Wood ash contains potassium as well as phosphate in small amounts, as well as trace amounts of micronutrients like iron, boron, manganese and zinc. The primary component of wood ash is calcium carbonate. This acts to make the soil more alkali, much as adding lime would do. Have your garden soil tested at your local cooperative extension office to determine its pH level, then add wood ash to garden soil that is acidic or neutral to adjust the pH and enhance the soil's nutritive content.
Test your garden soil pH using a test kit from your local cooperative extension service office. If your soil pH is higher--more alkali--than 6.5, do not apply wood ashes to the soil.
In late winter, wearing eye protection and a dust mask, slowly shovel wood ashes from their metal can or bin into a metal wheelbarrow using a manure shovel. Inspect the ashes as you shovel them to ensure there are no smouldering embers remaining in the ashes. Fill wheelbarrow with approximately 15 pounds of wood ash ir your soil pH measures between 6.0 and 6.5; fill wheelbarrow with proportionately more wood ash if your pH measures less than (more acidic than) a pH of 6.0
Transport the wheelbarrow to your garden area. Shovel the contents of the wheelbarrow onto garden soil and rake it out to an area about 10 feet by 10 feet square using a metal rake.
Turn the wood ashes in to the top 6 inches of soil using a spading fork. Be certain not to leave any patches of wood ashes remaining on the soil surface.
Repeat application of wood ashes to garden soil once a year for two more years. Retest soil pH the third year to determine whether it has shifted; if soil has become more alkaline than pH 6.5, discontinue wood ash applications for 2 years, then test soil again.