Chemistry without chemicals? Can we use light to measure lime requirement in soil?

There are a number of methods to measure the lime requirement in soil. Many of these need chemical reagents, and extraction or titrations to be carried out. An alternative using no chemicals is spectroscopy or ‘green chemistry’ Courtney Doyle, Konrad Metzger and Karen Daly from JOhnstown Castle have more information.

In the article Soil pH - What is it and how do we measure it? we described how soil pH is measured at our laboratories in Johnstown Castle. We also mentioned that this is often accompanied by lime requirement. There are a few methods in use to determine lime requirement Shoemaker-McLean and Pratt buffer method (SMP buffer), the Sikora buffer method, modified Mehlich, Ca(OH)₂ titration and the Roth Lime model (used in the UK). These methods are ‘wet chemistry’ meaning they all need chemical reagents, and extraction or titrations to be carried out. In this article the alternative method we are going to focus on is spectroscopy or ‘green chemistry’ as it requires no chemicals, and is based on the interaction of light with the soil sample.

Why do we need an alternative?

The SMP buffer method has been in use in Ireland since 1965 and it is an effective and reliable method to determine lime requirement.  The drawbacks however are significant. The reagent in SMP buffer contains hazardous chemicals,  potassium chromate and p-nitrophenol. P-nitrophenol is acutely toxic, an aspiration hazard and an environmental hazard. Potassium chromate is acutely toxic to the environment, a marine pollutant and is categorised as carcinogenic (category 1B) and mutagenic (category 1B). This is a hazardous reagent to work with for lab staff and poses a risk to health and the environment. The European Chemicals Agency has designated potassium chromate a “Substance of Very High Concern” within the REACH Directive. This means the chemical is to be phased out of circulation due to its harmful nature, we need a reliable alternative to determine lime requirement. Ideally we’d like to find a sustainable alternative. This is where soil mid-infra red (MIR) spectroscopy comes in.

LR and pH are not directly quantifiable from spectroscopy, we cannot read LR directly from a peak or peaks we see in the image or spectra. The soil properties that influence acidity (pH) and the soils ability to ‘buffer’ against acidity are strongly dependent on soil constituents; carbonates, organic matter and clays, and these are what we can see as shown in Figure 1.

Figure 1: Soil spectra (averaged) showing peaks of interest in the regions where they occur (Metzger et al., 2020).

Once we have this data from the spectrometer we can use it as a tool to build a database which is simply a large collection of results that combines these data to build a model. The model is like a computer program that can predict the LR of soils that we have not carried out a chemical extraction. Our recent model for LR is shown in Figure 2 which shows the regions in MIR that the model is using in Irish tillage soils. 

Figure 2. Areas in MIR that the model can use to predict LR in soils. Metzger et al., 2020.

Currently at Teagasc Johnstown we still use SMP buffer to measure LR. It is a robust and accurate method despite the negatives clearly outlined above. We have been forced to look for an alternative method because of the REACH directive and the obvious harms that potassium chromate can cause. The benefits of finding an alternative are significant; it will greatly reduce our environmental impact and make carrying out LR testing much safer. So far, we have been successful at predicting LR for tillage soils which means that it should be possible to extend to grassland and eventually move toward more ‘Green Chemistry’ soon.