Estimating plant-available nutrients with XRF sensors: Towards a versatile analysis tool for soil condition assessment

The timely diagnosis of plant-available soil nutrient contents is crucial in enhancing agricultural intensification and bridging yield gaps. There is a global demand for a practical and easy-to-use analytical tool capable of predicting the nutrient status of agricultural soils to make the soil chemical diagnosis faster, cheaper, and environmentally friendly. A growing body of research has highlighted the potential of energy dispersive X-ray fluorescence (XRF) sensors for monitoring the condition of agricultural soils. This study critically reviews current knowledge on the feasibility of using XRF sensors and suggests ways forward to predict plant-available soil nutrients. The review finds that some challenges need to be addressed, including: (i) mitigating the matrix effect in XRF spectral libraries and (ii) calibrating models that can capture the local context of the ratio between total and available nutrient content (T/A ratio). This study further discusses knowledge gaps related to the above-mentioned challenges and proposes the following future research areas: (i) understanding the impact of soil management on the temporal stability of T/A ratio and XRF model performance; (ii) assessing advanced predictive modelling strategies to address the challenges related to XRF spectral libraries, i.e., to deal with matrix effect and local context of the relationship between total and available content of nutrients, and (iii) evaluating data acquisition and modelling strategies that optimize the in situ application of portable XRF. Understanding these points is critical to advancing the technological maturity of predicting available nutrients in situ to fulfil plant nutrient requirements along with its development. Finally, portable, easy-to-use analytical tools are key to enhancing soil health/condition monitoring and proposing best management practices in agricultural areas worldwide, particularly in regions with limited infrastructure of soil laboratories. Soil monitoring is critical to preserve, sustain and recover soil condition/health, one of the main manageable drivers of soil and food security.