Sure... the ashes contain almost all of the nutrients contained in the wood which was burned. Ashes contain a fairly high amount of potassium (the nitrogen was eliminated, however, through the burning process). Your application should be limited though, since a heavy application can cause leeching of the salts remaining and increase the salinity and decrease the acidity of the soil. About once per year and no heavier than about 1/4 inch on freshly tilled soil. If you use them with existing plants, don't let any of the ash settle on leaves since it can, under the right circumstances, burn the leaves...