Soil Spectroscopy at Teagasc Johnstown Castle

ENVIRONMENT: Spectroscopy is an analytical technique that uses light to analyse a sample. Johnstown castle use the interaction of light with soil to give useful information

Courtney Doyle and Karen Daly

Soil analysis is a changing field. Since the 1950s, routine chemical and physical analysis has been used to assess soil function and quality. Many of these old methods are still used today.  For farmers, advisors and researchers knowing soil pH, soil texture, mineral content, and lime requirement are all very important values to help make decisions about land management and research practices. Be this for financial reasons, sustainability reasons or environmental reasons. Traditionally, we obtain these results through wet chemistry laboratory analysis which consists of many costly chemicals, single use plastics, high water usage and can be very time consuming. The alternative to these harsh analysis methods have been termed ‘green chemistry’ and an example of this is soil spectroscopy. Spectroscopy itself is not a new method, it has been around since the late 1800 used by physicists to understand the interaction between light and matter. Over the last 30 years it has been shown to be useful in soil analysis. Parameters such as pH, mineral content and organic matter have all shown the potential to be predicted using spectroscopic techniques.

What is it?

Spectroscopy is an analytical technique that uses light to analyse a sample. In our case we use the interaction of light with soil to give us useful information. The information we get comes in the form of a spectrum, which shows the soil samples component parts. Depending on the type of light and detector, we can see a range of information from clay content to soil organic matter, mineral content and more. At Teagasc, we use spectroscopic techniques like UV-vis spectroscopy, IR spectroscopy, X-rays and laser to analyse soil but in this article we will focus specifically on diffuse reflectance spectroscopy. With this technique infra-red light in the near IR and mid IR range is utilised by a FTIR (Fourier-transform infrared spectroscopy) spectrometer.  Diffuse reflectance spectroscopy can predict soil properties such as pH, carbon, iron, aluminium, potassium, magnesium, percentage organic matter and percentage sand, silt and clay.

Figure 1: Simplified light path diagram

How it works

The light source in the spectrometer emits infra-red radiation over a wavelength range (for example in the mid infrared region we scan over a range of 12,000 – 4,000 wavenumbers) that is directed to the sample by a series of mirrors. This infra-red radiation hits the soil sample, part of the light is absorbed by the sample and part is reflected back through the instrument, again by a series of mirrors, to the detector. The detector responds to the signal (the difference between the ingoing and outgoing light energy) which is converted into a peak at that wavenumber. There is a peak at every wavenumber over the chosen range which creates the spectrum. The information from the detector is sent to the computer which uses a series of mathematical equations to convert this information into a spectrum. Examples of 3 spectra are shown below in figure 2. Each coloured line is one soil sample.

Figure 2: An example of a soil spectrum. Soil samples from 3 field sites 

The resulting spectrum gives us a chemical profile of the soil with information about the organic and inorganic constituents. We then interpret the spectrum to get information on soil properties. When we correlate our spectra with reference laboratory data from conventional measurements, we have the potential to predict a multitude of soil attributes from a single spectrum.

Why it’s useful

The important thing to know about soil spectroscopic analysis is that from the spectrum we cannot read concentrations of soil nutrients, pH or lime requirement directly. The spectrum below (figure 3) shows a sample of what can be identified directly.

Figure 3: Labelled peaks on spectrum, 3 field sites 

To turn the spectrum into meaningful results we use chemometrics. This extracts the information from a spectral signature with the use of maths and statistics to correlate to physical and chemical parameters from a soil data base. This allows us to predict multiple soil properties from a single spectrum.  

Advantages of FTIR spectroscopic analysis

• Fast, reliable, non-destructive, cost friendly
• Environmentally friendly (no harsh chemicals or digestions required)
• Needs a very small amount of sample (<0.5 g)
• Very little sample preparation required ( soil dried and sieved <2 mm)

In conclusion, diffuse reflectance spectroscopy for the analysis of soil has a lot of advantages, and lots of potential. With the use of models and large and growing soil libraries, we have the possibility to replace old labour intensive and hazardous extraction techniques that will allow us to move to more sustainable and environmentally friendly soil analysis while continuing to assess quality and function of agricultural soils. It is particularly important to continue to monitor our soil at this time with more and more research showing that globally soil quality is declining and in our Johnstown Castle labs the Terra Soil project with the Geological Survey Ireland is creating a spectral library of soil across Ireland that will in time be available to use for chemometric modelling.

Figure 4. Scanning soils in Johnstown Castle to build spectral libraries with GSI.