Field perception of the boundary between soil and saprolite by pedologists and its differentiation using mathematical models

Abstract

Saprolite plays a central role into hydrologic and nutrient cycles. Despite that, saprolite research is scattered and uses heterogeneous, sometimes conflicting, methods and concepts. During field work, it is difficult to assign the boundary between soil and saprolite. This paper aimed to identify the subjacent logic that pedologists use to assign to a regolith volume its soil or saprolite nature. To achieve this goal, a tree algorithm was used to build a hierarchy of physical and chemical properties of a set of regolith profiles. Such hierarchization expose the inner, subjective criteria used by researchers during the assignment of a certain profile zone as saprolite or soil. The following variables were measured: total porosity (TP); bulk density (Bd); particle density (Pd); total Fe2O3, Al2O3, CaO, MgO, K2O, Na2O, P2O5, and TiO2; selective extraction of iron by ditionite-citrate-bicarbonate (FeDCB) and ammonium oxalate (FeOA); and the FeDCB/FeOA ratio. These measurements were done in a set of 25 regolith profiles (137 horizons and layers), located in the Southeast region and Northeast region of Brazil. The decision tree algorithm was applied using the recursive partition method to identify which of the measured property was most strongly associated with the field assignment of the pedologists to a certain profile zone as saprolite or soil. The Bd, FeDCB/FeOA, MgO, CaO, TP, and P2O5 explained 93% of the pedologists choice, being Bd responsible for 81%.