OUR APPROACH

We have proposed a scheme for the use of spectral libraries as a tool for building risk-based approaches to soil evaluation (Shepherd and Walsh, 2002). The first step is to widely sample the soils from a target area and scan the samples through the spectrometer. The ability to rapidly and non-destructively characterize soils using reflectance spectroscopy permits thorough sampling of the variation within a target population of soils. The spectral data space is then systematically sampled to provide a small subset of soils for further characterization. Soil properties or attributes of soil functional capacity are then measured only on this selection of soils. These attributes can include laboratory measurements (e.g. aggregate stability) or field measurements (e.g. infiltration rate, crop response to phosphorus application).

THE PROJECT

Calibrations are made between the soil attributes and the reflectance spectra. If, on the basis of cross-validation or holdout validation methods, calibrations are found to be insufficiently accurate for user requirements, the calibration sample size can be increased. The resultant calibrations between soil functional attributes and soil reflectance are then used to predict the soil functional attributes for the entire soil library and for new samples that belong to the same population as the library soils. Poorly described soils, whose spectra are not representative of the library spectra, are further characterized and added to the calibration library. In this way the value of the library is iteratively increased.

Conventional assessments of soil capacity to perform specific production, engineering or environmental functions rely on local calibration of observations on soil functional attributes to measured soil properties. However, soil analyses are expensive and dense sampling is required to adequately characterize spatial variability of an area, making broad-scale quantitative evaluation difficult. Infrared reflectance spectroscopy, especially near infrared spectroscopy (NIR), is now routinely used for rapid non-destructive characterization of a wide range of materials in industry. Although soil scientists have investigated reflectance spectroscopy for several decades, the technology has not been widely taken up and routinely applied in soil studies.