Ann,
 
There have been a number of site investigations in southern Maryland that
have utilized soil chemical analysis as an integral part of the project.  The
first was Bob Keeler's study (1978) of soil chemical distributions at the St.
John's site in St. Mary's City.  This was followed by several others carried
out by the Historic St. Mary's City folks (Henry Miller, Garry Stone, Sandy
Morrison) over the years.  Their results have not been extensively published
(so what else is new), but I did write a review article on the subject that
used some of their findings, combined with the results of my work at the
King's Reach Site, that appeared in the Journal of Northeast Historical
Archaeology some time back (citation below).  All of these sites date to the
17th century and were located in agricultural fields.  The patterns were
quite distinctive, and our interpretation is that the recent chemicals spread
by farmers probably simply raised the readings across the board.
 Unfortunately, we never generated a "control" sample by testing a site that
had not been treated.
 
I hope that this is helpful.
DJP
 
Pogue, Dennis J.  Anthrosols and the Analysis of Archaeological Sites in a
Plowed Context: The King's Reach Site.  Northeast Historical Archaeology Vol.
17:1-15 (1988).