When the flames of the winter's warming fireplaces and wood-burning stoves die down, a pile of wood ash remains. These remains can be added to vegetable garden plots, but only in moderation.
Wood ashes are a source of nutrients, primarily potassium or potash. Adding wood ash to a garden can make the soil good for growing vegetables, unless you add too much. Potash is a stable nutrient and doesn't degrade quickly or dissolve once added to soils.
Adding wood ash to a garden plot also adds calcium and magnesium to the soil, similar to applying lime. Like lime, this will increase the pH level and once the pH hits a proper level the soil is slightly acidic. Adding more ashes is going to skyrocket the pH. As a consequence, vegetable plants will yellow because the plants will no longer be able to absorb the nitrogen, and they essentially starve themselves to death. Slightly acid soil at a pH level of 6.5-6.8 is perfect for most vegetables and ornamentals.
To safely add wood ash to a home vegetable garden, add no more than five pounds of wood ash per 1,000 square feet per year. To check the pH level, take a soil sample to your local Extension office. When the pH level reaches 6.8, hold off on adding more wood ash.
Wood ash should not be added to compost piles. Compost needs an acidic environment and the wood ash may slow down the microbes; the same is true for adding lime.