At the Elkhorn Slough Visitor's Center at the head of Monterey Bay, California, is a diorama that shows a shorebird standing in a bucket of mud taken from the adjacent slough. After asking visitors to estimate how many and what kinds of organisms might live in this much mud, the diorama offers an estimate of 500 billion bacteria, [Page 265]500 million diatoms, 50,000 protozoa, 50,027 worms, 5,000 crustaceans, 39 clams—plus the bird. While we cannot vouch for the ballpark accuracy of the estimate, what we know about soil organisms and numbers suggests it may be low. But in pondering the diorama, several questions are raised. Does slough mud qualify as soil? If so, what is soil? How many organisms, and what kinds, live in soil? ...