I really like Vallejo Earth Texture, you can water it down to your fit, its basically a paint/fine sand mixture. Apply with a modeling tool before or after priming depending on if you like the existing earth tone. While its wet you can sprinkle some small rocks/more sand, large pieces you should glue it down first. I use to do the glue and sand, but this pretty much put the 2 steps into 1 and its less fussy as before, as I had to worry about applying thin glue around the feet of the model.
AS for jungle base finishing, you can add some swamp/jungle turf, lichen, skulls, other appropriate bits.