Because the wood ashes act as a liming agent, they reduce the acidity of the soil. Plants that prefer acidic soil such as blueberries, azaleas or rhododendrons will not thrive if wood ashes are applied.
[header] How to use ashes as fertilizer [title] Use wood ashes as a soil amendment in the early spring while the soil is dry and before the plant life has begun to actively grow. [substeps] Nearly all plants benefit from the potash content of wood ashes. Other components of the ashes are beneficial to the soil and plant growth as well.