Phosphorus Removal from Agricultural Drainage Ditches Using Gypsum Filter Structures.

Phosphorus is an essential nutrient for crop production.  However, when excess phosphorus concentrations are present in aquatic ecosystems algal blooms and eutrophication can result. One potential way to reduce nutrient loads in agricultural drainage ditches is to install filter structures containing phosphorus sorbing materials (PSMs), such as gypsum to remove phosphorus directly from ditch flow. One of the projected advantages of such a system is that the PSMs can potentially be land applied to provide valuable 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 four 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, 7, 30, 60, and 120 days after saturation, and analyzed for total N and P, nitrate (NO₃-N), ammonium (NH₄-N), water extractable P (WEP), and Mehlich 3 P. The results of the incubation study will be discussed.