Hydrophobic Soils: Exploring the Mechanism for Reversible Hydrophobicity.

Arable soils can develop seasonal hydrophobic properties. Current models, for hydrophobic organic matter (OM) on soil particles, do not explain soil wetting behavior reversibility. We present a new conceptual model for OM structure which describes moisture dependant hydrophobicity. Dynamic contact angles (CA) of soil samples were correlated with water content (WC) and grower-observed lower crop productivity. Data fell into two distinct groups: (1) higher WC, lower CA, less degraded; (2) lower WC, higher CA, more degraded. High CA soils had more total organic carbon (TOC). The zonal model for OM identifies mineral surface OM that consists of amphiphilic molecules which screen non-polar functional groups from the soil solution. Amphiphilic compounds change their orientation exposing the hydrophobic layer as soil dries. This mechanical hydrophobicity is reversible with addition of soil moisture. This model is supported by successful demonstration that the CA is determined by two parameters: soil WC; soil OM.