Phosphorus Removal From Agricultural Drainage Ditches Using Gypsum Filter Structures.

Agricultural drainage ditches can provide a direct connection between agricultural fields and surface waters. Certain drainage ditches have been shown to deliver high loads of phosphorus (P) to sensitive water bodies. One potential way to reduce nutrient loads in agricultural drainage ditches is to install filter structures containing P sorbing materials (PSMs) such as gypsum to remove P directly from ditch flow. One of the projected advantages of such a system would be the potential application of PSMs to agricultural fields to provide nutrients for crop production. The purpose of this study was to evaluate in the laboratory the feasibility of such a strategy. Gypsum was saturated at two levels on the mass basis of P, and applied to two soil types, a silt loam and a sandy loam. The solution was applied at both a high and low rate. The treated soils were incubated at 25° C, samples were collected 0, 1, 7, 28, 63, 91 and 119 days after saturation, and analyzed for total N and P, nitrate (NO₃-N), ammonium (NH₄-N), water extractable P (WEP), Mehlich-3 P, and CHN analysis. The results of the incubation study will be discussed.