Infrared Soil Analysis laboratory - Adelaide, South Australia

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What is mid infrared (MIR) spectrometry in relation to soil analysis?

Spectrometry is the combination of spectroscopy and chemometric (multivariate statistical) methods, in order to identify and/or quantify chemical species.

Infrared radiation light and heat energy in a continuous distribution of varying wavelengths, with frequencies in the near infrared (NIR) range 16600 to 4000 cm^-1 (600-2500 nm) and in the mid infrared (MIR) range 4000 to 500 cm^-1 (2500 to 20,000nm). Frequencies in the NIR are generally overtone and combination bands from the fundamental vibrations seen in the MIR. When this radiation is focused onto a sample, the molecules in the sample will increase their vibrational energy by absorbing specific frequencies depending on the molecular geometry, bond strengths and atomic masses. The resulting radiation is thus modified, resulting in a spectrum or “signature” of the molecular composition with peaks at the absorbing frequencies.

In practice, for soils, this is most conveniently and rapidly done using diffuse reflectance, where the incoming radiation is focused onto the soil sample surface, often in the form of a dry powder or <2 mm microaggregates, and is the reflected radiation passed back into the spectrophotometer, detected and analysed.

Both the MIR and NIR can be used to analyse soils. The NIR has some advantages in portability, remote sensing and samples can be run in glass containers and water, but the many combination and overtone peaks are often weak, complex and difficult to interpret. In contrast, the MIR spectra are much more intense and usually easier to interpret, but until recently the instruments have been less portable and use easily damaged optical materials.

The resulting MIR spectrum is dependent on the composition of the soil; particularly on specific vibrational signatures for organic matter (OM), quartz (sand), kaolinite and smectite (clays), carbonates (lime), gypsum, and iron and aluminium oxides.

The combined contributions from the various soil components can result in a very complex spectrum, difficult to analyse by eye, but multivariate computer models can be used to derive accurate qualitative and quantitative relationships or models between the spectral signatures and many chemical and physical soil properties.

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Predicting soil properties

MIR can predict a wide range of chemical and physical soil properties that are closely related to the bulk properties of soil (clay, organic matter, moisture content, cation capacity, mineralogy). It must be remembered that the MIR technique predicts many soil properties; it does not measure them directly.

Table 1: Typical soil properties predicted by MIR

Table1:Typical soil properties predicted by MIR

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DISCLAIMER Soil analysis using mid infrared (MIR) technology

CSIRO do not take any responsibility for decisions or actions that might be taken as a result of soil analyses obtained using MIR predictions of soil properties. The predictions are experimental at this stage and pending full validation. Although every effort has been made to accurately predict soil properties, some soil spectra may not be well represented by the calibration model used for prediction and result in prediction errors exceeding those reported for the calibration sets. If actions are to be taken based on the results of this soil analysis, conventional soil analysis should first be undertaken for confirmation. (We would suggest 10-20% of the total submitted).

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