Why I Use Hella Charcoal In Cactus Mixes, Experience and Rationale
If you stop the combustion of wood after flaming stops, and before it turns to ash, you are left with a shell of mostly carbon. Charcoal has a unique combination of attributes, useful in soil amendment and especially suited to growing cactus.
Before I launch into this article, I just want to say that I think this is an idea and practice that is going to have staying power. I’ve done enough experimenting with charcoal in horticulture that I’m pretty well sold on the idea and in all likelihood will continue to use a lot of it. This is exactly what I like doing. Finding something with potential, taking it somewhere practical and sharing it out to encourage its broader application. I hope this article and idea has lasting ripple effects in the cactiphile community, especially as charcoal becomes more commonly used in agriculture and more widely and cheaply available. References are provided to other videos on making charcoal and some extremely interesting research on early American and European use of charcoal as a soil amendment for those interested in horticulture in general or who are just nerdy. It is pretty fascinating stuff.
Some years back I became interested in Charcoal as a soil amendment. In the intervening time, I’ve done some very interesting historical research, made a couple thousand gallons of char, made youtube videos on making it by simple, accessible methods which have been viewed by hundreds of thousands of people, used a bunch of it, done experiments, and pondered a lot on it’s use and how to create systems for sustainable char production and large scale soil amendment. When I started growing cactus a lot, I figured charcoal would made a great addition to cactus mixes and began using it in quantity. I also put together some tests growing cacti in straight charcoal.
The use of charcoal as a soil amendment is an ancient practice and examples of its historic and prehistoric use continue coming to light. Anyone should be skeptical of information, especially when it is about something that sounds too good to be true, and is alleged to be able to save the world. Biochar (defined by me here as any charcoal used as any kind of a soil amendment, though some will disagree) is definitely in that it-will-save-the-world category. It is easy to see why people become so enthusiastic about it, but over-hype and religious devotion to biochar aside, when the available information is looked at, the probability that some charcoal, in some quantities, in some soils, can have profound results in improving plant growth and health over centuries or even millennia appears to be astronomically high. Between the fascinating historical record, the functional persistence for centuries of fertile, human modified soils that are chock full o’ char, the analysis of those soils, agricultural experiments and the personal experience, you have to bend over backwards to actually disbelieve in the potential benefits of charcoal as a soil amendment. The rest of the details, like growing what, where, how and which climates and soils are best for it, are the meat to be fleshed out on those very intriguing bones.
Suffice to say, that I’m a big fan and could use literally tons upon tons of the stuff in soils and potting mixes. So with that setting, let’s look at the properties of charcoal as relates to its use in horticulture, and how those properties might be used to good effect in growing cactus in particular. We will talk mostly about using it in potting mixes, but using it in soil is not that much different. We are just not likely to use as much in the ground as in pots.
Charcoal seems fairly unique in the number of properties in embodies that are useful in potting mixes. It combines some of the properties of organic matter and some properties of grit/aggregate materials. But on top of those properties, it also serves as a sort of sponge and reservoir for nutrients. Further, it is also likely a superior substrate for beneficial soil microorganisms. Aside from the science and a rational analysis of its properties, it just seems to work, and that is what really matters. Let’s look at charcoal’s potentially useful properties in soil mix.
DRAINAGE AND WATER RETENTION: Wait, which is it, water drainage or water retention? Oddly, I think it is both. Charcoal is both a drainage aggregate and water holder. It has a huge amount of pore space and can absorb a large amount of water like a sponge. Sponges work by filling air spaces with water and that is just what happens with char. But unlike some materials, it does not get soggy. If you have a lot of fine powder in a mix, it can get snotty and mucky, but char doesn’t get slimy or gooshy with water if it is not powdered. Charcoal is solid and remains so when saturated. It doesn’t have the potential to be a soggy wet mess like vermiculite or most organic matter. It can simultaneously grab and hold water for use, but still provide drainage as an aggregate. You can grab a pile of saturated vermiculite and squeeze water out of it. It is soggy and messy, but not so with charcoal. Once the extra water drains away from it, it cannot be squeezed out. Being hard, there is always space between the pieces for drainage and air. It could be likened in this way to pearlite, burnt clay pieces and pumice, but I’m pretty sure it holds more water than any of those. Regardless of how it would fare if you tested it against other materials for water holding capacity, it also has other very interesting attributes.
