Wood ash is very basic like drain cleaner. Vinegar is weakly acidic so mixing the two will neutralize caustic ash and make it safer to apply to any plants. Generally, wood ash contains less than 10 percent potash, 1 percent phosphate and trace amounts of micro-nutrients such as iron, manganese, boron, copper and zinc. Trace amounts of heavy metals such as lead, cadmium, nickel and chromium also may be present. Wood ash does not contain nitrogen.