Spectral Library 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). 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. |
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Reflectance Measurements
We measure diffuse reflectance spectra using a portable spectrometer (Analytical Spectral Devices) with a spectral range of 0.35 to 2.5 µm.
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In earlier studies, air-dried soil samples were packed into petri dishes and viewed from above using lamps (method details).
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In current studies, we scan samples through glass petri dishes use a High Intensity Source Probe. With this method, a single operator can comfortably scan hundreds of samples a day. |
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We are also researching the feasibility of direct field use of the spectrometer using a High Intensity Reflectance Probe. |
Test of Spectral Library Approach
The spectral library approach provides a coherent framework for linking soil information with remote sensing information for improved spatial prediction of soil functional capacity. Remote sensing of soil properties directly from space platforms is hampered by problems such as atmospheric interference, shade and shadow effects, mixtures of materials within pixels, and variation in soil moisture content. Studies on the effect of soil moisture content on calibrations between soil functional attributes and soil reflectance would help to evaluate the potential of reflectance spectroscopy in the field. Future studies should explore approaches that combine soil spectral libraries, and other geo-referenced information, such as from digital terrain models and field observations, with information from multi- and hyper-spectral remote sensing imagery (e.g. Shepherd and Walsh, 2002).
Methods and results of the test of the spectral library approach on 1000 African topsoils. |