Soil Analysis by Infrared Spectroscopy

By Dr Craig Russell, Research Fellow, Centre for Natural Resource Management, Albany, Western Australia, and Les Janik, Infrared Analytical Services Pty Ltd.

New technologies that provide rapid analysis of soil properties are in urgent need across the globe. Such technologies will promote the quantitative assessment of large-scale land management problems, and satisfy the environmental and economic need to manage agricultural land at a much smaller scale and promote the more efficient use of agricultural inputs.

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

The advantages of infrared techniques over other analytical techniques include:

(a) minimal sample preparation

(b) a short turn around time at the laboratory

(c) the need for only basic infrastructure

(d) minimal training of staff

(e) simultaneous determination of several constituents in every sample and

(f) the ability to analyse samples remotely. ie spectra are acquired electronically and can therefore be transported electronically.

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

Recent 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:

a) 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 carbon is linked to soil chemical, physical and biological health, and is strongly correlated with soil nitrogen supply.

b) pH: Soil acidity is Australia's greatest land degradation issue, and is currently limiting our agricultural production. Techniques that promote the measurement of soil pH, the determination 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.

c) 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 (ie percentage sand, silt and clay).

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.

Current research: Mid infrared and near infrared spectroscopy

Two infrared techniques are currently available for soil analysis, but their specific advantages and complimentarity have never been determined. These techniques are mid infrared (MIR) and near infrared (NIR) spectroscopy, and they are currently employed on separate instruments.

From a theoretical standpoint, MIR spectroscopy may be superior to NIR spectroscopy because it detects fundamental features as opposed to their overtones, and is sensitive to quartz. MIR soil absorption spectra are intense and display numerous peaks that can be readily identified, good for both qualitative and quantitative interpretation. NIR soil absorption spectra are of low intensity and exhibit few distinct peaks, yet these spectra have also been proven good for quantitative purposes.

Nevertheless, infra-red theory would suggest there are advantages in utilising both spectral regions. MIR energy absorption is linear for low energy absorbing properties, good for the detection of minor constituents. NIR energy absorption is linear for high energy absorbing properties, good for the prediction of major constituents.

Furthermore, due to the recent rapid adoption of NIR technology globally, NIR instruments are able to scan larger soil samples and are currently better supported with specialised software for calibration development than MIR instruments. Another advantage of NIR is that the same instrument can be used for plant and organic resource quality analyses (ie grains, fodder, green and animal manures, organic residues), as is currently available across Australia on instruments purchased for cereal chemistry assessment.

Australian scientists are seeking to explore the strengths and weaknesses, and thereby the best mix of these two infrared technologies for the routine purpose of soil testing. This knowledge will also be of significance in evaluating the usefulness of dual purpose instruments that may soon be commercially available.

Glossary

Electromagnetic spectrum : the complete range of wavelengths and frequencies of electromagnetic radiation extending from gamma rays to the longest radio waves including visible light.*

Infrared : specific wavelengths on the electromagnetic spectrum that heat an object they strike.*

infrared spectroscopy: the spectral analysis of compounds using radiation in the infrared region.*

Spectra : plural of spectrum

Eutrophication : excessive algal bloom, accumulation of nutrient in sediments, and low levels of dissolved oxygen in an ecosystem.** In recent years, this process has been accelerated by an increase in environmental pollution from such sources as detergents containing phosphorus, the leaching of fertilisers, sewage and toxic dumping, and heated water from the cooling systems of power plants and other industries.

Internet Sources:
*http://www.sgia.org/glossary

**http://www.peel.wa.gov.au/content/thePDC/strategy/environment_objectives.cfm

www.soilhealth.com