Field Evaluation of the Texas Phosphorus Index.

The Texas Phosphorus Index was evaluated in the field using rainfall simulations and edge of field studies.  Rainfall simulations were conducted on 90 sites across Texas in the dairy, poultry and beef cattle areas.  Data collected from the simulations included runoff weight every minute, runoff samples every 5 minutes and a composite after 30 minutes of runoff and soil samples from the 0-5, 5-15 and 0-15 cm depths.  Analyses of the runoff samples included pH, EC, P, K, nitrate-N, Ca, Mg, Na, Fe, Mn, Zn, Cu and B.  Soil sample analyses included pH, EC, nitrate-N,  Mehlich 3 extractable P, K, Ca, Mg, Na, and S.  The dairy and beef cattle areas were very similar in soil test P concentrations versus amounts of runoff soluble P.  The addition of extractable Mg into the PI increased the predictability of potential for P runoff from 20% to 35%.  The poultry areas had higher concentrations of soluble runoff P vs extractable P than the cattle sites.  The addition of drainage classes increased the predictability of potential P runoff from 21% to 35%.  Based upon our data, the cattle and poultry areas need to be separated in the PI.