DRYING: While charcoal has oodles of pore space to hold water, it also has a very high capillary action and seems to dry out relatively fast. I’m not completely sure on this, but I highly suspect that it drys faster than clay and most if not all organic matters. That of course could be a problem in some cases where very high amounts of charcoal are added to soil or potting mix. But with cactus, fast drying is generally a good thing right? Cacti are adapted to use water quickly when it’s available. When the water stays too long, many are prone to rotting. In this regard charcoal seems pretty ideal for cactus and succulents.
The only place I’ve seen this quick drying out effect to possibly be a problem is in garden soils using very high amounts like 1/4 to 1/3 char. But in cactus mix, 50% is the minimum I prefer to use. When using 100% charcoal, I have had some trouble keeping the San Pedro complex cacti wet enough to maintain rapid growth relative those in mixes with some organic matter. San Pedros like more water than most cactus though, so that is not surprising. Desert cacti in straight charcoal seem to do extremely well as long as they are fed adequately. Overall, I think this draining and drying effect is a benefit when growing cacti and succulents.
All but one of these small desert cacti are growing it straight crushed charcoal. They seem to be thriving. The ones that are not doing great had issues already. The rest have grown a lot and flower freely. Some have been in charcoal for several years now. They require adequate fertilizing, but so do any cactus in any potting mix eventually if you want to them to grow and thrive.
NUTRIENT CAPTURE: Charcoal has a very special property of attracting and holding many substances. This property is taken advantage of in everything from traditional healing to industrial uses, to modern medicine. It is used in filters, not just because it mechanically filters stuff out, but because it attracts and binds chemicals. In modern medicine, it is used to take on toxins in the digestive tract. I had a student who said that he once ate a deadly poisonous mushroom. In the hospital he was fed large amounts of charcoal to absorb the poison until he was pooping black. This ability to hold onto stuff extends to plant nutrients, which might seem like a bad thing, But it appears that once well charged with nutrients, some of those nutrients become available to plants. If you dump out a cactus plant growing in a mix with charcoal, you often see a lot of charcoal chunks clinging to the roots, because the root hairs have grown into the pores to get the nutrients back out.
The roots cling to the charcoal and seem to send roots into the pores. Over time, the pieces may become more encased in roots and root hairs. If there is much fine charcoal present, the roots will often be coated with it and not wash off easily.
This absorptive ability likely means that when we fertilize, we lose less nutrition out the bottom of the pot. This effect is not the same as a sponge, though charcoal does that too as I said. The effect is chemical binding, known as adsorption with a D. Absorption with a B is what a sponge does, filling pore space with liquid. Adsorption is a chemical bond. A pot full of high charcoal potting mix should function something like a charcoal water filter, but instead of filtering out toxins, filtering out nutrients and holding them for our plants so they don’t just run out the bottom of the pot. Other substances do this as well, notably clay. But the power of charcoal to do it is very high. If you want to know more, research Cation exchange in biochar.
MICROBIAL HOTEL: A common assertion is that much of the benefit of charcoal as a soil amendment is due to the char being so porous that it can house a lot of soil microbial life. Charcoal has a tremendous amount of surface area inside its pores, so it seems likely that this effect is legitimate. How much this factor contributes to the actual effectiveness is due to this factor I couldn’t say. It has been studied if you are motivated to sleuth out what available information. I would think it is at least as good as anything else, and that’s good enough for me.
