You should integrate large(r) fake stones (well, at least that's what I'd do), and set the density to 0.9 or perhaps even 2. Feed them (surface shader) a stack of 2-3 PFS with different grey-brown colors and scales, the 'in between PFS' with only the high color checked (perhaps even an orange lichen color, small scale, high contrast and roughness, color slider at -1.5 or less, no displacement, to produce small patches), the top one with both colors (greyish) checked, with displacement (experiment) to give the rocks some slightly rough form. You might of course merge some smaller stones in, but one rock layer would do wonders.
Good luck.
---Dune
