Using charcoal

[feralpossum]

G'day all,

As it's been bloody cold in the south east we've been having a lot of fires. I recently passed the fire ash through a sieve and was left with a whole lot of fine charcoal. I know that there is growing interest in the use of 'biochar' as a soil additive. Does anyone have any experience using charcoal as a component of bonsai substrate? If so, was it beneficial? Detrimental? Make not difference? Any feedback would be welcome.

[SuperBonSaiyan]

I haven't used it for a few reasons.

1) It crumbles apart too easily.
2) I can't think of a horticultural benefit. True, charcoal can filter water, but not if it's spread in the mix.
3) Haven't found a cheap source - it's more economical to use pine nuggets than charcoal.

I do throw my charcoal scraps into my garden bed, but that's mainly to suppress weeds.

[shibui]

Charcoal is a traditional additive to potting mixes.
We rely on wood stove to provide most of the winter heat for the house so I also have plenty. I tried some in bonsai mixes for a couple of years but seemed to have more troubles with it V without so discontinued using.

Good charcoal has good physical characteristics. It does not usually break down quickly and should last for years as a part of potting mix and maintain good particle size.

Charcoal also has good cation exchange capacity which, in layman's terms means it is like sponge that can absorb and release nutrients. Higher CEC in potting mix means less nutrient leaching when we water. Zeolite is another potting mix component that has high CEC.

Charcoal is traditionally used to 'sweeten' the soil. That simply means charcoal is alkaline so adding charcoal raises pH. That's great if you have an acid soil or acid water. Much of Australian soils are acid but not commercial potting mixes which are pH adjusted before being sold. In addition most treated water supplies are adjusted to be slightly alkaline so the water will not corrode the pipes and tapware or dissolve lead out of soldered joins in copper pipe. Using town water will slowly move pH in the pots upwards toward alkaline. Fertiliser use usually makes the soil slightly acid so the water usually balances fertiliser acidity. In some places water is even more alkaline and growers need to use an acidic fertiliser to ensure soils stay withing the pH plants tolerate. Obviously adding another alkaline substance will affect that pH balance. Do you know what pH your potting mix starts at? What it is after a year or 2 years? Just adding random amounts of charcoal to potting mixes without being aware of the consequences can have unforeseen consequences. Be very wary of using charcoal in pots with acid loving species like azalea.

Biochar is probably beneficial because of it's pH adjusting character, especially in naturally acid soils or soils that have become more acid through fertiliser use and cultivation.
Charcoal is also mostly carbon which tends to disappear from intensively cropped soils. Australian farmers have recently become aware of the need for returning carbon to the soils and are working on a number of different ways to do this. Biochar is just one way to add carbon.

Carbon is one of many critical components of healthy soils.
Soil carbon provides a source of nutrients through mineralisation, helps to aggregate soil particles (structure) to provide resilience to physical degradation, increases microbial activity, increases water storage and availability to plants, and protects soil from erosion. Ultimately, increasing soil carbon levels can lead to better plant establishment and growth. While increasing soil carbon is highly desirable, it is also easily lost, so maintaining what you have is important. Climate is a strong driver, affecting accumulations and decomposition of soil organic matter in soils.

So while charcoal has a number of benefits to plants and soils it's pH can potentially be detrimental in the confines of pots unless you really know your plants and potting mix. Use sparingly at first to assess any positive or negative reactions.

I spread the leftover ash and charcoal from our heater thinly on the grow beds a couple of times each year where it should help reduce the natural acidity of the soil and provide added carbon and CEC to the soil. Larger amounts of soil will buffer any pH changes more effectively than smaller quantities of potting mix in a bonsai pot so is safe at the quantities I'm applying. An additional benefit should be the potash supplied by the ash component. Always spread ash and charcoal thinly. Dumping piles in one place can change soil chemistry enough to poison nearby plants.

[Raniformis]

Charcoal provides a habitat for beneficial micro-organisms, large surface area, high porosity, etc. Not much weight to the stuff though (assuming weight is needed to keep the pots from toppling over).

[tgooboon]

I started using charcoal a couple years ago for beneficial bacteria/micro-organisms, and zeolite for better fertilizer retention, but reading this sounds like the charcoal will benefit fertilizer retention also. I sieve all my potting medium between 3-7mm.

I have observed fine roots growing into the charcoal where I can see them near the surface. The trees seem to be healthier, but most likely a coincidence. When I remember, I have been adding a cup or 2 of biochar same amount of zeolite to a 10L bucket when I mix up some fresh potting mix.

