Assessing the Accuracy of Permanganate Reactive Carbon Fraction Analysis In High Carbon Content Soils.

The National Soil Survey Center (NSSC) provides a portable kit for field lab testing of potassium permanganate-reactive carbon (KMnO4-C) in soils, a fraction considered sensitive to management practices and thus a good indicator of general soil quality. The fraction that reacts with the 0.02M KMnO4 solution in the test is thought to be various biochemical compounds (carbohydrates, lipids, amino acids) that are easily oxidized by biological activities. The test involves a reduction reaction that bleaches the color from the violet-hued KMnO4 solution, using a colorimetric measurement against a calibration curve. Because this is a bleaching reaction, when total KMnO4-C in a 5-g sample exceeds the theoretical limit, the sample size must be reduced to fall within the colorimetric range of the instrument for measurement. However, our results from small samples (<2 g) with high KMnO4-C show inconsistent reactions and non-linear responses, which suggests that the reactive fraction interacts with KMnO4 solution in a predictable thermodynamic relationship where the lower concentrations of KMnO4-C enhances reactivity of that fraction with the solution. We present results of sequential reactions of 0.02 M KMnO4 solution with maximum (5 g) sample sizes of a high KMnO4-C NSSC standard soil in comparison with single exposure reactions with smaller sample sizes (2.5 g, 1g). The inconsistent results of smaller samples indicate that the best results are obtained from using maximum sample size and exposing the sample to repeated additions of KMnO4 solution until color is measurable, thus providing the best estimate of this indicator value.