Tree Soap at base of tree
Photo: Wisconsin Dept. of Natural Resources
By Kevin McKeon, Maine Master Naturalist
Why Trees Sometimes Get in a Lather
A previous Out in The Woods article talked about the sudsy foam we often see along the streams during our trail walks. There’s another foam, tree soap, that we’ll see at the base of trees after a long-awaited rain. This is formed a bit differently and closely resembles real soap. So, a bit about what soap is, and how Grandma might have made it.
Back in the day, before we were able to decide what we wanted to do with our lives, we had to do the stuff needed for survival. Getting and keeping stuff clean seemed advantageous, so cleansing agents evolved. Along the way, it was discovered that wood ash mixed with water produced a strong alkali called potassium hydroxide — a type of lye — and when a fat like tallow or lard was added, an effective cleansing material that became known as soap was produced. As we humans evolved, we became more able to determine what we wanted to do, as opposed to what we needed to do. Bartering and trading offered the opportunity to relegate some of the “need-to-do” stuff to folks that were good at that stuff and wanted to do that stuff, so trades and specializations grew. Your ancestors no longer needed to make their own soap, in other words.
As civilization refined, various laws were enshrined to define and regulate accepted norms of the trades, including soap-making. The Food and Drug Administration now requires soap to meet three criteria: It must be composed mostly of “alkali salts of fatty acids,” those mixtures must be the sole cleansing agents, and the product must be labeled as soap. However, if its intent is either moisturizing the skin, adding a pleasant scent, or deodorizing, it’s not technically a soap, it’s a cosmetic. And if germ-killing to treat or prevent disease is intended, it’s not a soap, it’s a drug. But regardless, all those items can still have “soap” on the label.
Anyway, back to our tree soap. Tiny airborne particles of dust and plant matter continuously settle everywhere, including on the twigs, branches, and bark of trees. These deposits contain the salts and acids found in soap. And during dry spells or droughts, these deposits — which normally wash off during rains — can accumulate. The fat constituent comes from the tree, which has evolved various protective oil and wax coatings; also, various insects secrete oils as they feed on trees. Add rainwater to all the above ingredients, and we have the basic recipe for soap resting on our trees.
Next comes the mixing. Potassium hydroxide solutions have interesting molecules: One end likes water, while the other end likes fats and surrounds them — lifting them from stuff like our hands and clothing. Then, the water-loving end acts to emulsify — effectively keeping the lifted fats in solution — so we can now wash the fats (dirt) down the drain. Another thing potassium hydroxide does is create bubbles when mixed with water.
So here comes our welcome rain, and it washes down the tree. On its way to the ground, the water absorbs the salts and acids to form lye. The lye then reacts with the fatty deposits on the tree, essentially becoming soap. The rainwater solution’s turbulent flow over rough bark and ridges effectively agitates the mixture, aerating it to form a frothy and persistent bubbly mix — a process called stemflow mixing. These bubbles tend to be more pronounced on trees with rougher bark and deeper ridges — typically found on older and larger trees — allowing a higher level of agitation to form more bubbles that amass at the tree’s lower trunk. Sometimes this phenomenon happens on roads, with the agitation caused by tires rubbing on the pavement forming bubbles.
Stream Foam: https://sanfordspringvalenews.com/out-in-the-woods-7/
Editor’s note: Did you see something unusual last time you were out in the woods? Were you puzzled or surprised by something you saw? Ask our “Out in the Woods” columnist Kevin McKeon. He’ll be happy to investigate and try to answer your questions. Email him directly at: kpm@metrocast.net
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