A history of rhizosphere research – roots to a solution

The availability and accessibility of water and nutrients to plants, and the interactions of roots with soil, continue to be subjects of active research. Hiltner's insight that there was a volume of soil, the rhizosphere, over which the roots had influence, and in which a range of processes occurred, was a major advance in thinking. Soil science, though, took some time to incorporate this notion into its mainstream thinking. Nutrient and water availability to plants was defined throughout the early 1900s in equilibrium terms; for nutrients with chemical extractants, and for water by the equilibrium concepts of field capacity and permanent wilting point. It was not until the mid 1950s that ideas of water and nutrient mobility superseded ideas of thermodynamics and equilibria with measurements of convective and diffusive movement of resources to roots demonstrating the size of the zone of root influence. The size of the rhizosphere differs spatially and temporally depending on the resource considered, with microorganisms experiencing strong gradients. For the future, while direct management of rhizosphere properties to enhance the efficiency of N fixation is already a reality, interventions that will improve the recovery of nutrients and other resources are still a major challenge.