The remains of Tea bags at the bottom of the jar, when added to crushed up old herbs or other scenic stuff, makes for an effective damp dirt by roadside. If too dark, tone down with other pastel dust, paler brown or even green for a mould or grass effect to blend in if under hedges etc. Looks good so far on my dio.
For easy mud, with more 'body' or mud or dirt, add a bit of fine grit or plaster, with PVA white glue. Gives a nice clumpy mud. Cheap and easy.
NB the instant 'wall repair' plaster in bags in UK is more bulky and gritty, a good base maker.
Wilkinson's exterior plaster is also of a dirty or coloured tone, mine is like original Italian mud, a grey-green, but that is a few years old.