How to Apply Wood Ash
Before you apply wood ash to your vegetable garden soil, it's best to do some soil testing. Most plants prefer a pH level between 6.0-7.0. If your soil is alkaline (with a pH of above 7), wood ash is not a best choice for your garden. Use less, or avoid using it altogether.
For fertilization, apply a thin dusting of wood ash over the garden bed, orchard floor, grass, etc. Make sure the ash is cool with no live embers before applying.
Application guidelines vary from 5-10 pounds per 1,000 square feet up 15 to 20 pounds (approximately a five gallon pail) per 1000 sq. ft. per year. More isn't necessarily better. In fact, too much ash can throw off the soil pH and nutrient balance.
As an example, some years ago my brother applied a light layer of ash in his apple orchard. My nephew saw the ash, and, wanting to help, added a lot more ash. The trees got “burned” from the excess nutrients and their growth that year was stunted.
In our garden, we apply a dusting of ashes in late fall or early winter. This gives the ashes time to break down. We also apply soiled coop bedding at the same time for nitrogen.
Chicken manure needs time to age before it's safe to have in contact with plants. Top dressing the garden beds in fall effectively acts as sheet composting.
While that is some seriously good advice for gardening, it does not address why people might wish to add ashes to their garden in the first place. Ashes contain a large amount of minerals that have been absorbed from the soil by the plants that made the wood. These minerals include calcium, potassium, manganese, magnesium, iron, and a lot of other good things. Except that they are usually formed as carbonates or oxides in the wood ash. This form of the minerals generally makes a very basic solution when mixed with water, which you know is going to happen out in a garden. So it becomes a balancing act as to whether to add plenty of minerals using some acid reversing cheap wood ashes, or to manage the mineral problems with extra fertilization at a greater expense.
information I extracted from an internet source on this topic:
"average wood ash would be about 0-1-3 (N-P-K)"
"Wood ash has a liming effect of between 8 and 90 percent of the total neutralizing power of lime, and can increase plant growth up to 45 percent over traditional limestone."
"Most of Georgia's soils are naturally infertile, acidic and low in organic matter. Therefore, additions of nutrients are required for adequate plant growth. Nitrogen fertilizers often lower the soil pH, which makes neutralizing agents such as lime or wood ash a necessity on most Georgia soils. Liming improves crop growth by increasing the availability of nutrients such as phosphorus, providing calcium and magnesium to crops, and decreasing the availability of certain metals. While dolomitic and some calcitic limestone are the most common liming agents used in Georgia, wood ash has many of the same effects. It also can replace many of the macro and micronutrients removed during plant growth and harvesting."
"Several studies compare plant growth using traditional limestone and wood ash. Most conclude that ash gives better growth responses than limestone. While some studies have reported detrimental effects at extremely high application rates, these responses were explained by the drastic increases in soil pH beyond the plant's optimal level. As long as the soil pH is maintained at the proper level, productivity will be enhanced by using wood ash as a soil amendment."