Semiquantitative Evaluation of Secondary Carbonates via Portable X-ray Fluorescence Spectrometry

Secondary CaCO3 is commonly found in soils of arid and semiarid regions in variable states of development as Bk, Bkk, Bkkm, Ck, or, more rarely, Ak horizons. Historically, a qualitative scale featuring various stages of development has been applied when evaluating carbonate-laden soils. By contrast, this study used portable X-ray fluorescence spectrometry to determine the soil Ca concentration of 75 soil samples from four US states in relation to developmental stage, as determined independently by five pedologists from the USDA-NRCS Soil Survey Staff. Although they were experienced, the evaluators unanimously agreed on the carbonate development stage on only 22.6% of the samples while evaluating the samples ex situ. Portable X-ray fluorescence-determined Ca content generally increased from Development Stage I through VI for intact aggregates as well as ground soil samples. The widest variation in Ca content was found in Stage III for both conditions. No substantive differences in Ca content were observed between Stages V and VI. A strong positive correlation was observed between the Ca content of intact aggregates vs. ground soil samples (r = 0.89). Both support vector machine classification and interpretable rules were used to classify secondary carbonate development stages using total Ca concentrations for intact aggregates and ground soil. Using scans of both conditions offers stronger predictive ability than either condition independently. Portable X-ray fluorescence provides an important analytical tool available to field soil scientists for evaluation of soils containing Ca as part of CaCO3.