I am no expert in soil science. Could some of you pros explain how this can happen? I have never seen prismatic structure IRL and I had always assumed if that occurs it could only happen on the A horizon?
this post was submitted on 04 Oct 2025
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In the A you usually get granular structure (if it's high in carbon, like mollisols/chernozems), or platy if it's an elvuiated A, like you would find in a spodzol/brunisol
In B horizons structure is caused by shrink-swell action in 2:1 clays. Water gets in the clay sheets and spreads them apart, and the sheets contract when they dry out. Freezing can also have an effect. Since a hexagon is the most efficient shape, you get prisms forming. Think I'd the Giant's Causeway - columnar basalt, but with much weaker and different forces.
You might ask how do I get structure in non-clay soils? Clay is in every texture almost but in different proportions. sandy loam has clay in it too, for instance
The platy structure in the A horizon I mention earlier forms from the B shrinking and swelling as well.
Thank you very much for this elaborate explanation! I was even aware of that clay part, since I know the grain size distribution "triangle"? I don't know, what this could be in English.
You're referring to the texture triangle.
But yes, all textures are just different ratios of the size fractions (sand 2 mm to 0.05 mm, silt 0.05 mm to 0.002 mm, clay <0.002 mm).
Not all clays will swell - kaolinite is a 1:1 clay for instance but 2:1 clays like montmorillonite will hydrate and swell to a greater degree.
Ah, yes texture triangle! Thank you for all the details. You seen to know your soils!
I would hope so! I hold a M.Sc. in soils and did 10 years of soil surveying