LASTING POWER: I’ve often been asked if charcoal really lasts as long as it is claimed to. Yes, it really can. It does break down eventually, and not all charcoal is going to be equal. Some is soft and some quite hard and durable, depending on what wood it is made of and how it is made. But extremely old charcoal from archaeology sites is very common. There are also very old human modified soils full of charcoal. If I recall correctly, the African dark earths are as much as 800 years old and South American soils are older. Cactus OG Ben Kamm of Sacred Succulents told me that he has run into charcoal rich dark earths in South America while on cactus study and gathering trips. Around here I have found charcoal deep in the ground when digging holes. And here is an interesting one for you. My friend found a piece of sandstone with both petrified wood and pieces of charcoal embedded in it together! The petrified wood is hard, but the charcoal is still soft enough to crush with a fingernail, so it appears it did not petrify in the way that wood does. Whether charcoal will eventually decay or not is just not a factor to be concerned about in potting mix. For our purposes, it lasts indefinitely. That means it can be recycled and used over and over again. It is a suitable replacement for any other mineral aggregate, just with more benefits.
Does charcoal last a long time? Ask an archaeologist. My friend found this piece of charcoal embedded in a fucking rock! The durability of charcoal is not going to be an issue in potting mix, unlike the common coconut coir, peat moss and shredded tree barks that form the bulk of commercial potting mixes. Even cactus specific mixes still have a fair amount of organic material. Charcoal’s durability means less “soil shrinking” in pots over time due to the decay of said organic matter. Sure you could use other minerals in your mixes that might last a long time, but charcoal is a long lasting aggregate with benefits. As such, it can replace much of the organic matter commonly used. All of which makes it a good investment.
Remember that char also embodies some of the properties of organic matter. But organic matter is prone to breaking down and being digested by soil life. A main reason that coconut coir, peat moss and conifer tree barks are often used in potting mixes, is because they last longer than most organic materials. If you made potting mix from straw or leaves for instance, it would not last long at all and the volume of your potting mix would reduce rapidly, dropping the soil level in the pots within a year if not sooner. So with char, you get some of the benefits of organic matter, along with its other benefits, but unlike organic matter, it is going to persist longer than you are going to live. Other grit materials that are mineral based, like crushed shell and mineral additives like sand and rocks will also remain, but I don’t think you will find any that come close to bringing all of the other benefits of charcoal. I like to call charcoal an aggregate with benefits.
PH: I have been told repeatedly that charcoal raises PH. Some think it is due to ashes in the char, which are quite alkaline, but the assertion is that it is a long term effect, and ash does not provide long term PH shifts. I know, because I’ve used immense quantities of the stuff on my gardens for decades. I have no desire to research this point, but if it is true, that is probably a benefit when growing cacti, which tend to grow in alkaline soils anyway. I have used large amounts of oyster and egg shell, with large amounts of unwashed charcoal to apparently great effect. One person told me san pedro like acid soils and that they actually add vinegar to their soils. All I know is that whatever char does to soil PH, it at least doesn’t seem to hurt to use a lot of it, because my cactus in 50% charcoal, with wood ashes and oystershell added grow very well. I feel no need to do any more research or change anything. It is not that I have no problems, but who doesn’t?
SO, WHAT ARE THE DOWNSIDES? At this time, unless you can make it, charcoal is more expensive than most potting soil constituents if you have to buy it. That is changing, so I hope it becomes closer to pearlite in price, or cheaper. I think it should actually be cheaper once it is common and technology and infrastructure is invested in. Eventually, we will probably see municipalities turning yard waste into char in small scale power generating systems and then selling the char. That is certainly what should happen. If you are really interested in using a lot of it and can’t make it, look into ordering a tote. Pacific Biochar in Santa Rosa California sells high quality biochar for 300 a ton on site (more if delivered by the truckload of 15 tons). That is really cheap, but delivery or ordering though a middleman will bring the price up a lot. If you order char, make sure you are not getting finely ground stuff or these will be no aggregate effect. I think putting in a good proportion of small ground char would be great if using other aggregates, but my tendency is to use the char for both aggregate and fines.
