Mineral Solubility Relationships in Fragiudalfs of Western Kentucky

Relationships between mineralogical and soil solution composition of nonfragipan and fragipan horizons were investigated in Fragiudalfs derived from loess and loess over sandstone parent materials in western Kentucky. Smectite and albite quantities were generally higher in fragipan horizons. Increased smectite quantities in fragipans are considered products of illuviation or neoformation while albite is residual. Compositions of soil solutions extracted from moist samples near field capacity indicated undersaturation with respect to amorphous SiO 2 in nonfragipan horizons and a near saturation or super‐saturation in fragipan horizons. High solution‐Si levels were accompanied by increased soluble Na and exchangeable Mg in all but one fragipan horizon. The solution data support the hypothesis of an amorphous SiO 2 phase acting as the binding agent between particles in fragipans, with the possible involvement of Na and Mg in the cementation process. Smectite and kaolinite were the stable minerals controlling Si levels in solutions of nonfragipan horizons.