Clay Dispersibility and Soil Friability—Testing the Soil Clay‐to‐Carbon Saturation Concept

Soil organic carbon (OC) influences clay dispersibility, which affects soil tilth conditions and the risk of vertical migration of clay colloids. No universal lower threshold of OC has been identified for satisfactory stabilization of soil structure. We tested the concept of clay saturation with OC as a predictor of clay dispersibility and soil friability. Soil was sampled 3 yr in a field varying in clay content (∼100 to ∼220 g kg −1 soil) and grown with different crop rotations. Clay dispersibility was measured after end‐over‐end shaking of field‐moist soil and 1‐ to 2‐mm sized aggregates either air‐dried or rewetted to −100 hPa matric potential. Tensile strength of 1‐ to 2‐, 2‐ to 4‐, 4‐ to 8‐, and 8‐ to 16‐mm air‐dried aggregates was calculated from their compressive strength, and soil friability estimated from the strength–volume relation. Crop rotation characteristics gave only minor effects on clay dispersibility and no detectable effects on soil friability. Dispersed clay correlated to soil content of clay, but the correlation increased if subtracting a fraction assumed protected by OC. This trend was less convincing for soil tensile strength and friability. Increased clay dispersibility and reduced soil friability for 1 yr of measurements could be ascribed to wet conditions for potato ( Solanum tuberosum L.) harvest and tillage the preceding year. Literature data indicate soils' content of clay and silt (Fines20) to be a better predictor of specific surface area than clay. We conclude that a clay/OC ratio of 10 and a Fines20/OC ratio of 20 may serve as corresponding thresholds for clay dispersibility, the latter probably best reflecting organo‐mineral interactions of importance to the soil physical properties.