This method provides a surrogate indication of soil pH, in this case based on correlations between multiple MIR spectra and measured estimates of soil pH by Method 4B (1:5 soil/0.01 M calcium chloride extract). Merry and Janik (2001) reported an indicative coefficient of determination of 0.88 for this surrogate soil pH test.
This non-destructive analytical option follows the combined development of spectrometer hardware and smart computing and statistical software targeted to MIR spectra of scanned soil samples. Absorbance spectra in the MIR range of 4000 – 400 cm-1 (2500–25 000 nm) or wider are used (e.g. Janik et al. 1998), as specific molecular vibrations are sensed and are strongly associated with functional groups common in soil minerals and OM. Scans take around 1–2 min/sample.
Typically, many soils (the more soils the better) from the nation, state or region are collected, prepared and analysed carefully by the relevant ‘conventional’ method, in this case soil pH in a 1:5 soil/0.01 M CaCl2 at 25°C (4B). The same soils, after fine grinding to <0.5 mm and drying to 40–45°C are then scanned across an appropriate MIR spectral range, sufficient to record spectral responses for such things as –O–S–O vibrations of SO42– (1260 cm-1), –OH stretch from water of hydration vibrations (3500 cm-1) and deformation vibrations (1690–1640 cm-1), plus spectral overtones and combinations of fundamental vibrations (e.g. Beech et al. 2003). The method relies on strongly correlated relationships involving the MIR spectra and the chemically measured values. False skill is minimised by leave-out-one cross-validation.
Stabilise and verify the set-up and operating performance of MIR spectrometer, usually incorporating an auto-focussing diffuse reflectance accessory or capability. Also confirm the ‘standard calibration’ for soil pH (Method 4B).
Load the instrument’s auto-sampler with ‘unknown soils’, previously air dried to 40–45°C and ground to <0.5 mm to assist with sample uniformity. Typically, the instrument or its integrated computer will integrate the spectral signals with the calibration equations to provide the result without further calculation.
Report pH (1:5 soil/0.01 M CaCl2) equivalent by MIR on an air-dry basis.