Thursday, November 16, 2006 - 9:25 AM

Land Application of Gypsum: Agronomic and Environmental Implications.

Chad Penn, Oklahoma State University, Oklahoma State University/USDA-ARS, 367 Agricultural Hall, Stillwater, OK 74078, Ray Bryant, USDA ARS Bldg 3702, Curtin Road, University Park, PA 16802, Andrew Sharpley, USDA-ARS, Past. Sys. & Mgmt. Res., University Park, PA 16802-3702, and Arthur Allen, Univ of Maryland E. Shore, 30921 Martin Court/crop & Aquacultu, Princess Anne, MD 21853.

Application of gypsum (CaSO4) to soils has been shown to improve soil fertility and physical conditions, as well as reduce phosphorus (P) solubility in manures and high P soils.  However, there has been less research on the use of gypsum for reducing P concentrations in runoff or investigating any potential deleterious agronomic effects under field conditions.  The objectives of this study were to (i) determine if and for how long gypsum applications to soils can effectively reduce P concentrations in runoff from high P soils, and (ii) investigate the mechanism of magnesium (Mg) deficiency induced by gypsum application.  A field study was initiated in 2002 on an Othello loam, under a corn-soybean rotation located at the University of Maryland Eastern Shore Research Station.  Gypsum was applied at the rate of 22.4 Mg ha-1 to duplicated 1 acre plots with surface runoff samplers located at field edge for runoff collection and P analysis.  Duplicate control plots received no gypsum.  Although gypsum applications significantly reduced runoff P concentrations for all 4 years (from > 2 mg L-1 to < 1 mg dissolved P L-1), corn and soybeans showed visible Mg deficiency symptoms after 2 years.  Soil and tissues samples confirmed Mg deficiency on gypsum amended soils.  Soils were removed from the control plots for further laboratory investigation, including a column leaching study and determination of ion exchange selectivity coefficient (Ca-Mg).  Ion exchange experiments indicated that Ca exchange for Mg was thermodynamically favored, while the leaching study showed that increasing gypsum application rates resulted in a greater loss of the original Mg present in the soil prior to gypsum amendments.  Therefore, gypsum can be an effective material for reducing excessive P losses to surface waters, but crops may require additional Mg fertilization since high gypsum application rates can cause excessive Mg leaching.