Nitrogen and phosphorus leaching as affected by gypsum amendment and exchangeable calcium and magnesium

The loss of N and P by leaching is an important issue, especially on agricultural fi elds with subsurface tile drainage. The objective of this study was to evaluate how gypsum amendment and soil exchangeable Ca 2+ and Mg 2+ could affect the movement of P, NH 4-N, and NO 3-N in infi ltrated water and soil. A column experiment was performed using a Miami silt loam soil, and the treatments were (i) control, (ii) gypsum applied at the surface, (iii) gypsum mixed into the 2.5-cm depth, and (iv) alteration of fi ve different target exchangeable Ca/ Mg ratios. A clear Plexiglas cylinder was fi lled with a 15-cm layer of soil; N, P, and K were applied in solution at the surface after the soil had been wetted and drained. Deionized water at a fl ow of 0.5 mL min -1 was applied and eight leachate fractions, totaling about fi ve pore volumes, were collected. Gypsum (5000 kg ha -1) applied at the surface and mixed into the 2.5-cm depth signifi - cantly decreased P and increased NH 4-N concentration but had no signifi cant effect on NO 3-N leaching. Exchangeable Ca/Mg ratio treatments did not affect soluble nutrient losses; however, leaching of particulate P was signifi cantly less in the Ca-treated soil than the Mg-treated soil. The overall practical conclusion of this study is that to control P transport, it is necessary to add gypsum even with a high soil exchangeable Ca/Mg ratio. The application of gypsum to the soil could be recommended as a best management practice to avoid water pollution by P; on the other hand, this could cause environmental problems by increasing the NH 4 soil mobility. © Soil Science Society of America.