I think I may have mis-read your post.
Are you using the parented fields to ALSO effect the softbody ground?

If so, don't

Just use the parented fields to effect the sand particles, and use separate fields, or even Blend Shapes to effect the ground

You can use my file (attached) as a reference, and if you notice, I DID NOT do the ground deformation, but the particle effect should be doable with a little more tweaking

I've included the movie in the zip as well

To see this, you need DIVX 5.0.3 or later: