Friday, January 30, 2009

Wood ash... the wonder trash.

There’s an old saying…"If you heat with wood it warms you twice. Once when you cut and split it and again when you burn it.” There is a final benefit to heating with wood if you are a gardener…. wood ash.

Wood ash is high in several plant nutrients, most notably potassium. Most references say that wood ash contains about 5 % potash… slang for potassium oxide, K2O in the fertilizer biz. That puts wood ash right up there with many conventional bagged fertilizers as far as nutrient content goes. It would be (is) a 0-0-5 fertilizer. No nitrogen or phosphorus but a nice slug of potassium.

Potassium is one of the “Big Three” or primary elements that plants need in order to grow and thrive. It is always listed in third place in the sequence of three numbers on a fertilizer bag.

There isn’t much “organic” about wood ash since all of the organic matter is burned off in your fireplace or wood stove. But most organic gardeners are more concerned about the natural part of stuff so wood ash is accepted. So everything is hunky-dory, right? Maybe.

First be sure that what you burned was wood, not stuff that might leave nasty remains. And that includes pressure treated wood, which until recently usually contained arsenic. Next, it is important to realize that wood ash is quite alkaline which means that it will increase soil pH (lower the acidity). It acts like a liming material. Still with me?

The practical matter is, while wood ash does contain essential nutrients, you can have too much of a good thing. If you have a big wood burner and a small garden you can drive soil pH into an extreme range and over apply potassium. Neither is good for plants. So how much should you apply? Depends on what you’re growing and the current status of your soil fertility. If you really want to know, get a Penn State Soil test. If you have to take a guess, don’t apply more than a pound or two of wood ash per 100 square feet until you get more information. If you just want to get rid of the dang stuff, sling it around the lawn. Wood ash is light and with the right technique you’ll be applying such small quantities that it will be hard to over apply it. Since wood ash is alkaline, avoid applications to areas where you are growing acid loving plants such as azaleas and blueberries.


Kathleen said...

I have just been reading about the use of wood ash to eradicate lesser celandine - a spring invasive plant that has moved into my garden, wooded area and lawn (western NY)- I am looking for more details on when to use, how much to apply and methods for application. Do you have any information on this topic? Thank you

Scott Guiser said...

I doubt that wood ash would eradicate Lesser Celandine. Think about it... we apply wood ash to our gardens because it supplied elements that are essential for plant growth and it is somewaht alkaline, thus reducing soil acidity. I'll take a guess that moderte amounts of wood ash would make lesser celandine grow better!Of course excessive amounts of wood ash might make the soil unsuitable for anything by making pH extremely high. Then what do you do?
I'd be interested in taking a critical look into the basis for the suggestion that wood ash eradicates lesser celandine. In the mean time, moderate amounts of wood ash appled to soil are OK. Of course the celandine will be fading away very soon. Don't mistake its ephemeral nature with "control " of wood ash.