I'm in the same boat as you!!
I have the same PSP section but have pondered the same problem.The only ideas I have come up with so far are these.
First one being for sand is to paint and weather the PSP as desired.Give it a wash with something close to the color of the sand and then use an eyedropper to apply a drop of thinned white glue to the holes and sprinkle sand on it theory being that the sand will only stay where the glue is when the excess is shaken off.Since there are literally thousands of tiny holes to fill with glue,I break out in hives when I think of trying that.
My other idea is to again paint and weather the PSP plates as desired.Then add a good coat of future or other gloss clear to seal it up.After the clear has cured completely,I was thinking of spraying the entire sheet with the sand color and immediately wiping the sheet with a cloth dampened with thinners( for whatever paint you use) while the paint is still wet.I would wrap the cloth around something flat so it only wipes the paint from the raised PSP sections and leaves the sand color in the recessed parts.The same thing could be done with a sludge wash which may be easier to work with and do in small sections as patience permits.After it's all done,hit it with a clear flat to tone it down.Maybe you could use something like a stretched scrub pad to stipple the surface with glue and sprinkle some sand or earth (depending on the desired base under the PSP) before the final clear flat as I'm sure the base medium would work it's way up through the plates in spots to be scattered on top.
Bear in mind that these are only ideas I've had and I have not tried either as of yet.Either should work the same for either sand or earth.Maybe I'll get brave and try to attempt mine soon.Good luck with it and I hope I was of some help to you.
Kevin