As long as you don't mess with the dirt too much, and do water changes whenever you rearrange stuff, you can actually have dirt without a cap. I've done that in a couple pico tanks. Another option is just leaving a fairly thin layer of gravel on the dirt. After a couple water changes, most of the particles that would be clouding the tank should be gone, and if not, a few more water changes wouldn't hurt. You do have to be gentle when adding new water, and all of this is a lot of work, but it's possible to not need a cap.
That being said, there's the possibility of removing everything, and redoing the tank starting with new substrate. It's much more possible for smaller tanks than larger ones.