Rootzone and topdressing sand size

Wouldn't it be convenient if topdressing or rootzone sands could be evaluated by a single number? It's not quite possible to do that, and I'll get into that below, but one number that is pretty useful on its own is the average particle diameter of a sand, referred to as the D50. This is the diameter of particles in a sand for which 50% of the particles (by mass) have a smaller diameter and 50% of the particles have a…