Application of visible near-infrared absorbance spectroscopy for the determination of Soil pH and liming requirements for broad-acre agriculture

Soil acidification is a major and growing concern in many cropping regions globally. Whilst spatial variability in acidification is a common consideration in the management of soil health and fertility at sub-paddock scale, insufficient focus has been directed toward the identification of this variability. Suitability of portable visible near infrared reflectance (vis-NIR) spectroscopy was assessed in this study as a potential technique to achieve rapid, precise, inexpensive and spatially specific quantification of key soil parameters to inform lime requirements. Spectral fingerprints were taken using a 1 ha grid sampling approach, with four sampling protocols investigated as follows, scans: (i) directly on cleared soil surfaces; (ii) on 0-100 mm undisturbed cores; (iii) on dried 0-100 mm cores; and finally (iv) on dried, ground, sieved and mixed cores. Data was analysed using a partial least squares regression (PLSR) model to identify the strength of linear relationship between reference chemistry data and predictions derived from spectral readings. Lime requirement maps using vis-NIR predictions were then theoretically compared against traditional aggregated sampling patterns while considering the trade-offs between accuracy, economics and agronomy associated with the identification of spatial variability. The vis-NIR measurements demonstrated moderate predictive capabilities in field for determining pH (R-2 = 0.3-0.5) and liming requirements (R-2 = 0.5-0.6) rapidly at high spatial resolution. Vis-NIR in field mapping techniques, which enable the use of site specific management of soil resources, were found to positively redirect lime resources from alkaline areas toward acidic areas of the paddock, resulting in minimal difference to overall expenditure on lime purchase and potential for increased agronomic benefits over the long-term. Further spectral library development, calibration, and research on in-field sampling methods is recommended.

Automated summary

Soil acidification is a major concern globally, and portable visible near infrared reflectance spectroscopy can be used to rapidly, precisely, and cost-effectively quantify key soil parameters to inform lime requirements. The technique demonstrated moderate predictive capabilities for determining pH and liming requirements at high spatial resolution, allowing for site-specific management of soil resources and potentially increasing long-term agronomic benefits. Further research on spectral library development, calibration, and in-field sampling methods is recommended.