Exposed soil surfaces are soon invaded by algae, fungi, bacteria and bryophytes.  If lush plants or leaf liter don't accumulate, a crusty community of microscopic (or nearly so) life develops after some time.  In arid regions these biological soil crust are of great ecological importance.  See the web sites below:

http://www.soilcrust.org/

http://helios.bto.ed.ac.uk/bto/microbes/cyano.htm#crest