Development and Cooperator Testing of An Active Carbon Field Kit.

The USDA-NRCS Soil Survey Laboratory (SSL) has developed a field kit for rapid, relatively accurate assessment of soil active carbon (AC) in response to a growing demand by land managers for soil quality information.  Active C is a chemically labile soil fraction comprised of management-sensitive soil organic matter. The kit analysis is based upon reduction of a violet-hued 0.02M KMnO₄ solution by AC.  The associated color change is measured with a hand colorimeter.  Before release to Soil Survey MLRA offices, the kit was evaluated by university and EPA cooperators across the nation in a 15 month study that included five seasonal sampling events.  Sample splits from the cooperators were evaluated in a double-blind assessment, with neither side knowing the others results prior to testing.  Cooperator and SSL analyses with the field kit were coordinated so that both occurred at approximately the same time, to minimize analytical biases.  The SSL also performed the standard laboratory analytical method (Method 6A2a1) in addition to the field kit.  Overall, the field kit returns robust data relative to the SSL standard method, with 76.1% of samples evaluated (n = 295) falling within ±25% error range, a range determined by repeated measure of two standard soils by the SSL and cooperators.  Seasonal variations were noted in data, with particularly high variability noted in the early to mid summer sampling (June-July; data matching within ±25% error = 55.3%), possibly attributable to AC response to elevated temperatures, soil moisture, and plant growth. The field kit has a relatively broad analytical range, from 50 – 6000 mg AC kg^-1, with greatest sensitivity at the 200 – 600 mg AC kg^-1. The kit is available on a request basis to Soil Survey MLRA offices to provide the field labs additional soil quality assessment capability with a rapid turnaround time.