Wood ash from a fireplace or woodstove can be a good source of plant nutrients calcium, potassium, and phosphorus in the garden. Wood ash is alkaline—about half as alkaline as lime—so it can be used to balance acid soil, but it should not be used on alkaline soils—as found in most of the western United States.
Use wood ashes from natural, unpainted wood or non-shiny newsprint (shiny newsprint can include toxic inks). Use ashes from hardwood and tree prunings, avoid ashes from treated lumber or fireplace “logs” or pellets made of pressed wood chips and sawdust which may have been chemically treated.
Wood ashes—depending upon the variety of the wood being burned—can contain small amounts of phosphorus and calcium and significant amounts of potash or potassium. The plant nutrient phosphorus is important for fruit development and plant maturation; potassium and calcium improves stem strength and aides winter hardiness.
Here’s how to use wood ashes in the garden:
Collect ashes from fireplaces and stoves during the wood-burning months. Store ashes in an ashcan, metal bucket, or metal garbage can with a watertight cover. Collect ashes only when they are cold.
Scatter ashes in the garden bed at the start of the planting season before you plant. Use ½ pound per square yard. Work the ashes into the soil before planting. Don’t spread ashes too early—rain and snow will leach away the nutrients in ash. To slow leaching, mix ashes into organic amendments and mulches.
Wood ash is similar to lime in that it will reduce soil acidity. Avoid using wood ash where the soil is balanced or alkaline; too much wood ash can result in soil too alkaline for vegetable and other plant growing. A soil pH test is a good idea before applying a heavy amount of wood ash.
Scatter ash as a side-dressing around the base of plants when they are already growing in the garden. Spread the ashes a few inches from plants. Avoid direct contact between wood ash and germinating seeds or new plant roots—the potassium can burn young plants. Wood ash will discourage root maggots that attack crops such as cabbage and onions. A line of ash around plants also will turn back slugs and snails.
Use wood ash around alkaline loving crops such as artichokes, arugula, broccoli-raab, Chinese vegetables, collards, and tomatillos. Place the ash around the base of the plant in a line a few inches from the stem; do not sprinkle wood ash on plant leaves or stems.
Do not use wood ash around crops that prefer acidic soil such as potatoes, blueberries, strawberries, and parsley.
Do not add wood ashes to compost piles; wood ashes do not decompose.
Wood ash can be used like lime to reduce soil acidity. Wood ash also can be used to deter plant pests.