Paul,
I don't know if this will help, but Robert D'Aiggle did his thesis on soils chemical analyzes of the Texas region expecifically SE Texas. I believe that it included phosphate analysis. You can get his thesis through interlibrary loan from the University of Houston. I believe that he never did publish his thesis.
Jorge
Jorge Garcia-Herreros, RPA
Principal Investigator
Archaeologist
BHE Environmental, Inc.
15311 W. Vantage Parkway, Suite 307
Houston, Texas 77032
Phone: 281.219.1200
Fax: 281.219.1211
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