Wood ashes provide a good source of potassium for plant growth. It also is an alkaline material that will reduce the acidity of the soil. It is not as effective as limestone but with repeated use, it can drastically raise the pH value of a soil, espe cially if the soil is primarily sandy in texture.
The ideal soil pH range for most plants is 5.8 to 6.5. Soils which are too alkaline will tie up many of the micro-nutrients such as zinc and manganese which are needed for good plant growth. Often at the soil testing lab, when we came across a soil s ample with a pH value in the range of 8.0 to 11.0, we would find out that it came from the garden of a homeowner who was routinely adding wood ashes during the winter as they became available from the wood stove.
I recommend that before applying wood ashes to the soil, that a soil sample be submitted to the Ag Service Lab for assessment of the current pH status. If the soil is already in the 5.8 to 6.5 pH range and no lime is recommended, it would be best to a void putting wood ashes in that area. If the soil pH value is below 5.8 and there is a lime recommendation on the soil test report then put a dusting of the wood ashes over the soil surface and work it into the soil. If you add the wood ashes, don't add the recommended lime or you will raise the pH too high. Since wood ashes are low in calcium compared to limestone, some supplemental gypsum (a source of calcium) will be needed if the calcium level indicated on the soil test report is medium or low. Ad d about 1 or 2 pounds of gypsum per 100 square feet to compensate for the lack of calcium.