What's Wrong with Soil Physics? III. Dynamo or Dirt.

Soils are complex, while physics is generally reductionist; thus the stage is set.  Consider a soil in place: the hydraulic gradient we characterize so simply in our equations is also affected by (among other things) temperature (which affects interfacial energy and viscosity), and thermal gradients (driven by radiation [with accompanying reflection and absorption], conduction, convection, and phase changes).  As the soil surface dries it gets lighter (increased albedo); this is just one of many nonlinear feedbacks.  The soil’s other fluid, air, is compressible, a carrier of heat and water in the form water vapor, and dynamic at essentially all scales.  This illustration also ignores the soil’s dynamic shrinking and swelling which play havoc with our water retention curves.  In an effort to cope with this complex system, we collect soil samples and bring them to our laboratories, analyzing the soil as dirt: inert material without context.  Laboratory experiments are necessary, because the complex coupled processes in the field may overwhelm the signal of the one fundamental process we are attempting to measure.  But laboratory experiments by themselves are dead ends, because understanding simple processes in isolation falls short of understanding the complexity of soil in place.  However well we understand dirt, we must not pretend it fully represents the dynamo.