Fragipan Bonding by Silica and Iron Oxides in a Soil from Northwestern Italy

An Aquic Fragiudalf developed from a fluvial terrace of Pleistocene age in northwestern Italy was investigated to determine the possible role of different compounds in fragipan bonding. Fragipan characteristics appeared only in yellow, goethite‐containing zones of the fragipan horizon but not in gray, essentially Fe oxides‐free zones. Citrate‐bicarbonate‐dithionite (CBD), but not acid oxalate, extracted more Si in the yellow zones than in the gray zones. These data, and the fact that citrate‐bicarbonate (CB) without dithionite extracted very little Si demonstrate that CBD extracted a Si compound specifically associated with the Fe oxides (mostly goethite). In the fragipan the ratio CBD‐extractable Si goethite reached a profile maximum suggesting that the Si compound associated with the goethite particles could, if present in sufficient amount, cement goethite particles together thus providing a framework to mechanically hold other soil particles; alternatively, that compound could bind goethite with other soil mineral particles. Although the amounts of CBD‐extractable Si were low, a simple calculation showed that they were clearly in excess of those needed to saturate the theoretical sorption capacity of goethite, giving a surplus of Si able to promote bonding. Selective extraction methods provided no evidence for implication of Al in bonding. Consequently amorphous silica could be the bonding agent.