Another way to get charcoal is by going into areas that are burned recently by wildfires and just spend time picking the stuff up. Over the years I’ve salvaged charcoal from firepits, pitroasts, wildfires, burn piles, woodstoves and anywhere else I can find it as long as the feedstock was clean wood with no paints of preservatives.
It also floats. It can be annoying to have a bunch of charcoal pieces floating up when you water.
APPLICATION: So those are the reason that I use charcoal. If you’re the kind of person that likes to have more evidence, knock yourself out doing the research. I can and do make a lot of charcoal and have seen apparently great benefit in my garden and am inclined toward leaning on experience, so I’m very happy at this point to continue to use a lot of it in my cactus mix. I typically use a minimum of 50%, but I would use even more if I had unlimited supplies and didn’t use it in my soils too. I am typically adding that 50% char to a potting mix that is already maybe something like 20% to 30% pearlite. I would say right now that my prefered proportions for a base mix would be about 30% coco peat to 70% char, 3/8” and down. That may seem like a lot, but remember this includes a lot of fines. Those fines act something like organic matter. Once I experiment more, I will not be surprised if I end up using something more like 80/20.
If I were to add other aggregate type stuff, it would probably be any of coarsely crushed oyster shell, pumice, pearlite or baked crushed clay. I’m not a fan of sand in potting mixes. Sand is heavy. One of the nice things about charcoal is that if you let it dry all the way out, cactus in large pots are easier to move because char is so light.
I currently prefer to use it sifted down to 3/8”, but if you roughly crush it, you’ll get everything from coarse to fine. I like using the straight run crushed stuff with lots of different sizes from a 1/2” to 3/8” max size down to fine powder.
Crushing can be an issue. On a small scale, laying out a piece of plywood and crushing it under your boots works well enough. I’ve done a lot of that and usually throw it on a 3/8 inch screen and recrush the big pieces, but you don’t have to. A garden shredder is great for crushing charcoal, but it come out a little fine in some cases. Dampen it lightly first to keep the dust down so you don’t get black lung. You can also fold it in a heavy tarp in the driveway and drive over it, though it is not as effective as you might think. That can be a good first step though, then sift it once and crush the siftings some other way.
Chunky charcoal also makes a nice top dressing. It allows aeration around the base of the plant where that is helpful. It’s a good use for the durable, chunky, hard stuff that you stift out after a couple rounds of crushing and sifting. It just looks good too.
One important issue to be aware of is that since charcoal absorbs plant nutrients, when you use raw char out of the fire, it will slurp up a lot of stuff and quickly make the soil poor in nutrients. Once it is charged with nutrients however, my observation is that char soils make better use of any nutrients that you add than non-char soils. The most common way to deal with this nutrient draining effect, is to pre-charge the charcoal with nutrients. Usually this is done in a nutrient solution that also contains beneficial bacteria, such as compost tea. Urine is a great way to charge up char since charcoal has a high affinity for nitrogen, which urine is very high in. There is a lot of information on pre-charging char out there, though it is usually heavy on dogma. No need to over complicate it. You can just soak it in a high nutrient solution, preferably with a lot of nitrogen and use it. A very easy way, is to put it in a bucket and pee on it until the bucket is full. Pour the liquid off onto something you want to fertilize and you’re done. If you add a handful or two of forest duff from your local environment, and a couple tablespoons of sugar or starch and leave it for an extra week, you’ll probably get a bunch of good bacteria too. A lot of the time I actually don’t do any of that.
