With both deciduous and evergreen trees dominating a large percentage of our coastal area, it’s no real surprise that many of us continue to heat our homes with wood. Those of us who cozy up to the warmth of a wood fire this time of year will validate that there’s nothing like wood heat to comfort us during our cool, damp winters.
An added benefit of burning wood is a seemingly never-ending supply of ashes. Although most gardeners long ago recognized the positive attributes of ashes as a source of fertilizer in the garden, the question often arises as to just how much ash can be safely added to garden soils? Furthermore, are there plants that should not have ashes placed around them?
Although wood ash does have fertilizer value, the amount varies with the species of wood being burned. Generally, wood ash contains less than 10 percent potash, 1 percent phosphate and trace amounts of micronutrients such as iron, manganese, boron, copper and zinc. Trace amounts of heavy metals such as lead, cadmium, nickel and chromium may also be present. Wood ash does not contain nitrogen.
The largest component of wood ash (about 25 percent) is calcium carbonate, a common liming material that increases soil alkalinity. Wood ash has a very fine particle size, so it reacts rapidly and completely in the soil. Although small amounts of nutrients are applied with wood ash, the main effect is that of a liming agent.
Increasing the alkalinity of the soil does affect plant nutrition. Nutrients are most readily available to plants when the soil is slightly acidic. As soil alkalinity increases and the pH rises above 7.0, nutrients such as phosphorus, iron, boron, manganese, copper, zinc and potassium become chemically tied to the soil and are therefore less available for plant use.
Applying small amounts of wood ash to most soils will not adversely affect your garden crops, and the ash will help replenish some nutrients.
Because wood ash increases soil pH, adding large amounts can do more harm than good. Keep in mind that wood ash that has been exposed to the weather, particularly rainfall, has lost a lot of its potency, including nutrients.
Specific recommendations for the use of wood ash in the garden are difficult to make because soil composition and reaction varies from garden to garden.
Acidic soils (pH less than 5.5) will likely be improved by wood ash addition. Soils that are slightly acidic (pH 6.0 to 6.5), which includes most of our local gardens, should not be harmed by the application of 20 pounds per 100 square feet annually, if the wood ash is worked into the soil about 6 inches or so. However, if your soil is neutral or alkaline (pH 7.0 or greater), find another way to dispose of wood ashes.
You should also consider your plants’ tolerance to alkaline soils. Some plants, such as asparagus and juniper, are more tolerant of slightly alkaline conditions than “acid-loving” plants, such as potatoes, rhododendrons and blueberries. Wood ash should never be used on acid-loving plants.
Donald Tapio is a WSU Extension regional specialist emeritus. He may be reached at firstname.lastname@example.org.