A cord of Douglas-fir ash supplies enough potassium for a garden 30 by 30 feet. One-half to one pound of wood ash per year is recommended for each shrub and rose bush. Spread ash evenly on the soil around perennial plants. Rake the ash into the soil lightly, being careful not to damage the roots. Never leave ash in lumps or piles, because if it is concentrated in one place, excessive salt from the ash will leach into the soil, creating a harmful environment for plants.
Lawns needing some lime and potassium can also benefit from wood ash.
Do not use if soil pH is more than 7.0 or if potassium levels are excessive. As a general rule, acid soils that would benefit from ash application are usually found in those places in Oregon that get more than 20 inches of rain per year. In compost piles, wood ash can be used to help maintain a neutral condition, the best environment to help microorganisms break down organic materials. Sprinkle ash on each layer of compost as the pile is built up. Ash also adds nutrients to compost.