If something is nearly dust, like dry pigment or fine turf.
For small areas brush on some wood-glue and sprinkle the material, then use paintbrush or dropper to soak the material with the Glue-Water mixture.
Just like you do with gravel.
For large areas use Spray-Adhesive to provide sticky surface for material to stay, and sprinkle the material, then use squirt bottle to mist the Glue-Water mixture to soak the area.
Also had experience with (PREVAL sprayer bottle) used to wash/ink large pieces of terrain with darker tones using acrylic and also apply watered down glue that way too.
Watch this one, turned-out very well. and He usually soaks everything. Before gluing a misting of isopropyl alcohol is used
https://www.youtube.com/watch?v=HEG3d0cMuv4&t=1s