dirt and water.

....and white glue if you intend for it to stick to your model or dio...#:-)

but iff you use water and dirt will is stick together?

You may try to mix white glue into that water, but ... there must be less cumbersome ways to imitate mud.
You won't believe this, but real mud probably won't look the right colour in your dio. That is because colors in full size are not the same as on scale, and you will want to put in some shading too. So next thing you know you'll want to paint that mud.
Cement works nice (for couch modelers who aren't ordered around the house to perform some real life 1/1 scale modeling to improve the interior : mix cement with fine sand and water). The result will look muddy like mud, but it'll be hard and you can paint it.
Who tried this before ?

Jan

I have also heard of modelers using cocoa in thier mix to give it a darker color and change the texture a bit..

I shall say it again... Baby powder! White glue is fine, but ultimately it settles to a smooth finish representative of almost watery, fluid mud, and that's not bad, depending on what type of mud finish you want; but if you want clumpy stuff that you want to simulate as fresh trodden earth, say on tracks, you can start with talcum powder mixed with paint to get the primary texture and build from there... As soon as I can take a decent photo to post I will show my results...