If you’re reading this, you’ve probably heard that wood ash is good for the garden.  I got to thinking about this and couldn’t figure out what the ingredients of wood ash would be.  It seemed to me that wood is mostly cellulose (which is really a complicated sugar) containing carbon, oxygen and hydrogen and that when combined with oxygen in the burning process would most likely form carbon dioxide and water (both given off as gas)…I mean what else could the combustion reaction create, right?  So what’s the stuff that’s left over and what does it contain that’s so good for plants? It turns out that all those other elements important to healthy cellular plant life (potassium, phosphorous, calcium, sodium, and chloride) form carbonates in the burning process so when you spread a bit of wood ash on your garden, you’re inoculating it with some of the elements most important to plant growth.

When you buy a bag of fertilizer, it has three numbers on the front separated by dashes.  This is the amount of nitrogen, phosphorous and potassium that the fertilizer contains by weight, so for example, a bag of 10-20-10 contains 10% nitrogen, 20% phosphorous, and 10% potassium by weight.  Nitrogen in plants acts like a rocket fuel and produces fast, vegetative growth.  Potassium and Phosphorous on the other hand are responsible for healthy intercellular exchange and both catabolic and anabolic reactions (building energy stockpiles and breaking them down) so are typically attributed to overall plant vigor.

What’s the chemical make up of wood ash?

So let’s go back to the fireplace and take a look at what’s going on.  We place a log on the flames and the majority of the wood, its cellulose, literally goes up in smoke.  Oxygen combines with the carbon and hydrogen in cellulose to form carbon dioxide and water vapor.  What little nitrogen is in the wood also combines with oxygen to form NO2. What’s left over, the ash, is any potassium and phosphorous that was in the plant, combined with carbon and oxygen in the burning process to produce carbonates of each element.   So, when we dump our wood ashes on our plants, we’re really giving them a big boost of two out of three essential ‘vitamins’ important to plant health, potassium (in the form of potassium carbonate or ‘potash’) and phosphorous in the form of phosphate (Po4).   Analysis of wood ash shows that it contains nitrogen, phosphorous and potassium by a % weight of 0-1-3.  A word of warning.  The chemical formula for Potash (the carbonate formed from the reaction of Potassium and Carbon Dioxide in the burning process) is K2CO3.  When this dissolves in water, you get Potassium Hydroxide KOH, a strong base.  That’s why you want to make sure you don’t add wood ashes to acid loving plants (like blueberries).

One more thing about wood ash.  Some people will combust wood at low oxygen levels to produce large chunks of charcoal.  They refer to this as bio-char.  It is believed to be good for plant health.  The charcoal bits act as an ADsorber and as an ABsorber. As an ABsorber, water is pulled inside the charcoal itself like a sponge holds water.  As an ADsorber the charcoal bit draws chemicals to its surface and hold onto them, the same way the above mentioned sponge holds breadcrumbs and dirt when you wipe it against your countertop. By holding chemicals onto its surface, charcoal evens out the soil chemistry and makes sure that excesses of chemicals are released evenly as the charcoal decays.