This test for TOC uses MIR diffuse reflectance spectroscopy. The instrument’s outputs should be related to one of the 6B methods, either using generic relationships from a spectral library or by establishing in-house relationships, ideally with 1:1 linear relationships and coefficients of determination of ≥0.9 across the expected soil TOC concentration range. The same technology is suitable for predicting the concentrations of other soil chemical and physical properties.
Stabilise and verify the set-up and operating performance of the MIR spectrometer, usually incorporating an auto-focussing diffuse reflectance accessory or capability. Also confirm the ‘standard calibration’ for the 6B-SOC method of preference. For example, if significant carbonate is known to be present, the preferred ‘conventional’ method is 6B3.
Load the instrument’s auto-sampler with ‘unknown soils’, previously dried at ≈40°C and finely ground (<0.5 mm) to assist with sample uniformity. Typically, the instrument or its associated computer will integrate the spectral signals with the calibration equations to provide the result without further calculation, if the conventional method’s results were expressed on an oven-dry basis. If the calibration equations were based on conventional measurements expressed on an air-dry basis (≈40°C), then the MIR results will need to be adjusted for residual moisture using relevant air-dry moisture to oven-dry moisture ratios, guided by Method 2A1.
Report TOC (% C) by MIR on an oven-dry basis.