I’ve had multiple people comment on my biochar videos saying they like to use ashes in their gardens. that’s all well and good, but ashes aren’t charcoal.
Steven at SkillCult just posted a good look at the different uses of charcoal and ashes in the garden:
“If you apply enough heat to wood, it begins to break down and release gasses. If those gasses flare off, you get flames. If not, you get smoke. In the presence of oxygen, you are eventually left with a pile of white or grey stuff, which is the mineral content of the wood. If you stop the process, you have a chunk of light weight, easily broken, porous jet black carbon with no brown areas and no parts recognizable as wood. So charcoal is a shell of carbon left over once much of the substance and components of wood are destroyed by heat. Of course the charcoal still contains the minerals that are in ashes, but they are locked in this carbon matrix and not readily available.
Charcoal is stable and durable. It is capable of persisting in the soil for a very long time. Of course how long may depend on the type of charcoal, conditions, soil etc. but it is no doubt capable at times of persisting for millennia. While some of the minerals in ashes may be persistent, ash is essentially a very short term fertilizer.
First off, there is some debate about whether it is appropriate to call black cinders from the fire charcoal, or “biochar”. In video comments I’ve had people argue forcefully that any char produced in the presence of oxygen, the way I usually make it in open piles or pits, is not biochar, but just charcoal. Others argue forcefully that it is not charcoal, yet when asked what it is, they have no appropriate simple term. If I handed a chunk of that half burnt shell of carbon to most people, they would say it is charcoal. Words are not things and language is a product of living, changing culture.
If wood is heated under a very low to zero oxygen environment, it undergoes destruction, similar to an open burning fire, but more of the carbon structure is retained. It will be denser, harder and have a higher fuel value than wood that is burned with more available oxygen. At some point, all the gasses will be driven off and it will just stay red hot, without burning up, because there is no oxygen to finish the process. Once I was part of an iron smelting experiment. When we dug the kiln out the next day, I found pieces of charcoal embedded in slag. That slag, which is a collection of melted unwanted minerals melted from the iron ore, had been white hot, molten goo the night before, yet the charcoal survived it. It survived because there was no oxygen in that part of the kiln to finish turning that charcoal to ashes.
If I start a fire, then quench the coals at a certain point before they burn to ash, I end up with a softer and less dense product. This is closer to the way I usually produce char. The difference is quite real. I actually need to burn some of that low oxygen hard charcoal soon for my forge, because it burns longer. I can get away with using the softer stuff I usually produce, and it gets plenty hot, but I have to use more of it. The common argument is that char made by pyrolysis is better for agricultural use as well. That may be. The question that interests me more is whether less carefully prepared open burn char works and is it a viable option in some contexts. In my experience, it is. So grammarists and fundamentalists can argue the finer points or debate over the terminology all they want. I’m going to call it all charcoal or biochar, alternately, because that is what most people will understand. I think if the term biochar persists, it will come to mean all charred plants used for soil improvement and potting mixes. I am not particularly attached to what it’s called, I just use the language that is common. I would prefer to live in a society that has a more sophisticated nomenclature for chars, but I’m not sure I’m interested in trying to bring it about.
Charcoal has some interesting properties that make it potentially very useful in soil improvement.”
(CLICK HERE to keep reading over at SkillCult)
Steven was one of the gardens who first inspired me to try biochar in the garden. Since then, we’ve seen some remarkable improvements in the areas we amended.
My video on the attempt to make terra preta soil has proven quite popular:
Char is a well-known component of the black earths of the Amazon.
We use both charcoal and ashes in our gardens, and as Steven said – both have their place. The former is a good nutrient battery and improved the flavor of our produce.
On the other hand, ashes are a good source of fast minerals as well as a way of liming the soil to raise pH.
Finally, if you aren’t following Steven’s work on YouTube and at his excellent website, I highly recommend doing so.
Thanks for this post. I have often wondered if it’s worth rigging something to make biochar in a lower oxygen environment. So far, I haven’t bothered, and I don’t think I will in the future, either. Too many other things on the list, to justify messing around with something that’s already doing an adequate job for our needs.
Wood burned in the presence of oxygen does, indeed, produce charcoal. The difference between this and char (wood burned with reduced levels of oxygen) is the charcoal typically contains some oils that, primarily because of its relative ease in igniting, makes it more suitable as a home grill fuel. Unfortunately, the oils makes the charcoal less than desirable for use in a garden.
I haven’t tried it, but I am curious if hardwood charcoal such as Royal Oak and others that are advertised as having no additives, would be suitable for the garden. Has anyone tried it?
I watched a video of a youtuber doing just that. I think it was MI Gardener that did it, if I recall correctly.
Good write-up, David. Definitely gives me something to think about in my beds.