nutrient diffusion into substrates

George and I have been conducting an interesting discussion on the
merits of clay, laterite and vermiculite in substrates. I was saying 
how vermiculite is so wonderful because it increases the permeability 
of the substrate so well whereas fine clays would restrict it. I made
the claim that vermiculite substrates might not even need any substrate
flow enhancers such as slow RUGF or heating coils since diffusion of
the nutrient ions via concentration gradients may be sufficient. George
countered saying Dupla laterite is not a fine clay and challenged
my scientific foundation for this "ionic diffusion" claim.

Jim Kelly refers to three mechanisms for nutrient uptake by roots:

    "1.  Root interception:  as roots penetrate the soil they
         come in contact with colloids having adsorbed 

     2.  Mass flow:  nutrients move into roots as water is 
         absorbed.  [I have no idea of the degree to which 
         this occurs for submerged aquatics.]

     3.  Diffusion:  nutrient absorption in root vicinity sets
         up a concentration gradient, and ions diffuse toward
         root surfaces."

To put that in context, Jim is talking about really local diffusion
gradients probably less than a millimeter. Does anybody with a
chemistry background know if ions rely upon actual circulation of
water through a substrate or if diffusion through the substrate
is predominant?? I believe or suspect that ion concentrations in a
solution reach equilibrium very quickly, not dependent upon mixing.