Roots: The Big Movers of Water and Chemical in Soil

Root water uptake and the dynamic availability of water to plants is a phenomenon that tends to be overlooked by soil scientists, despite the often dominant role of roots as a sink for water in the soil. Water taken up by roots is vital for plant growth and the productive management of soils. In addition, trapping and consuming water in the rootzone means there is less remaining to act as a vehicle to carry chemicals beyond the grasp of roots and consign them to either receiving groundwater bodies or surface reserves. Here we review landmark developments in the theoretical description of the availability of water to roots. By the 1960s, thanks to Wilford Gardner and others, we had achieved a good theoretical understanding of the interplay between soil characteristics and water availability for a single, isolated, semiinfinite root embedded in a cylinder of soil. We discuss the incorporation of these simple theoretical notions into the multitude of comprehensive simulation models of rootzone functioning that followed the democratic spread of computers throughout the 1970s. Much still remains to be done in linking rootzone form to root functioning, but technology is coming to the rescue. New techniques are providing improved means by which we can better observe both the changing spatial form of roots and the temporal pattern of their functioning. Time Domain Reflectometry for measuring soil water content close to roots and near the soil surface, in tandem with instantaneous monitoring of sap flow directly within roots, is providing a sharper view of root functioning. Mean while improved rhizotrons, along with new techniques of image analysis and topological description, are providing better descriptions of root system form. We feel that progress in understanding the role of roots as the big movers of water and chemical in soil will, in the near future, remain driven by improvements in our ability to observe the link between rootzone form and function.