What do you guys do for your stalls to keep the amonia smell at bay? Ours are built on dirt, no concrete or mats underneath.

We use lime at least 2 times a year. Strip the stall then lightly coat the floor with it and cover it with sawdust. 

the stalls at my parents place (where my horses are curretnly stalled) are dirt floors also. We've used lime successfully and that helps quite a bit but you can also buy Sweet PDZ (I get it from Co-OP) and it works really well also...

I put the link to Sweet PDZ down below if you want to read on it :)

http://www.sweetpdz.com/