From Open Source Ecology
For a nice hard smooth floor...of dirt - tamp it down with a heavy object, then rub it with something acidic, like citri-solve.
From The $50 and Up Underground Housing Book by Mike Oehler (link is to a 115 page pdf file) -
"Some folks in the southwest sweep linseed oil into their floors, wait a day or so, and repeat the process. Though still allowing "give" this sets the earth nearly like concrete; you can wipe up a dropped egg without getting any earth on the rag.
Others go to more elaborate and artistic extents: they mix up adobe, pour it six inches thick on the floor, smooth it down, wait for days or weeks for it to dry and crack into the mosaic of dried mud, mix up another adobe mixture and fill in the cracks, wait for it to dry, sweep in linseed oil and wait for it to dry and then repeat with linseed oil again. The resultant floor looks like tile, sets up like concrete yet still has give to the walker and will not poke through except when walked across by someone wearing spike high heel shoes or loggers calked boots. Even then, repair is as simple as mixing up a small adobe mixture and filling the holes. "