Well, let’s take a quick look at the variables:
1. Wood ashes will raise soil pH because the largest component of wood ash (about 25 percent) is calcium carbonate, a common liming material that increases soil alkalinity. So generally use them only if your soil pH is under 7.0 based on a soil test.
2. The safe rate of wood ash application to lawn or gardens is 15 to 20 lbs. (7 to 9 kg) per 1000 square feet (93 square meters) per year, which is approximately a 5 gallon (19 l) pail.
3. Remember, a little wood ash is beneficial, but a lot is not because increasing the alkalinity of the soil does affect plant nutrition. Nutrients are most readily available to plants when the soil is slightly acidic. As soil alkalinity increases and the pH rises above 7.0, nutrients such as phosphorus, iron, boron, manganese, copper, zinc and potassium become chemically tied to the soil and less available for plant use.
4. Wood ash does have some fertilizer value, the amount varying somewhat with the species of wood being used. 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.
5. Trace amounts of heavy metals such as lead, cadmium, nickel and chromium also may be present. Wood ash does not contain nitrogen.
6. Applying small amounts of wood ash to most soils will not adversely affect your garden crops, and the ash does help replenish some nutrients. But because wood ash increases soil pH, adding large amounts can do more harm than good.
7. Crop tolerance to alkaline soil also should be considered. Some plants, such as asparagus and juniper, are more tolerant of slightly alkaline conditions than “acid-loving” plants, such as potatoes, rhododendrons and blueberries. Wood ash should never be used on acid-loving plants.
8. The best thing with wood ash is to use it sparingly, and test your soil pH regularly to make sure you are not causing any problems.
Have great week!