IR diffuse reflectance spectroscopy is an emerging, non-destructive, analytical technology that relies on statistical relationships between IR soil spectra and results obtained from ‘conventional’ measurements, in this case for TSN. The instrumentation typically has a small laboratory foot-print (Figure 7.6). For more details on the technology, see Janik et al. (1998), Shepherd and Walsh (2002), plus Method 6B4. The same issues and opportunities applicable to the measurement of soil C by IR apply to soil N.
Malley and Martin (2004) have summarised multiple examples of the use of NIR spectroscopy for TSN, incuding goodness-of-fits for ‘training-set’ data from conventional methodologies (r2 values 0.53–0.96; median = 0.85). Methods for TSN using both NIR and MIR are provided. Sequential scans typically take around 1–2 min/sample, most commonly 1 min. The soils should be ≈40°C air-dry and <0.5 mm. This technology is also suited to predicting the concentrations of other chemical and physical properties of soils.