Bill and Douglas,
 
thanks for the replies - this really was just an idle question.  I've been
wondering about this for quite a number of years, but have never pursued it.
 Clearly, it depends on the context, but I was curious to see if anyone had
done any controlled studies in fertilized agricultural fields.  If the level
of fertilization had been constant over the entire site, then I assume the
chemical signatures of human activity would maintain their relationships and
still be readable.  But, what happens if only portions of the site had been
under fertilization?  What about a field that had been spread with manure for
the past 50 years?