Analyzing Soil Acidification Using Mid-Infrared Spectroscopy

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A recent case study in southern Australia shows how mid-infrared (MIR) spectroscopy can analyze overall soil quality by measuring its acidity.

According to a recent study published in Precision Agriculture, mid-infrared (MIR) spectroscopy, when used in conjunction with partial least squares regression (PLSR), can measure soil acidification accurately (1). This development comes at a time when the agriculture industry is combating the effects of climate change, resulting in the need to use analytical methodologies to ensure the continued success of the industry.

Farmer holding soil in hands close-up. Male hands touching soil on the field. Agriculture, organic gardening, planting or ecology concept. | Image Credit: © maxbelchenko - stock.adobe.com

Farmer holding soil in hands close-up. Male hands touching soil on the field. Agriculture, organic gardening, planting or ecology concept. | Image Credit: © maxbelchenko - stock.adobe.com

Soil acidification is a process that occurs in soil where the pH levels decrease over time (2). It poses a significant challenge to agriculture, particularly in terms of crop yield and soil health (1). Traditional methods of managing soil acidity have primarily relied on the application of lime (calcium carbonate, CaCO3). Soil acidification occurs when the pH of the soil drops, making it less conducive to plant growth. This condition often necessitates the application of lime to neutralize the acidity (1). Traditional laboratory methods for measuring soil acidity and the effectiveness of liming treatments, while accurate, can be time-consuming and costly (1). Thus, there is a growing need for faster, more cost-effective approaches.

A recent study led by Ruby Hume at The University of Adelaide explored this issue, proposing a new cost-effective approach using MIR spectroscopy for measuring soil acidity. By analyzing soil properties using light absorption patterns, MIR, when combined with PLSR (MIR-PLSR), can potentially provide accurate assessments of soil acidity and related characteristics in a timely manner (1). Hume's study specifically aimed to evaluate the performance of MIR-PLSR against traditional laboratory methods in detecting the effects of lime treatments on acidic soils.

The research process involved a single field trial at The University of Adelaide, where soils received lime treatments varying in rate, source, and method of incorporation. The research team then evaluated several soil properties, including pH (in both H2O and CaCl2), exchangeable and extractable aluminum (Al), cation exchange capacity (CEC), and organic carbon (OC) (1). These measurements were then compared between MIR-PLSR predictions and traditional laboratory results.

The results indicated that MIR-PLSR predictions for soil pH showed strong correlation with laboratory results, with R² values of 0.739 for pH in H2O and 0.788 for pH in CaCl2 (1). Therefore, the conclusion derived from this was that MIR-PLSR can reliably detect changes in soil pH following lime treatments.

However, the performance of MIR-PLSR in predicting other soil properties varied. For organic carbon, the model achieved an R² of 0.881, indicating a high level of accuracy (1). In contrast, predictions for CEC and both forms of aluminum (exchangeable and extractable) were less reliable, with lower R² values and higher RMSE (1).

Although MIR-PLSR demonstrated potential as a method that could be widely applied in the agriculture industry, Hume and the team acknowledged that their method was not perfect. As an example, the researchers noted in their results that MIR-PLSR was less effective in detecting treatment effects on CEC and aluminum, which suggests that certain nuances of soil chemistry might be lost in the spectral data (1). Therefore, the team underscored the importance of further refinement of the prediction models, so that all measured properties can generate more reliable data.

By providing a faster and potentially more cost-effective method for monitoring soil properties, MIR-PLSR could enable more precise and timely interventions, ultimately supporting better crop yields and soil health. However, further research and model improvements are necessary to fully realize this technology's potential.

References

(1) Hume, R.; Marschner, P.; Mason, S.; et al. Using Mid-Infrared Spectroscopy as a Tool to Monitor Responses of Acidic Soil Properties to Liming: Case Study from a Dryland Agricultural Soil Trial Site in South Australia. Precision Agric. 2024, 25, 1340–1359. DOI: 10.1007/s1119-024-10114-3

(2) Queensland Government, Soil Acidification. Queensland Government Website. Available at: https://www.qld.gov.au/environment/land/management/soil/soil-health/acidification#:~:text=Soil%20acidification%20is%20a%20process,the%20surface%20soil%20and%20subsoil. (accessed 2024-06-06).

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