A protocol for in field collection of soil spectra and alignment with laboratory based SSL’s

The large majority of research studies and commercial examples on soil spectroscopy are based on spectra collected on dried and sieved soil samples brought to the laboratory. These preparatory steps are the same for traditional laboratory methods, and so far in the process no rationalisation is gained. But these steps can be replaced by collecting spectra directly in the field to gain full potential of spectroscopy. Unfortunately, this introduces a number of additional to address. For example, soil surface and moisture conditions vary greatly in the field and these are factors with a profound influence on spectra. This induces questions on how to best collect spectra in the field and on how calibration data sets should be built and used. To build field based spectral libraries that cover also these dimensions is hardly realistic. To use the existing, and under continuous development, laboratory based soil spectral libraries is a much more attractive alternative, despite the most likely miss-match between spectra. In the EJP SOIL project ProbeField, we address these issues. The final outcome of the project is a procedure protocol incorporating the project advances. The protocol consists of a decision tree that, based on soil properties of interest, available instrumentation, location, cost benefit, etc. guides the user to an adequate line of procedures. The next part deals with procedures in the field that will result in the best possible spectra to be collected regarding structure, moisture and contamination caused by for example residues. This is followed by the pre-processing of spectra for correction of remaining moisture and other artefacts, and alignment with SSL’s. Finally soil property estimation and the potential use of covariates is discussed.