Method 6B4 contains an outline and several references on the use of this emerging analytical technology. In brief, it relies on statistical relationships between MIR spectra and results obtained from conventional soil test measurements.
There is good evidence (e.g. Janik and Skjemstad 1995; Janik et al. 1998) that different absorption bands exist in the MIR region, which permit the non-destructive estimation of many soil components. These include exchangeable Ca2+ and Mg2+ and CEC, corresponding to results obtained following extraction with alcoholic 1 M ammonium chloride at pH 8.5 (Method 15C1). Unfortunately, present findings (D Lyons, pers. comm.) are that MIR is unable to reliably estimate exchangeable Na+ and K+ concentrations equivalent to those from Method 15C1. Soil samples should be ≈40°C air-dry and finely ground (<0.5 mm). Sequential scans typically take around 1–2 min/sample.
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 this method, which should provide results that correlate strongly with results obtained by Method 15C1 on similar soil types.
Load the instrument’s auto-sampler with ‘unknown soils’, previously dried to ≈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 (preferred).
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 the equivalent of alcoholic exchangeable Ca2+ and Mg2+ and CEC (cmolc/kg), expressed on an oven-dry soil basis.