Russell et al. (2002) reported positively on the potential of NIR diffuse reflectance spectroscopy to predict PMN in soils used for Australian-grown rice. For example, similar wavelengths of NIR soil spectra were correlated with both plant-N uptake and N mineralisation.
NIR reflectance spectroscopy for PMNKCl relies on good relationships between absorbances in the NIR spectral region (from overtones and combination bands of fundamental absorbances) and corresponding results obtained by ‘conventional’ method 7D1a or 7D1b. Ideally, continuous relationships should be linear, with coefficients of determination ≥0.85–0.9 across the expected soil concentration range.
Stabilise and verify the set-up and operating performance of the NIR spectrometer, usually incorporating an auto-focussing diffuse reflectance accessory or capability. Also confirm the ‘standard calibration’ for the ‘conventional’ Method 7D1 of preference.
In common with Method 7A6a, load the instrument’s auto-sampler with samples, previously dried to ≈40°C and finely ground (e.g. <0.5 mm) to minimise soil heterogeneity. Typically, the instrument or its associated computer will integrate the spectral signals with the calibration equations to provide the result without further calculation.
Obtain PMNKCl by NIR and Mineral N from Method 7C2b (both expressed as mg N/kg air-dry) then calculate as follows:
Net PMNKCl = [NIR Measured PMNKCl – Mineral N from Method 7C2a/b (or equivalent)], with all expressed as mg N/kg, air-dry.