Transferability of a large mid-infrared soil spectral library between
two FTIR spectrometers
Abstract
Large and publicly available soil spectral libraries, such as the USDA
National Soil Survey Center – Kellogg Soil Survey Laboratory
(NSSC-KSSL) mid infrared (MIR) spectral library, are enormously valuable
resources enabling laboratories around the world to make rapid estimates
of a number of soil properties. A limitation to widespread sharing of
soil spectral data is the need to ensure that spectra collected on a
local spectrometer are compatible with the spectra in the primary or
reference library. Various spectral preprocessing and calibration
transfer techniques have been proposed to overcome this limitation. We
tested the transferability of models developed using the USDA NSSC-KSSL
MIR library to a secondary instrument. For the soil properties, total C
(TC), pH and clay content, we found that good performance (RPD = 4.9,
2.0 and 3.6, respectively) could be achieved on an independent test set
with Savitzky-Golay (SG) smoothing and first derivative preprocessing of
the secondary spectra using a memory-based learning chemometric
approach. We tested three calibration transfer techniques (direct
standardization (DS), piecewise direct standardization (PDS), and
spectral space transformation (SST)) using different size transfer sets
selected to be representative of the entire NSSC-KSSL library. Of the
transfer methods, SST consistently outperformed DS and PDS with 50
transfer samples being an optimal number for transfer model development.
For TC and pH, performance was improved using the SST transfer (RPD =
7.7 and 2.2, respectively) primarily through the elimination of bias.
Calibration transfer could not improve predictions for clay. These
findings suggest calibration transfer may not always be necessary but
users should test to confirm this assumption using a small set of
representative samples scanned at both laboratories.