I have never got really worried about testing ph or ballancing ph of soil mix, but i should research this more as I know from testing fish tank water, my ph is extremely alkaline due to town water.

[treeman]

Green man Biochar is a supplier of graded charcoal. I think they have up to 40 litre bags available but you'll have to check.
Most plants seem to love charcoal. I haven't read a really good answer why that might be but trials show that they can do better with it. It needs to be washed to remove as much residual ash as possible. That's the alkaline part.
https://greenmanchar.com.au/

[treeman]

Soil carbon provides a source of nutrients through mineralisation,

I have no idea what that means.

helps to aggregate soil particles

Not the type of carbon that charcoal provides. Only the carbon in humus.

increases microbial activity,

Not the type that charcoal provides. Organic matter (food) does that.

increases water storage and availability to plants,

Only if it actually increases water holding capacity. (again, humus)

and protects soil from erosion.

Not charcoal.

Climate is a strong driver, affecting accumulations and decomposition of soil organic matter in soils.

That has nothing to do with charcoal.

[treeman]

.

[feralpossum]

Thanks everyone for your input. I might experiment with a small amount in a robust and non acid-loving tree. Then spread the rest in with the vege beds.

[Dendrophile Dave]

Great topic. 'Charcoal' is used in orchid culture a fair bit for its light weight and high porosity but mostly for its ability to raise pH (other orchid mix components such as sphagnum moss and pine bark tend to acidify overtime), also charcoal is high in potassium which is necessary for flowering, my understanding is that potassium is incredibly resilient hence why it is not destroyed by fire and remains within the charcoal. It can accumulate salts overtime which can be problematic (not sure if this is due to the very high cation exchange capacity or porosity/filtration abilities or a combination?)

Now for more questions, how does regular wood heater 'charcoal' from seasoned aussie hardwood compare to 'horticultural charcoal', 'bio-char', and 'activated charcoal' ? My understanding is that all these products are very different structurally and chemically with varying positive and negative qualities with regards to horticulture.....

[Jan]

I'm a long time fan of charcoal and have been using it in my potting mixes since the 1970's.

Like shibui, I spread the wood ash and charcoal from the heater thinly onto garden beds rather than in my Mixes; the gardens seem to love it.

I use weathered charcoal, collected a few months after burning our gum tree windfalls (Brittle Gums are always providing plenty of fallen limbs). I give the charcoal exposure to the elements/rain before collecting it. I break down the size of the particles to suit the mix particle size for bonsai and use it as larger chunks for "spreaders" for branches and as orchid potting mix. The orchids love it; I have Sydney Rock orchids and some native dendrobiums flowering in charcoal at the moment.

I think that as the charcoal holds water it acts as a "water well" for the smaller pots and keeps the roots in a humid atmosphere. It also make my larger pots a bit lighter (I need all the help I can get to lift some of them!).

Each to their own but I'll stick with what works for me.

[shibui]

Now for more questions, how does regular wood heater 'charcoal' from seasoned aussie hardwood compare to 'horticultural charcoal', 'bio-char', and 'activated charcoal' ? My understanding is that all these products are very different structurally and chemically with varying positive and negative qualities with regards to horticulture.....

I believe that biochar is made by heating wood without (or with very little) oxygen. Not sure whether that makes bio char different from charcoal from the fire. I think that's just so they can maximise production. When we burn wood with O2 most of it turns to grey ash. Reduced O2 leaves the wood mostly as charcoal but, essentially it is still the same stuff. The old time charcoal burners burnt logs in a covered pit to keep out as much air as possible which would leave most of the wood as charcoal and less ash.
One source listed biochar as having pH between 7.1 to 10.5 (depending on what wood, etc it is made from) so still pretty alkaline just like charcoal from the fire. Ash is even more alkaline at 11-13 pH.
Just skimmed an article saying that there are a couple of ways to produce biochar - high temp or low temp - and the biochar from each has quite different characteristics. Chemical make up also depends on the original feedstock - type of wood, paper, crop waste, coconut husk, etc so it appears that biochars are not equal. Effects on soil and plants also varies depending on the size of particles. I'd be pretty confident that would also apply to charcoal sourced from fires.

I just looked up activated charcoal. As far as I can work out it is still charcoal but there's an added 'activation' stage where the charcoal is heated with air, steam or certain chemicals. Appears that those processes open up more microscopic pores that can then trap the stuff that is supposed to. Appears that all charcoal can act as a filter but 'activated charcoal' has a bit more capacity.

Way more questions here than answers I think.