Stephan studied soil science and agronomy and spent a considerable part of his career in international research in Africa and Asia. His main research interests as a systems agronomist are focused on plant nutrition, soil health, and the relation between crop nutrition and crop quality for human consumption. Apart from his own research in a team of ten colleagues and PhD students, he is heading the Soil Science Section/Department at Rothamsted Research. He is also leading the spectral laboratory at Rothamsted which investigates new methods for soil and plant analysis. Currently, he is contributing to several ongoing projects funded by the BBSRC (Growing Health, Resilient Future Farming), ESA (Earth Observation for estimating and predicting crop nutrients) and OCP (New Soil and Plant Diagnostic Tools).