Wait on Wood Ash

Last night I attended a meeting about developing a wood heating system for the school and municipal buildings. Experts said that it makes economic sense (saves money, creates jobs) and wood suppliers and the local forester say the fuel supply is sustainable. The system under discussion is said to be smokeless (see Garn Wood Heat Systems) so air quality will be unaffected. But there will be ash - quite a bit of it. Let's see...according to information on the University of Connecticut horticulture site,

An average cord of wood, depending on the efficiency of combustion and wood type, will yield approximately twenty pounds of ashes or the equivalent of one five-gallon pail.

Preliminary numbers suggest that we would annually use at least 700 cords of wood to heat the school. That's 14,000 pounds of ashes or 700 5-gallon buckets. The presenter caught my attention when he said that the school in Darby, MT spreads its ashes on its playing fields. Can we do that too?

The answer, like most answers, is "very judiciously." Wood ash is like limestone, only different. It raises the alkalinity of the soil, only faster. And for plants, and lawns, and playing fields, faster may not be better. Limestone takes up to six months to take effect; wood ash is a very fine textured material, is highly water soluble, so quickly changes the soil pH. (pH stands for Potenz Hydrogen. Now you know!) Wood ash raises the alkalinity of the soil, usually measured on a scale of 1 (acid) through 7 (neutral) to 14 (alkaline).

According to Purdue University Consumer Horticulture, wood ash should never be used on acid loving plants - like potatoes, rhododendrons and blueberries. But applying small amounts to most soils will not adversely affect garden crops. So, how small is small? You have to know if your soil is acidic, slightly acidic, neutral, or alkaline, remembering that the acidity-alkalinity of your soil can vary from one corner of the garden or field to the other.

Acidic soils (pH less than 5.5) will likely be improved by wood ash addition. Soils that are slightly acidic (pH 6.0 to 6.5) should not be harmed by the application of 20 pounds per 100 square feet annually, if the ash is worked into the soil about 6 inches of so. However, if your soil is neutral or alkaline (pH 7.0 or greater), find another way to dispose of wood ash.

Now, maybe there was a typo in the University of Purdue information, because the University of Connecticut site says this:

A safe rate of wood ash application for a garden or lawn area would be twenty pounds per thousand square feet or a five-gallon pail full of wood ash.

Uh oh. That's a big difference - one 5-gallon bucket on a 10 x 10 plot, or one 5-gallon bucket on a 100 x 10 plot? Looks like some clarifying questions are in order. I'll see what I can do.

Regardless of the rate of application, it looks like it's imperative to determine the pH of your soil. You can get a fairly reliable measure with the home tests you can pick up at a gardening store, as long as you use distilled water. Turns out that tap water has a pH too (probably headed in the direction of alkalinity) and you will skew your results if you don't use distilled water.

There are also soil pH indicator plants. Dandelions indicate acidity; chickweed indicates alkalinity. And if you have them side by side? Definitely put those wood ashes somewhere else!