Use of biochar/charcoal in the potting mix

[mmcavall]

Hi folks I'm new to the forum (and also to cacti as a "serious" hobby) , although not new to plants as a whole. I'm reforming my collection of cacti and succulents and get to the question of the potting mix. Its winter in Brazil and I'm preparing the things to the Spring.

I read too much about it here and you people always talk about coir, pumice and perlite, three components that are not easily found here. So I'm looking for substitutes (is there any Brazilian reading it that could help me?).

First, instead of pumice/perlite, I found a basaltic gravel of about 2 mm to 5mm which is very cheap. Thinking in use it as about 60% of the potting mix, or more...but to me its a bit weird doing a mix so "rocky". (I think its just because I had never done this way before).

The other 40% would be sandy top soil, but I'm considering using about 10% of biochar / charcoal , also very easy to find here.

Biochar is being increasily used in agriculture for its properties. Its a good moisture and ions holder.

I know that the potting mix must dry out quickly, but maybe some biochar coarsely chopped could work as the coir you guys use: giving some structure and holding some moisture and nutrients.

Specially in the clay pots, which dry out very very quickly, this could be a good component of the potting mix.

My plan is: 3 parts of basaltic gravel: 2 parts of top soil : 1 part of biochar/charcoal

Does anybody has some experience with it or could give me any advice about my future potting mix?

(please remember I dont have perlite, pumice or coir!)

Thanks in advance!

[mmcavall]

Its easy to find info about biochar on the web. I still thinking in use it, unless someone disencourage me.

Some information about biochar obtained from http://www.maraseeds.com.au/biochar_pro ... nefits.php" onclick="window.open(this.href);return false;:

"How does CharcharcharBIOCHAR work for soil and plants?

An increase in production from soil and plants is gained because the Biochar attracts plant available water and plant nutrients to its Carbon molecules, these would normally drain through the soil and away from the plant roots. The stable form of carbon (non-labile) in Biochar ensures that the soil structure is improved because of the binding and aeration qualities that this material adds to the treated area.

The microscopic image of biochar is a lattice-like structure showing the phloem and xylem cells that were formerly the veins of the plant. As this porous vascular structure is still intact after the pyrolysis process it allows the soil water to impregnate the char, a colony of beneficial soil organisms that live in the soil water can inhabit the pores of the biochar and assist with distributing nutrients and water for plants from the Biochar particles. When good Compost is added to biochar and soil it can improve the diversity of organisms that live in the biochar and increase the production of plant nutrients. The labile carbon (available) of compost assists with the growth of plants as a digestible material whereas the biochar is generally indigestible and serves as a habitat."

[mdpillet]

Welcome to the succulent hobby!

As someone with a reasonable background in plant ecophysiology and some soil science, I can tell you that the description you're reading is, well, mostly rubbish. One aspect of charcoal is that it increases cation exchange capacity of the soil. Good for most plants, but for cacti, it doesn't matter as much. Your nutrients also need to come from somewhere. Thus, you will still need a finer organic material to add to your soil to deliver those nutrients, or add fertilizer with micronutrients. Save your money, and go with a low-fertility compost instead, or at the very least design an experiment so you can actually see for yourself if charcoal helps.

[mmcavall]

Hi mdpillet, thanks for answering, I feel better now someone answered

I would like to disscus almost every sentences of your answer!

"the description you're reading is mostly rubbish"
Indeed, as long it is part of a marketing thing of a biochar brand, we really must distrust. However, there are many scientific articles reporting the benefits of using biochar / charcoal in plant cultivation. What we must understand is whether it is good or bad for cacti.
I have a friend which is phd in plant nutrition and soil science. She develops a research on the use of biochar as soil component in arid regions of Brazil , and she reccomend its use, although, she make it clear, she does not have any idea about using it for cacti cultivation.

"it increases cation exchange capacity of the soil. Good for most plants, but for cacti, it doesn't matter as much"
I would like to understand it. Why cation exchange capacity of the soil is not important for cacti? (considering I do not want to do the "hidroponic" thing)

"Your nutrients also need to come from somewhere": yes, sure! I dont think on biochar as a source of nutrients. Soil and fertilizer would be part of the plan!

"Save your money": that is what I'm trying to do. Remember we dont have perlite, pumice, cat litter, or anything light in weight here. About 60 "liters"of charcoal costs about U$3,00. It is very cheap. This would not be a problem.

"design an experiment": yes, you're right. I had already started, and in the near future I would like to show it and discuss. With the material I have here, I tested some mixes for water retention and porosity, following a Iann thread.

According to results, coarsely chopped charcoal increased porosity and water retention. I'm a bit afraid of that water retention capacity. I dont know to what degree it is good or bad for cacti, and sure it have different consequences for plastic and clay pots. But I noticed that my previous mix, which was 2 parts of sand to one part of soil, was very very bad, with higher water retention and low porosity.

Now I'm testing the new mix (2 parts basaltic gravel, 2 parts of top soil, 1 part of coarse charcoal) on some plants, and comparing to other mixes. Let's see what will happen.

I still having some "faith" in charcoal, but your answer helped me keep one eye opened to its possible bad properties. If there are something I'm not seeing, please tell me.

[greenknight]

It might be a pretty good substitute for pumice, "increased porosity and water retention" describes pumice as well. As long as it retains air as well as water it should be fine for cacti, and in my experience charcoal absorbs water rather slowly. Only way to find out how well it works is what you're doing, giving it a try.

