What's Wrong with Soil Physics? I. Perilous Presumptions.

A soil’s water retention and hydraulic conductivity are largely determined by the pore size distribution and the arrangement of those pores.  But even ignoring the wiggle-room of “largely”, neither the pore size distribution nor the pore arrangement can be characterized readily and unambiguously.  Here we examine the most common way of obtaining a pore size distribution – from the water retention curve –identifying various pitfalls along the way.  Pitfalls we consider include wetting angle and pore shape, capillary bridges, surface films, gravitational gradient, air accessibility, and sample size.  Some of these pitfalls have been addressed in theory, fewer in practice, and yet others remain mired in confusion and denial.  In the end, we consider how much the derived pore size distribution deviates from the true pore size distribution, and how much it matters.