Use caution with wood ash on your lawn and garden

Applying fertilizer
Applying fertilizer. Photo: Tom Gentle
Last Updated: 
January 1, 2011

CORVALLIS - Although wood ash is a valuable fertilizer for most lawns and gardens in western Oregon, use it with caution.

Wood ash is alkaline, which means it has a high pH level. Use the same precautions with it as when handling other strongly alkaline materials, such as household bleach, according to Dan Sullivan, soil scientist with the Oregon State University Extension Service.

Here are some ways to protect yourself from the health hazards of wood ash.

Wear eye protection, gloves and a dust mask. Do not scatter ashes in the wind. Apply recommended amounts to moist soil and rake lightly to mix.

Do not use ash from burning trash, cardboard, coal or pressure-treated, painted or stained wood. These materials can contain potentially harmful substances. For example, the glue in cardboard boxes and paper bags contains boron, an element that can inhibit plant growth if found in excessive levels.

Do not use wood ash on alkaline soils; or where potatoes will be planted; or on acid-loving plants such as blueberries, azaleas, and rhododendrons.

Never leave wood ash in lumps or piles. If it is concentrated in one place, excessive salt from the ash can leach into the soil and create a harmful environment for plants.

Do not apply ash at time of seeding. Ash contains too many salts for seedlings.

Do not add ash with nitrogen fertilizers such as ammonium sulfate (21-0-0-24S), urea (46-0-0) or ammonium nitrate (34-0-0). These fertilizers lose their nitrogen as ammonia gas when mixed with high pH materials such as wood ash. For a lawn, wait at least a month after wood ash application before applying nitrogen fertilizer to allow time for the soil to reduce the alkalinity of the ash.

Author: Peg Herring
Source: Dan Sullivan