Environment & Environmental Analysis | Agricutlural- and Soil Analytics | Suitability Determination of Soil in Construction as well as in Agriculture and Forestry

Spectroscopic Analysis of Soil for Pre-Categorization and Quality Determination

Soil is vital to human life on earth, provides habitat for animals and serves as media for growth of all kind of plants. But not all soil is alike. The use of soil depends on its composition. An exact determination of the quality of soil is not only crucial for farmers or gardeners, but is also relevant for building or road construction. Improper use of soil can result in it losing its function, being degraded or even destroyed.

An upfront determination of the quality of the soil to be used in agriculture or construction is therefore paramount. The type and amount of detectable substances in the soil allows conclusions to be drawn about its composition and thus its suitability for a particular application. On behalf of the Federal Ministry for Economic Affairs and Energy, Pedologists at institutions such as the Federal Institute for Geosciences and Natural Resources (BGR) analyze rock samples in their mineralogical laboratory, using spectroscopic measurement methods, and publish their results in soil distribution maps and databases.

Cross Section of Soil

The Situation

Mid-Infrared Analysis for the detection of minerals: accurate but time-consuming

Many decisions can be made based on the results of the mineralogical analysis of soil, and hence the identification of existing mineral phases in areas under investigation. One interesting example is in determining the suitability of the host rock to act as a long-term repository for waste materials, amongst many others. At the BGR an infrared (IR) spectrometer is used in the laboratory to identify the individual mineral phases. By irradiating a sample with specific wavelengths of infrared light the molecular bonds within the different minerals will be exited to a resonant state; this absorbs energy from the infrared light. Because the molecular bonds in different minerals are unique, then each mineral will absorb specific wavelengths from the IR light. By monitoring the energy absorbed at different wavelengths (as an absorbance spectrum) the height of specific absorbance peaks will reflect the concentration of different minerals in the sample. In this manner the minerals of the Chlorite group and Kaolinite, among others, can be easily differentiated.

Mineralogical Analysis of a MIR Spectrum | Source: BGR

The most common spectroscopic method in the MID-IR range is the so-called KBr method, whereby a small and finely grounded part of the sample is mixed with anhydrous potassium bromide and pressed into a transparent tablet; this is then analyzed by means of a transmission measurement that produces an absorbance spectrum. The location and intensity of the measured absorption maxima is an indicator for the chemical binding, and often for the mineral which possesses that binding. The intensity of the absorption indicates the proportion of the bond (mineral) in the sample, or the degree of order of the bonds in a crystal. Carbonate, Sulfate, Quartz and Kaolinite can easily be detected using this method.

The Challenge

The traditional KBr method is not applicable for large quantities of sample material

The KBr method, based on transmission measurement, is very precise and reproducible but at the same time is very complex and inefficient, because only a very small amount of soil material can be pressed and analyzed simultaneously. However, to make a reliable statement about the nature of an entire soil area, a large amount of sample material must be investigated!

It is therefore important and essential to apply a second testing method that can be used to pre-categorize the samples based on a rough analysis of the soil sample. If, based on this pre-categorization, it immediately emerges that the soil is not suitable for the intended kind of use, due to a substance that has been detected, a further, more precise and time consuming analysis can be dispensed with.

The Solution

Some solids can be detected by means of reflectance measurements in the Near Infrared Range (NIR)

Hellma Reflection Probe Gladius XP for Quick Testing of Soil Samples

An alternative to the complex MIR measurements are reflectance measurements in the NIR range. Some solids such as quartz and other substances that occur in large quantities in the sample material can still be detected in this wavelength range. The specific measurement method applied for this purpose is diffuse reflectance. It serves as a quick method and to pre-categorize soil samples.

Diffuse reflectance avoids the surface dependent effects found in specular or direct reflectance and so can be used to measure the absorbance of specific wavelengths of NIR light, to determine the constituents of the sample. Using a fibre-optic diffuse reflectance probe such measurements can be made in the field or on collected bulk samples, enabling a large area to be checked quickly. Setting the angle of the fibres in the probe, not only avoids the specular reflectance but also focusses the light so that the beam can penetrate the surface of granular or powdered samples. The reflected signal, less the absorbed wavelengths , is collected by the receiving fibre(s) and processed by the connected spectrometer.


Labormessung einer Bodenprobe in Reflexion im NIR Bereich

Proof of suitability with little measurement effort

Some substances that occur in the soil can be detected in the NIR spectrum. In many cases this proof, which can be obtained by a simple diffuse reflectance measurement with the Gladius XP reflectance probe from Hellma, is enough to make a statement about the suitability of soil for a particular type of use. As a result, only a fraction of all soil samples have to be measured in the IR range. This enables institutes like BGR to efficiently determine the nature of large-scale ground areas much more quickly than in the past.



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