Mid‐infrared spectroscopy to estimate ratios of soil organic carbon to clay

Abstract

The index based on the ratio of soil organic carbon to clay concentration (SOC/clay) has been confirmed as an effective tool for assessing the organic matter status of mineral soils. The index contains four classes: very good (SOC/clay ≥ 1/8), good (1/10–1/8), moderate (1/13–1/10) and degraded (≤ 1/13). Conventional analyses of SOC and clay concentrations are resource intensive, however. Here, we investigate the use of mid-infrared spectroscopy (MIRS) as an alternative for quantifying SOC/clay ratios and the index. We use data from the National Soil Inventory of England and Wales (n = 1301). We obtained RMSE values of 3.6 and 57 g kg−1 for estimations of SOC and clay concentrations by MIRS, respectively. We found that a simplified three-class index was satisfactory for predictions using MIRS. The accuracy of clay concentration estimations was a limiting factor, particularly for the combined middle class (approx. 60%–70% success). Over 80% of the samples in the other two classes were correctly predicted. The probability that a sample was in the estimated index class can be used to filter out estimates likely to be misclassified. The filtering removed at least twice the number of misclassified samples as the number of correctly classified that had below-threshold probability. We found that uncertainty estimates are more informative if calculated from the component estimates of SOC and clay rather than the SOC/clay ratio. An important source of uncertainty is the mismatch between MIRS estimates of clay concentration based on clay mineralogy and conventional estimates based on particle size fraction. The additional information on clay mineralogy given by MIRS means it provides a better characterisation of SOC–clay interactions affecting soil functioning.