I put the ashes in my compost pile and mix with the various carbon products of leaves, spent plants and scraps from the kitchen. Only those gardeners with acid sandy soil should use the ashes directly on the soil.
Ashes are a valuable source of potash. Wood ashes generally contain from 1 to 10 percent potash and 1-1/2 percent phosphorus. They can be mixed with other fertilizing materials or side-dressed around growing plants in acid soils. Apply about 5 to 10 pounds per 100 square feet. They can be used in any soil if mixed with organic matter and composted first. Avoid using wood ashes around blueberries or other acid-loving plants and don’t use heavily even in alkaline soils. Only wood ashes, and not coal ashes, should be used in the soil or compost. Wood ashes are also an effective tool in controlling slugs.
Wood ash can be a beneficial soil amendment in acid soil but can be a problem for alkaline soils unless mixed into the compost pile before using. Even then it should make up more than about 10% of the pile. When the ashes are buffered with the other ingredients in the compost pile, the resulting product can be helpful. Having the pile and the soil tested by Texas Plant and Soil Lab in Edinburg would be the wisest and safest thing to do.