Wood ash is an organic source of potash. The wood ash also act as a liming agent, they reduce the acidity of the soil. The average nutrient content is 1.50 per cent N 2-4 per cent P2O5 and 3-12 per cent K2O.
Wood ash will produce lye (sodium hydroxide) and salts if it gets wet. In small quantities, the lye and salt will not cause problems, but in larger amounts, the lye and salt may burn your plants. Hence, wood ash has to judiciously use in vegetables.