Soil analysis by infrared spectroscopy

This material in this section first appeared as an article in the Australian Soil Club Newsletter 1(3) in February 2004 by Craig Russell (The University of Western Australia) and Les Janik (Infrared Analytical Services).

This section contains three questions.

  1. What is infrared spectroscopy?
  2. What are the advantages of infrared spectroscopy for soil analyses?
  3. What soil properties can infrared spectroscopy be used to measure?


Question 1. What is infrared spectroscopy?

Infrared spectroscopy is based on the fact that individual materials are defined and therefore identified by their reflectance or absorbance of infrared light. Infrared spectroscopy is currently used routinely for the rapid characterisation of a wide range of materials. It is used in Australia in the areas of grain quality and leaf tissue testing, food chemistry and mining.

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Question 2. What are the advantages of infrared spectroscopy for soil analyses?

The advantages of infrared techniques for soil analyses over other analytical techniques include:

  • minimal sample preparation
  • a short turn around time at the laboratory
  • the need for only basic infrastructure
  • minimal training of staff
  • simultaneous determination of several constituents in every sample and
  • the ability to analyse samples remotely. I.e. spectra are acquired electronically and can therefore be transported electronically.

Infrared technology offers the potential of a more precise and standardised soil testing service. Soil analyses derived from standard chemical methods from different laboratories, or at different times from the same laboratory, can be difficult to compare. This can be due to operator error and, or, differences in analysis conditions across laboratories, or even across batches within a laboratory. Infrared techniques will support the further development of precision agriculture by providing information at higher spatial resolutions cheaper and faster. They may also be available for on site analysis in the near future.

All of these advantages contribute to a reduced cost of analysis. Consequently, infrared spectroscopy is being rapidly adopted across Australiaʼs primary industries.

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Question 3. What soil properties can infrared spectroscopy be used to measure?

Laboratory research has demonstrated the capacity of infrared spectroscopy to predict soil physical, chemical, and biological properties. Quantitative predictions of several important soil properties have been made. These properties are important in assessing soil fertility, agricultural practices and land degradation. They include:

  • Organic carbon: Soil organic carbon is an indication of soil organic matter content, which acts as both a source and sink for nutrients. Soil organic car- bon is linked to soil chemical, physical and biological health, and is strongly correlated with soil nitrogen supply.
  • pH: Soil acidity is Australiaʼs greatest land degradation issue, and is currently limiting our agricultural production. Techniques that promote the measurement of soil pH, the de- termination of the rate of lime required to achieve an acceptable pH, and the quality of lime products, will greatly aid the management of soil acidity.
  • Iron and aluminium oxide content: Soil iron and aluminium oxides bind phosphate that may otherwise be displaced from the soil rooting depth. Displaced phosphate is not only a loss in potential crop productivity, but in many regions results in the eutrophication of wetlands and waterways. Cheaper determinations of iron and aluminium oxide content will promote better phosphorus management and help alleviate nutrient pollution.

Other soil properties that have been predicted with infrared technology are total nitrogen, carbonate, lime requirement, cation exchange capacity and soil texture (i.e. percentage sand, silt and clay).

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