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Clay minerals have the capacity to bind and exchange H+ ions, and as I understand it the clay content in soils has a rather significant effect on soil PH. Apparently sandy soils naturally have a lower ph requirement than loam/clay soils.

Also it seems that clay and organic material form a chemical complex that increases soil fertility. This complex forms a nutrient buffer in the soil and also buffers the hydrogen ions, stabilizing the soil PH and preventing souring of the soil.
My source also says that soils without this clay-organics complex can never be optimally fertile.

This is at the outer limits of my understanding though, maybe that someone that knows more about this can elaborate.

Stefan

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