Soil mix questions

Hi All, based in South Africa some inorganic soil components are impossible to find - I’ve managed to get hold of Crushed Leca, Silica Stone and Perlite. If I added chipped bark would it provide the necessary CEC? I was thinking 40% Leca; 15% Silica; 15% Perlite and 30% bark. Does this sound at all suitable and should there be an ingredient added to increase ph? Any advice would be greatly appreciated.

Interesting question! I have had no experience using any of the components you mention but i am familiar with their physical characteristics in general. I will be interested to hear how well this combination works. I would consider the following; particle size should be the same or close to the same. Varying the percentage of chipped bark will affect moisture retention and PH range the most.
For my climate that percentage of chipped bark would result in too much moisture retention, that might be required in your situation though. Let us know how that combination works and how you adjust it to suit your circumstances.

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Hi Frank - thanks so much for your feedback - I stay in a coastal town where temperatures rarely reach beyond 37 ºC in summer, humidity levels are quite high, so would you suggest reducing the amount of bark to15 or 20%? My only concern is that this is the only component that offers any CEC. Also would you suggest an addition of lime to increase PH? I’ve prepared 2 sets of particle sizes - 2 to 5mm (more refined trees) and 6 to 10mm (collected). Thanks a mil

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The baseline mix of Akadama/ pumice/ lava rock has component CEC of 31.4/ 75/ 10-30
Your mix components CEC:
Calcined clay 15-40 (likely about 30)
course sand 0
perlite 1.5 -3.5
2.8mm pine bark 24.3

You need something to raise the CEC of the mix. I like pumice other choices are charcoal 200+, vermiculite 85+ or peat moss 75-180.

If the Leca does not break down, you will not get the benefit of reducing particle spacing for reducing root size which results in reduced branch size like you get with Akadama. I use Akadama mix for refinement and bite the bullet on cost. Nursery bonsai I use Pine bark instead of Akadama, and not much pumice; plus instead of lava rock I normally use expanded shale (Stalite brand) as they are very similar and the production facility is a 30 minute drive from where I live in North Carolina (SE USA) it costs about $90/ton USD. For pre-bonsai I combine the 2 formulas based on what I am trying to accomplish before refinement. Pine bark breaks down much slower than Akadama.

Hope this helps,

Hi Shaun
Stay with the amount of bark you suggested and monitor the plant health and water retention. i would not add lime at this point, Just monitor the situation!
The best guage will be your experience and judging the plants response. As others have suggested you may wish to fine tune for refinement purposes. But i would get a handle on a basic mix that works well for development and recovery purposes first. once you have a handle on basic care then you can tweak your local resources for better results.

David and Frank - really appreciate your assistance with this. My resources are unfortunately limited to Leca, Perlite, Bark, Charcoal, Vermiculite and Peat moss. I’ve heard the water retention of Vermiculite and Peat moss is excessive, therefore not ideal? However judging by the CEC maybe including a small amount of Vermiculite and charcoal instead of Perlite may be a better option?

Do not hesitate to use Peat Moss in appropriate ratios for your purposes. Really it is about working out the best approach for your resources and purposes. Just because it holds more water than other components does not mean it will not be useful in lower percentages for moisture retention and CEC purposes.