|left: pure ohia ash right: 50% ash & 50% bone|
Wood ash is a valuable item, in my opinion. I use it for the garden on a regular basis. And since I use wood to heat my house and cook food, I create my own and can control what sort of wood makes the ash. Very importantly, I can prevent trash, plastics, and other nasties from contaminating the ash.
Ash has a rapid liming effect. It raises the soil pH. Since I have to deal with acidic rain due to being downwind from an active volcano, I find that this liming effect is very beneficial. Ash also provides potassium, a plant nutrient. And there is small amounts of various trace minerals.
When my ash is created, I also add bone to the woodstove. The heat from the embers bakes the bone, making it fairly easy to crumble by hand or crush with a hammer when it is cool. Thus I am able to add bone to the ash, adding calcium and phosphorous, both needed plant nutrients. Beside bone, I also bake coral. Once cool, I crush it with a hammer. This adds more much needed calcium to the ash. Soils here are extremely deficient in calcium.
To use the ash, I normally add it to my compost, lightly dusting the various layers as the pile is being created. In this way, ash is gradually but constantly being added to the garden soil. I rototill or dig in compost with each new crop.
I regularly check the pH of the soil prior to adding the compost. This lets me know if a little more ash should be dusted on. So far I've never had to withhold the ash, not even with potatoes. But I take care not to add extra ash to areas that I plan to plant potatoes next. I know that the books say no ash with potatoes, but I don't have a problem with it so far. I guess that is because of the active volcano. And poosibly because I rototill in extra mulch to keep the soil easier to dig at harvest time. The extra mulch tends to bind up the excess nitrogen from the compost as the mulch rots down. Plus it also helps to keep the soil evenly moist, something that potatoes really do well with. I always check the pH before doing the ash thing.
I don't have any special formula for how much ash. As I said, I lightly dust the layers of a growing compost pile.