"Never" is a word I don't often use in reference to gardening and landscaping; when we're talking about living plants in an ever-changing environment, "never" has a tendency to come back to haunt us. So, you know I'm serious when I tell you the following:
Never add wood ashes to your compost pile in the Desert Garden.
Never, never, never, never, never, never, never.
Wood ash is extremely alkaline, and has high levels of calcium carbonate. While our desert-adapted plants do fine in our native alkaline soil, increasing the alkalinity with wood ash will damage their ability to take up and utilize soil nutrients. Our non-desert-adapted plants — those on which we are likely to use compost — are already struggling in our alkaline soils, and compost with wood ash in it is worse than adding no compost at all. Most vegetables, for instance, do best in soils with a pH ranging from around 6.0 to 7.0, while our native soil tends to be around 8.0 and above; adding wood ash raises the pH to a yet higher level.
One of the challenges we have in gardening and landscaping in the desert is that much of the gardening tradition and research in this country derives from and is centered on Northern and Eastern climates with higher rainfall. Often, when we pick up a gardening book, or read gardening advice on-line, we can easily discern the parts that don't apply to us here in the Desert Garden: dealing with deep snow, or excess rainfall, for instance. We don't as easily pick out the more subtle differences.
Of those subtle differences, one of the most important is our soils. Almost nothing that you read about soils in a gardening source is relevant to the Desert Garden, unless that source is specifically targeted at the desert. And even then you need to read with a skeptical eye.
A few days ago, one of our major desert newspapers ran a column by their garden writer declaring that wood ashes are appropriate to add to a compost pile. The columnist cited a government publication from the Northeast as the information source.
Shredded newspaper, by the way, is an excellent addition to the compost pile.