How can I make molded fake dirt? I'm planning to sell small pots and as part of my display I'd like to "plant" fake plants in some of them. I'd like it to look as real as possible, so my goal is to have the plant "growing" out of a solid block of what looks like soil. I'd like to be able to grab the plant and lift it out along with the attached soil bottom then plunk the whole thing into a new pot as I sell them. 

I'm not sure exactly how to make the dirt bulb for the plant. I was thinking of molding it by pouring something granular (granulated + dyed cork maybe?) and glue or resin into a sacrificial Tupperware of similar size to my pots, sticking the plant in and letting it dry. The only thing I'm worried about is making sure the whole things stays in shape without crumbling once it's out of the Tupperware. Any suggestions on fake dirt materials and/or glues to use?
Why not use real dirt?
You can buy (or find) potting soil quite easily, which is relatively 'clean'.

An alternative is coffee grounds. To make it as dark as regular soil, you can add a little black ink to it and mix it with water.

In both cases it's best to let it dry first, then mix it with the resin or (maybe slightly watered down) glue inside a Tupperware container or a paper cup, which is both easier to cut away and less wasteful.

The resin or glue has to be waterproof (in case the plant accidentally gets watered), transparent (to let the (fake) dirt steal the show), hard curing (to keep everything properly in place), and preferably matte drying (since glossy dirt will look unrealistically wet or muddy).
Many resins and glues can be used for this - even wood glue, for example, will work fine. Visit your nearest hardware store, and look around or ask someone.