Theory 37/Geometry & Scale/Soil physics
Water Retention Follows Grain Size
The claim
Soil water retention is set by the ratio of grain size to pore-throat size rather than by total porosity, so two soils with equal porosity can hold very different water.
01Mechanism
Capillary retention depends on pore-throat radius, which scales with grain size and packing rather than with the void fraction.
02Prediction
Retention should track the ratio across textures.
03Refutation
Retention tracking porosity would remove the pore-throat term.