ash, sift it to remove any large charcoal pieces and active embers. This will ensure a finer consistency and better distribution in the soil.

4. Apply Recommended Amounts

Apply wood ash based on the recommendations from your soil fertility test and the nutrient needs of your plants. Excessive amounts of wood ash can lead to nutrient toxicity or deficiency issues. As a general guideline, limit applications to a maximum of 15 to 20 pounds (approximately a five-gallon pail) per 1000 square feet per year.

5. Timing and Application Method

Spread wood ash evenly over the area to be treated, such as a vegetable garden bed, established perennial flowerbed, lawn, or other landscape area, during the winter. Avoid applying wood ash on windy days to prevent it from being blown away. Whenever possible, apply wood ash to moist soil. In the case of a vegetable garden, work the ash into the soil using a spade or rake in early spring.



By following these guidelines, you can effectively utilize wood ash as a natural fertilizer and soil amendment in your garden. Just remember to consider the specific needs of your plants and consult with a soil testing lab for personalized recommendations. With proper use, wood ash can contribute to healthier soil and more abundant plant growth in your garden.