Ned,
 
Two comments.  First, I'm currious about the potassium signiture in your
recent studies.   Did the structure or related activity involve burning of
organics (e.g. house fire or charcoal kiln)?  Or was there some other
activity which produced the K-salts?
Second, re: the slaughterhouse.  Often sawdust is used in great quantities to
soak-up the liquid wastes and provide traction on the floors.  If green or
fully saturated, and disposed of on site, localized anaerobic conditions in
the soil would be likely.  Increases in many reduced elements and compounds
would be likely including amonium if young or limited to atmospheric O.  The
hay bedding common to most barnyards (excluding prime stock like horses where
the bedding is rarely allowed to saturate) would initially foster similar
soil reduction, but would not be as extensive, nor as long lasting.
 
Doug