[MartP]

It's a good idea to reduce basaltic gravel use in the mix, but that also decreases much needed drainage. Now you've got 40% of gravel, 40% top soil and 20% biochar? This is just fine for some kinds of cacti, but no mix should be uniqe. You need to adapt to the cacti you have. I recommend a mix of 1/4 basaltic gravel, 1/4 of "garden loam" which can, in your case, be topsoil, 1/4 of "low-fertility compost" as advised by mdpillet and the last fourth is optional.
The last fourth is :

cacti: basaltic gravel
succulents: 1/2 of topsoil and 1/2 of compost
living stones: sillica
planted seeds: basaltic gravel
Epi-cacti : biochar
There might be a bit too much of the basaltic gravel in here, so perhaps use 3 parts gravel and 1 part biochar in the last fourth? I'm not sure.
That's the mix I can recommend you. I'm not sure how it will work out for you, but as far as the mix is well-drained, it's a good mix for most cacti.

[Steve Johnson]

It might be a pretty good substitute for pumice, "increased porosity and water retention" describes pumice as well. As long as it retains air as well as water it should be fine for cacti, and in my experience charcoal absorbs water rather slowly. Only way to find out how well it works is what you're doing, giving it a try.

Sorry that I didn't enter this discussion sooner, but what greenknight says is exactly what I was thinking when I saw the first post. While there may be a certain fad aspect to biochar, I think this would be worth some experimentation if its porosity, water retention, and CEC properties are similar to pumice. (Good cation exchange capacity is the reason why pumice works well when we apply fertilizer in soil-less mixes.) I'll be interested to see where the conversation goes here.

[DaveW]

Never used charcoal, but they used to claim when I started it was a "soil sweetener", meaning it reduced acidity of the potting soil. But whilst most North American cacti tolerate alkalinity, many South American ones prefer soil on the slightly acid side of neutral. Anyway found these on the Web:-

http://biochar.pbworks.com/w/page/9748050/Soil%20pH" onclick="window.open(this.href);return false;

http://www.eprida.com/hydro/ecoss/backg ... nefits.htm" onclick="window.open(this.href);return false;

Probably the truth is it makes little difference.

[MartP]

Most basalt is also on the alkaline side, making this soil mix good for nothern cacti, but not for southern cacti. You can still use it on both, but if you want to get serious, you can add more organic components and less basaltic gravel and biochar, as long as the drainage is still good.

[mmcavall]

Thanks for all replies.

First of all, it is clear to me now that there is no universal potting mix which can be good in all situations. As I'm a beginner, I should make some tests if I want to, but probably I can be satisfied with a well-draining mix, and along the time I can make the amendments necessary (probably there exists a “syndrome of the beginner”, which freaks out trying to find a perfect mix without having yet the plants).

About the charcoal: in the experiment I made here, it stored its own weight of water, although being very coarsely chopped. I filled up one plastic pot with charcoal (it has big pieces, very irregular but some pieces were of about 5 cm or more, so you can imagine how much free space was left for air) and it weighted 50g. Therefore, with bottom holes closed, I filled it with water, and it weighted 150g. After opening the bottom holes, I left it drain for two hours (unnecessary, because it drained in 10 secs). After two hours, it weighted 100g. I repeated it 3 times independently. I don’t have practice, but I can see that this is a huge water storage capacity. How good or bad it will be for cacti and other succulents will depend on factors such as: how many charcoal added, how fine or coarsely chopped is the charcoal, how often I water the plants, etc.

But if I control my OCD about watering the plants and be careful to put little charcoal in the mix, it may have beneficial properties. In nature, when soil dries out, these plants can find moisture in niches scattered in the soil (until these sources dry out too). So it may works fine, moreover considering the good CEC and good drainage of this material.

About the pH: it may be a problem, both charcoal and basaltic gravel. But in Brazil most soils are slightly acid and water is not alkaline as in other places (no need to add vinegar, for example). So the effects of charcoal and basaltic gravel may not be a problem to my plants. But I’ll follow the recommendations of some of you and try to add some organic matter to my mix. Also, I’m looking for substitutes to basaltic gravel.

I don’t know if I covered all issues from your replies, hope yes!

[mmcavall]

1/4 of "low-fertility compost"

[

what is a "low-fertility compost"? I dont think I cant find it here. In which aspects it is different from top soil so I have to mix both?

thanks!

[greenknight]

See how much water the charcoal retains if the drain holes are open.

[DaveW]

Low Fertility Compost is the purely mineral mixes that consist of gravel, pumice, cat litter etc but no soil or humus content, therefore need to be regularly dosed with half strength liquid fertilisers. The old soil based cactus mixes did not need any fertiliser for at least a year until the soil became exhausted, but were then usually repotted into fresh soil instead.

[MartP]

Since you're looking for cheap stuff and said pumice isn't availible, think of a mineral material that will increase drainage and isn't organic.

[mmcavall]

See how much water the charcoal retains if the drain holes are open.

Thanks for the suggestion. I made the experiment and 50g of charcoal retained 25g of water. I weighted the pots one minute after water passed throught the charcoal, and then weighted after 2 hours. Results were the same: 50g of dry charcoal => 75g of wet charcoal


Thanks everybody for the all replies, I had many insights from it