Reducing P Export from Agricultural Soils in the Maumee River Basin to Lake Erie Using Gypsum Application.

Phosphorus (P) from agricultural and other sources has continued to contribute to water quality problems in Lake Erie. Research has shown that gypsum (calcium sulfate dihydrate  or CaSO₄^.2H₂O) is effective in reducing P loss from agricultural soils by forming insoluble calcium phosphate.  A demonstration project in the Maumee Basin of western Lake Erie is evaluating field-scale applications of gypsum for reducing P export in surface runoff and tile drainage water. The initial project began with 4 farms in 2012 and was expanded to 8 farms in 2013. On each farm, separate fields with similar hydrology and management histories, or separate sections within one large field, were either treated or not treated with gypsum at a rate of generally one ton per acre.  Tile drain water samples were collected from areas with or without gypsum and analyzed for P (total and orthophosphate).  There were 20 total sampling events (collection of samples on one farm on one date) from May 2012 through November 2013.  After the initial sampling in May 2012, dry conditions during the remainder of the growing season prevented additional sampling events until December 2012. Average reduction in orthophosphate for gypsum-treated areas was 39%, with median reduction of 48% and range from 0 to 82%. Orthophosphate concentrations (mg/L) in drain water ranged from 0.01 to 0.11 (mean = 0.04) in gypsum-treated areas and from <0.01 to 0.44 (mean = 0.10) in areas without gypsum. Recent results have shown that P reductions in tile drainage water persist at least 20 months after gypsum treatment.