Reconstructing pH of Paleosols Using Geochemical Proxies

AbstractSoil pH is essential for understanding weathering, nutrient availability, and biological-edaphic relationships. However, standard pH measurement on soils requires friable material, thereby excluding most paleosols. In this article, bulk geochemical proxies for pH are developed for soil B horizons using indices that track pH-dependent mineralogical transformations. Geochemical relationships within a continental-scale modern soil data set (n=619) reveal a close association between pH and log-transformed CaO and little influence of refractory oxides on pH. These results guided the formulation of three geochemical indices that consist of ratios of Fe2O3, TiO2, and Al2O3 to CaO, herein referred to as FeCa, TiCa, and AlCa. After careful screening for anthropogenic influence, pedotransfer functions relating each index to pH were derived using sigmoidal regressions on a calibration data set (n=305). Each index has similar predictive capacity for pH (r2=0.70–0.74, root mean square error=0.83–0.88). The mod...