Evaluating the accuracy and usefulness of commercially-available proximal soil mapping services for grassland nutrient management planning and soil health monitoring

Grasslands play an important role in global food security. However, there are increasing pressures to improve the sustainability of ruminant farming. Precision nutrient management tools (e.g., proximal soil sensors for soil mapping) offer opportunities to improve nutrient use efficiency through spatially-variable nutrient application rate maps. Despite little research validating these technologies on grasslands, commercial companies promote these technologies to grassland farmers. In this study, the accuracy of commercial companies offering these services was evaluated by comparing soil pH, P, K, Mg and SOM measurements derived from conventional soil sampling and laboratory analyses to measurements derived from the commercial operators, across a range of soils that are typical found in UK grasslands. Results showed that soil mapping services utilising gamma-ray spectroscopy (GRS) were not sufficiently accurate to predict soil pH, P, K and Mg on grasslands, and subsequently inappropriate for nutrient management planning for variable rate lime and nutrient application. Conversely, both GRS and visible-near infrared spectroscopy (Vis–NIR) accurately predicted between-field SOM variations in grassland soils but not within-field variation. This study emphasises the need for further research to explore the limitations of, and opportunities for, the universal application of these technologies across different soil types and/or land uses before their commercial application. It is therefore highly recommended that commercially-available soil mapping services are subject to certification, similar to centralised soil testing laboratories, to ensure data are accurate for soil management interpretation. The lack of reliability of such systems risks farmers’ confidence in the value of soil mapping, which could severely hinder future adoption of potentially valuable technologies.