Full‐pattern fitting of X‐ray powder diffraction data to determine allophane and organic matter contents in soils

Full‐pattern fitting of X‐ray powder diffraction (XRPD) data using reference materials has attracted increasing attention as a high‐throughput method to determine the contents of long‐range‐ordered (i.e., crystalline) materials in soils. We hypothesized that this XRPD method can also be used to quantify short‐range‐ordered (SRO) materials with low crystallinity, despite their much weaker and broader diffraction. To obtain soil samples with a wide range of allophane and organic matter (OM) contents, surface and subsurface soils were collected from five profiles in northern Japan. A full factorial design was used to identify the optimal combination of reference SRO materials, including allophane, OM, and phytolith, for XRPD method using full‐pattern fitting. The allophane and OM contents determined by XRPD were broadly but unexpectedly consistent with those obtained by conventional methods. In addition, the allophane and OM contents determined by XRPD showed significant positive correlations with phosphate absorption coefficient and cation exchange capacity, respectively (Spearman's rank correlation, p < 0.001), consistent with the relationships observed by the conventional methods. These results suggest that, although accurate quantification of samples with low contents (i.e., OM < 10 wt.%) remains challenging, state‐of‐the‐art XRPD method using full‐pattern fitting alone provides a promising new approach for rapid evaluation of soil properties and functions through the quantification of both crystalline and low‐crystalline materials.