30.07.2021 •

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. 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 (pXRF) spectrometry to determine the Ca concentration of 75 soil samples from four US states in relation to the developmental stage, as determined independently by five pedologists from the USDA–NRCS Soil Survey Staff. Although experienced, the evaluators unanimously agreed on the carbonate development stage of only 22.6% of the samples while evaluating the samples ex situ. Portable XRF‐determined Ca content generally increased from Development Stage I through VI for intact aggregates and 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 XRF provides an important analytical tool for field soil scientists to evaluate soils containing Ca as part of CaCO3.

Introduction

Soils featuring secondary carbonates are common in semiarid and arid regions. CaCO3 is a mineral of limited solubility, often originally derived from the mineral calcite or Ca‐containing rocks. As mineral and rock forms of CaCO3 degrade, they can serve as parent material for various soils. For example, Alfisols, Mollisols, and Inceptisols of the Southern High Plains of Texas are largely eolian in nature, and all contain substantive subsoil secondary CaCO3, originating from the northern Chihuahuan Desert (via prevailing winds from the south). The solubility of CaCO3 is influenced by variable CO2 pressure, pH, temperature, and salinity. Often in the presence of active acidity inherited from atmospheric sources (e.g., H2CO3) or plant respiration, CaCO3 dissolves to free Ca2+ and HCO3 ions that readily translocate within the soil as part of the soil solution [1]. As water in the soil solution begins to evaporate, the solubility product is exceeded, and CaCO3 begins to precipitate, leading to the development of lower portions of the solum as Bk, Bkk, Bkkm, Ck, or, more rarely, Ak horizons. Collectively, these horizons often constitute diagnostically recognized calcic horizons [2].

The Soil Survey Staff commonly uses morphological attributes (e.g., identifiable carbonates) and qualitative descriptions of soil horizons for establishing the quantity of secondary carbonates observed either in a fine earth or coarse fragment matrix in the field (Figure 1) [3]. Similarly, previous studies have investigated carbonates in soils, recognizing and describing up to six carbonate development stages [4–6]. Although useful, these descriptions lack the objectivity of being quantitative measures. In contrast, CaCO3 can be measured in the laboratory; however, the two most commonly applied methods—gasometry and titration—do not lend themselves easily to field quantification [7].


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