The over‐estimation of clay and the under‐estimation of pores in soil thin sections

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Abstract
SUMMARY: The amounts of clay visible in four thin sections from paleosol horizons (clay contents 9–32%) were measured by point counting. These sections were used as references for visual estimates, without point counting, of the amounts of clay in 14 further sections. The measurements and estimates were recorded on pore‐free and gravel‐free bases so they could be related to gravimetric analyses. Amounts of clay measured and estimated by area in these thin sections were between 1.6 and 3.1 times those determined gravimetrically.The reasons for these differences are discussed. Firstly, clay in soil thin sections contains much pore space beyond the resolution of the optical microscope and has an apparent density dependent on the amount of submicroscopic pore space. An edge effect also causes over‐estimation of clay in soil thin sections and under‐estimation of visible pore space. Components (clay, mineral grains, pores) are usually sloped at their borders with each other within the standard 25–30 μm (three‐dimensional) thin sections. Clay, for instance, may overcap a pore at its border, hiding some pore space. The need for true two‐dimensional images of soil pore patterns in image analysis is indicated.A nomogram has been constructed, using apparent density, to allow estimation of clay in thin sections of soils with any clay content. Coarse/fine ratios can be determined more accurately and estimates of clay can also be recalculated to provide illuvial to total clay ratios that are more meaningful. It is recommended that the micromorphological identification criterion for the argillic horizon be reassessed and that 4% illuvial to total clay in a soil thin section be adopted as the diagnostic cut‐off.