Understanding soils

Wow - what an interesting presentation!
I don’t know if where it goes from here - but certainly opened up a new level of understanding.

Thank you Ryan.


Ryan, Check the solubility of gypsum in water. Not sure it is as soluble as you might think.

The graphs I found indicate a maximum solubility of anhydrous CaSO4 of 0.015 millMol per kg of water. That is about 2 g of anhydrous CaSO4. Most of the granular gypsum we get will be hydrated to some extent and it looks like the hemihydrate CaSO4-0.5H2O is more soluable.

I believe the low solubility is one of the reasons it is use to help granulate clay soils - it dissolves and then binds to the clay particles holding them together rather than just flushing through the soil.

Pretty much a rehash of what Ryan said…but sometimes reading is easier than hearing.

Gypsum alters soil chemistry. Soil particles and organic matter have negative charges on their surfaces. Mineral cations (with positively charged surfaces) are attracted to these negative charges on the soil particles. The number of exchangeable cations that a soil is capable of holding and available for exchange with the soil water solution is called the CEC. This is an indication of the level of nutrients the soil can hold. Clay soils tend to have a high CEC.

Gypsum is calcium sulfate (CaSO4). It dissolves (dissociates) with moisture into free calcium and sulfate. The sulfate (SO4) attaches primarily to excess magnesium (Mg), aluminum (Al) and sodium (Na) in the soil complex, making soluble compounds that move down and out of the crop rooting environment. The remaining Ca from the gypsum then attaches to the exchange sites, replacing the excess Mg, Al, Na, etc. in the soil complex. This is what sets the stage for improved soil structure.

Calcium is a positively charged ion called a cation. Cations are absorbed by the plant roots and also held on exchange sites in soils. The positive charges of calcium are attracted to negative electrical charges found on the exchange sites on clay particles and Organic Matter (OM). The more clay and OM, the larger the attraction. This attraction of the positive Ca with negative charges in the clay particles – binds the clay soil particles together so they become flocculated and resist dispersing and soil structure breakdown.

Within the realm of typical soil components, our pumice is highly silicic (SiO2) and acidic, our lava (basalt scoria) is a basic rock composed of calcium, iron, aluminum and magnesium rich silicate minerals, and Akadama is a high aluminum (aluminosilicate) clay.

Taken together, gypsum can be expected to react with the readily available aluminum in the relatively weakly bound Akadama clay, some impact on the ferro-magnesium minerals in the lava, and almost no impact on the pumice. If organic components are added to the soil, they too would be impacted by the addition of calcium sulphate.

Now, what does it all mean? What exactly are those effects on potted plants? Can gypsum be used to improve the availability of nutrients? Can it be used to improve the drainage in Akadama rich souls that have weathered in the pot for some time…rejuvenate it for a bit longer?

Time and experimentation will tell.


Not sure that strict solubility limits are much of an issue unless you are only trying to apply it in dissolved phase. Maybe that is the reason for the graph?

If applied in granular form, water available with each watering provides fresh solvent.

As reaction with soil components occur, it would remove dissolved calcium sulfate from the water to drive further dissolution. Standing water in pot will still drive reactions forward.

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Regarding the video, if you want to deliver 16g of gypsum in solution to your plants in a 2 gallon soil container with a drench, this should be no problem.
Calcium sulfate:
Solubility in water 0.21g/100ml at 20 °C (anhydrous) 0.24 g/100ml at 20 °C (dihydrate)
My watering can is 8 liters (2 gallons is 7.6 liters, a similar size for watering cans), and I use horticultural gypsum dihydrate. .24 x 10= 2.4g/ liter= 19g max. at 20 degrees C (water from the tap is 19.4 degrees where I live).

Gypsum is a common horticultural soil additive, very useful to displace sodium in soil, and of course to add sulfur and calcium. Personally, my main use is to flush sodium, the added calcium is a bonus.

Here’s an interesting quote from that article that made me think about Ryan’s latest Asymmetry podcast with Andrew Robson where he talks about finding amazing amounts of mycorrhiza in pots of trees that weren’t necessarily very healthy or vice-versa (almost as if there was little correlation between the two):

Herrera-Estrella pointed out, though, that past research has discovered that the symbiotic relationship between plants and AM fungi is most active in soil with low phosphate availability and suppressed in soil with high levels of available nutrients. That means cultivated crops that are highly fertilized see a severely reduced or completely suppressed impact from mycorrhiza.

Maybe with the amount of fertilization used on bonsai trees they don’t rely on fungi the way they have to in their native habitat?

It seems like heresy to suggest that mycorrhiza doesn’t matter to bonsai, or at least not much, but there are definitely plenty of areas where we find bonsai react counter-intuitively to our traditional horticultural knowledge…

I’m not sure what I think, but I thought this would at least spark some good discussions!

I certainly can’t pretend to be an expert in this area - but it is my understanding that mycorrhiza are part of the communication system. In a forest, there may be a mother tree with a whole colony of trees in communication. A tree isolated in a bonsai pot is not able to communicate with other trees no matter how much mycorrhiza is present.
Look at the recent inspirational video posted or check the work of Dr Suzanne Simard on TedTalks. She talks about a tree sending nutrition to other trees after it has been felled. I suspect wood chips may be supporting communication through mycorrhiza to heeled-in trees in a similar way…

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I’m not sure I’d use the word “communication” but it certainly connects the roots of a tree to a vast network of other organisms and transports nutrients from a much larger area than a tree can reach just using its roots. Perhaps that’s another reason mycorrhiza has less of an effect on bonsai, since there isn’t much area the mycorrhiza can reach that the tree can’t reach itself.

This is probably also why trees grown in root-control bags planted in the ground grow faster than trees in root-control bags above grounds even though both root systems are restrained. Mycorrhiza interacting with the roots in the bag can deliver nutrients the roots cannot reach giving it more resources than the tree not planted in the ground.

This is an interesting article I came across talking about adding acids to remove Bicarbonates in irrigation water.

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Impressive thread on understanding soils, for bonsai. This is the precise information I hunger for while I journey on my path to understanding how to bonsai and expressing myself through bonsai. Thank you to all the contributors to this thread.