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Clay Quality affecting CEC - Montmorillonite vs...

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Hi all!

Been a while, hope you are all well.

I’ve been busy geeking out on agronomy books with some farmer friends trying to understand the soil fertility cycle better and came across an absolute gem of a manifesto: Claude Bourguignon’s “Regenerating the Soil: From Agronomy to Agrology”.

Seems ole Claude in France there was well ahead of his time, his writing (of 70’s vintage) is much like that of modern Restorative/Regenerative authors.

Anywho, he drives home an interesting concept about not just clay and humus driving CEC (cation-exchange capacity) but the fact that it’s a complex of clay-humus providing this function. And that the type of clay is crucial to this. So crucial that he even recommends amending with these clays to provide for a permanent bonus in CEC, fertility.

Might make sense.

So, other authors (such as Dr. Arden Anderson) are mentioning the “all-star” type of clay that he favors: Montmorillonite clays. It appears to be related to the shape of the molecules and plating characteristics that provide the surface area for holding onto massive amounts of +’s...

Now, we don’t live in France, but are currently trying to get French friends to translate more info about this for us American bumpkins, but I want to know more..

Anyone with some knowledge of this?

I have been doing some amateur sleuthing and it seems Calcium Bentonite is related to or is a major component of these clays. Wikipedia describes Thai farmers using Ca Bentonite to great success in a rainy (leaching) climate, which we also have here in the PNW of the US.

And go!

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This is the stuff that can cause landslides where it gets soaked, swells, disconnects and then slips off a different substrate base. Engineers talk about it a bit. That implies it has great water holding capacity, so as a portion of a soil might be useful indeed. It is also used as a flocculant implying it has a high capacity for bonding with solids, and as a cat litter, so probably also bonds with gaseous molecules. To me it sounds like it would: increase water adsorption, provide nutrient storage (as opposed to runoff), raise CEC, and certainly seems a good contender for formation of a clay-humic crumb (tilth, aeration) as it would bond quite readily with humic acid.

The calcium content likely varies widely from site to site. If you can check what's growing at the source it might be informative.

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Location: Vilonia, Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
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Montmorillonite clays are clays that do have a fair amount of humus already incorporated as well as not such fine structure as most clays (more a mix of clay, silt and sand particles). As Dean brought up, this is the type of structured clay that causes land slides, this happens because the contained different sized particles along with the humus allows for super saturation and where there is contact with the second horizon the same sort of action that happens in an avalanche occurs. That is slippage and it usually results in total failure of the surface soil structure creating a disaster situation for anything down hill.

In several experiments where clays were being investigated for CEC capabilities, it was found that any pure clay (super fine silt) that is combined with silts, sands and humus materials in combinations where the clay is at least 5% of the mix, CEC was increased. It was found that when the clay material reached 50% that there was no more significant increase in CEC but there was a slow down in water infiltration. This occurs because there are no pure clays that are hydroscopic the infiltration of water in pure clays seems to be rather slow and constant, it is only when silt and other, larger sized particles are present that infiltration increases.

Most gardeners have found that a 10% pure clay gives good infiltration and water holding abilities to soil mixes, and some have this as high as 20% though these areas are closer to lake side or stream side plantings.
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Location: Greybull WY north central WY zone 4 bordering on 3
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Having grown up in an area where a lot of it is mined I would be leery of getting much of it in the garden.  Understand that by itself bentonite is basically water proof.  It needs organic to maintain a water pathway into the soil.  Now be aware if you are looking at plating that how it is milled makes a difference here.  
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