Wood ash is loaded with minerals. Back before the days of large-scale mining when wood was a common fuel wood ash was the primary source of some minerals.

One of the problems with wood ash is that it is extremely alkaline. People used to extract lye from ashes. Another problem is that is contains carbonates -- enough that it can fizz pretty actively when you drop acid on it. A third problem is that fresh, dry wood ash is chemically unstable. You can expect it to go through some difficult-to-predict changes when you put it under water. Yet another problem is that parts of it are soluble and will quickly end up mineralizing your water. Of course, there's some overlap between these problems, but I think you get my drift.

All-in-all, wood ash isn't very inert and isn't very benign. Experiments are always fun and usually worth doing for the learning experience alone, so I rarely try to discourage them. This is an exception. I have a hard time imagining that anything good could come from using fresh, dry wood ash in your substrate.

On the other hand, wood ash can make a pretty good amendment for soils that are low in mineral content and have a low pH. You just spread it out and dig it in.

If you want an inexpensive substrate that's known to work there's always dirt. If you're willing to sacrifice a little cost for some added manageability then there's kitty litter or kitty litter+peat. There are a number of other low-cost alternatives.


Roger Miller