Validation Testing of a Portable Kit for Measuring an Active Soil Carbon Fraction

Abstract

Increasing demands exist for information about properties related to soil quality and human-induced soil change, particularly soil C. To help address this need, the USDA-NRCS Soil Survey Laboratory (SSL) developed a portable kit for rapid and relatively accurate assessment of soil active C (AC), an easily oxidizable soil C fraction. This study determined the accuracy of a hand-held spectrophotometer to measure a colorimetric reaction induced by reduction of a violet-hued KMnO 4 solution by AC. Seven National Cooperative Soil Survey university cooperators analyzed samples for comparison with results obtained at the SSL. Kit results from participants correlated well with SSL results, with 78% of all participant results within ±25% of paired target results. Variations attributed to prevalent mineralogy affirmed the need to separate the soil sample from reactant solution within 10 min for reproducible measures. Seasonal variations were observed, with the most variability being noted in the June to July sampling period (57% agreement, ±25%), attributable to increased soil biological activity. This study confirmed that the kit provides a reliable means of assessing a vital soil C component under field office conditions. The kit will be a valuable tool in assessing a critical soil quality measure, adding needed soil quality data to the national database, and improving the reliability and precision of soil quality interpretations.