Contributors to this special 100th volume were carefully selected by the editor for their perspectives on not only the current state-of-the-art of their topic but also where their fields are going and what future opportunities exist to advance scientific frontiers. Originating in 1949, Advances in Agronomy is a venerable serial review featuring state-of-the-art reviews on crop and soil sciences over the years, and is highly regarded around the world. Volume 100 continues this long tradition of excellence, with cutting-edge and visionary reviews from distinguished scientists at the top of their respective fields. - Contributions from leading scientists in crop and soil sciences and allied fields including geochemistry, environmental microbiology, and microbiology - Dynamic illustrations - Extensive literature review - Grand research challenges and opportunities