In spite of the common assertion that you must “ALWAYS precharge your char”, I often do not for personal use in cactus potting mix, or when adding to garden beds. Instead, I add quite a bit of extra fertilizers for the first few months. If I’m making potting mix for personal use, I often just make it with raw char and add some extra amendments while mixing. Mostly though, I rely on using a lot of soluble fertilizers for the first few months. Pee, again, works great as many cactus people already know. When making mix for potting cactus to sell, or making flat mix to start seedlings, I pre-charge char with an organic liquid fertilizer in a big barrel, drain it, rinse it off with water, and then add it to the mix. The leftover liquid, I dilute and use to feed other plants. I don’t worry about the chargin solution being too strong. The charocal will adsorb as much as it wants, so I just use a strong solution and the left over isn’t wasted anyway.
Pee is a terrible thing to waste. I peed on this spot one winter, trying to make a distinct line as a demonstration. Yes, you can overdo it and kill plants, but it actually takes quite a bit to have any harmful effect. When charging pre-char, it doesn’t matter if it is too strong. The char will just slurp up what it can hold.
For me the bottom line in using char is that it not only makes sense for multiple reasons, but I’ve also used it a lot now and it just seems to work really well. The few other species of small desert cactus I that I put in straight charcoal that are thriving. They have to be fed, but if fed well and given occasional water, they seem to actually do exceptionally well without anything else to grow in. The drainage and drying are so good that you can bury the base of desert cacti and they don’t seem to have any problems at all. Char is also something that I can make fairly easily out of stuff I need to dispose of safely anyway for fire safety reasons. I have endless amounts of tree trimmings and brush that the land constantly produces in an area that was logged 60 years ago and has grown back into a messy fire hazard full of weak, light starved trees laden with dead limbs, dead trees and other debris. It would be an epic job just to clean up the forest here in a very basic way, let alone well, which could result in enormous quantities of char. The last large, top lit burn pile I did produced about 130 gallons of uncrushed char. At about 7.5 gallons per cubic foot, that is roughly in the neighborhood of 2/3 of a yard in one burn, though it is considerably less once crushed, so maybe half a yard worth. That is a lot of potting soil at 50% char, making what I think is a superior and very long lasting cactus mix that can be recycled indefinitely out of something most people would have burned to ash.
Pallets are an abundant source of wood for charring if you can have open fires. Just stuff some extra small wood in the spaces if you have it, and light from the top. Manage as necessary and start watering any part that is no longer flaming. walk on it a bit to break it up, and run a big magnet through it to take out nails. Just examine them carefully to avoid pressure treated wood and plywood.
As biochar/charcoal becomes more widely available and hopefully cheaper, I think it will find its way into potting mix in general at a goodly percentage, but especially high in cactus mixes. As I said, I think this practice has staying power and I hope you will try it. I hope the use of char in cactus culture spreads quickly and I think it will. Happy charing and happy growing.
Making char by the pit method. This is ready for another layer of wood, or when the flames stop, it can be dowsed with water. USE A LOT OF WATER! It takes more than you’d think to put something like this out. One ember left and you could come back to a pit full of ash the next day, or worse. Putting a sprinkler on it is a good way to be sure it is extinguished all the way. I use this method a lot, because if the pit is long (usually 9 or 10 feet, this one is short), you need to cut very little of the wood, and all you need is a shovel , a hose. and a match! Just start a fire in the trench or pit, add a layer of wood to cover when it looks like this, and keep going like that until the pit is full. See the videos linked below.
RESOURCES:
Most of my biochar videos on youtube in one playlist. https://youtube.com/playlist?list=PL60FnyEY-eJBzDtNl2YBuem2U9lL3TZfv
Fascinating research I compiled on using charcoal as a soil amendment in N. America and Europe in the 19th century and earlier. http://skillcult.com/blog/2012/05/18/some-citations-on-biochar-in-europe-and-america-in-the-19th-century
Some paper on Amazonian and African Dark Earths containing considerable amounts of charcoal, which I haven’t even actually read. http://steps-centre.org/wp-content/uploads/ADE_Africa.pdf