shule1

Wood ash is mostly calcium and carbon, and oxide, I believe. It has potassium, a lesser amount of phosphorous, and trace minerals. Although I've personally used it in soils that were probably already alkaline without issues, I wouldn't recommend it unless you're just experimenting and don't mind increasing the pH. I wouldn't use more than three handfuls per plant or so. Many of the minerals in wood ash are water soluble, and readily available to plants (so, even the trace amounts make a noticeable difference). High pH can cause mineral deficiencies and/or toxicities, however. Reducing the pH with sulfur can take a while.

If your pH is consistently high, I recommend amending with peat moss. It seems to make soil with too much wood ash in it growable (at least for a season or two, if not longer). I'm not sure of the long-term effects of peat moss on pH, but sulfur is usually used to reduce pH, and takes a while.

Growing stuff generally should help to draw calcium from the soil over time. Tomato plants seem to use a fair amount of calcium.

For your clay soil, I'd be a lot more concerned about how much organic matter you have, though. And I'd recommend mulching. Mulching or using black plastic also seems to make some tough soil more growable. Clay soil can insulate well, and stay cool, IMO; so warming it with black plastic may help the plants.

Your sandy soil is alkaline, too? That's interesting.

I wonder if calcium is what's keeping the pH high or if it's other minerals.