What's Wrong with Soil Physics?

Soil physics has a dual identity—it is both a branch of physics and a branch of soil science—but its legitimacy as a science depends on its claim to be physics; this implies a self-consistent structure of definitions and concepts underlying the equations we actually use. Upon examining some of our core concepts—specifically those relating the water retention curve to the pore size distribution, the unsaturated hydraulic conductivity relationship, and the convection–dispersion model—we find that all three are built on the notion that soil is composed of bundles of capillary tubes. This underlying conceptual model lacks both self-consistency (threatening our claim to be a legitimate science) and a firm connection to reality (threatening our ability to reason and predict successfully). We argue that many of our struggles during the last decades are artifacts of building from the flawed conceptual model of soil as a capillary bundle. We propose in its place a pore network concept, which can be applied using the mathematics of percolation theory. We must build on a sound and self-consistent conceptual model, in teaching, research, and application, so soil physics can be firmly based in both soils and physics, and meet society's many challenges in food production, hydrology, water quality, bio-energy, and climate change.