Gypsum amendment and exchangeable calcium and magnesium affecting phosphorus and nitrogen in runoff

Loss of N and P from soil to water is not only an agronomic but also an environmental and human health problem. The main objective of this study was to evaluate the effect of gypsum amendment and soil-exchangeable Ca and Mg on the concentration and loss of nutrients in runoff, sediment, and soil. The experiment was performed in the laboratory using a programmable rainfall simulator and erosion pans. The soil was a Miami silt loam, and treatments were (i) gypsum amendment, (ii) five target exchangeable Ca/Mg ratios, and (iii) a control. Nitrogen, P, and K were applied in all treatments based on soil test results and standard recommendations for corn (Zea mays L.). The fertilizers were mixed to a 2.5-cm depth before pre-wetting the soil. Gypsum was applied to the surface just before the rain. Two rainfall intensities (30 and 60 man h(-1)) were simulated, and infiltrated water and runoff samples were taken every 5 min over 2 h. Nitrate N, NH4-N, and P in runoff, sediment, and soil were determined. The results showed that gypsum, as compared with the control, significantly decreased the mass loss in runoff of dissolved reactive P (DRP), total P (TP), soluble NH4-N, and total N by 85, 60, 80, and 59%, respectively. The concentration of these constituents was also significantly decreased by 83, 52, 79, and 50%, respectively. Nitrate N concentration in runoff was not decreased by gypsum. Gypsum also affected sediment and soil water-extractable F. The only significant effect of exchangeable Ca/Mg ratios was observed in the sediment water-extractable P, where Ca-treated soil decreased by 50% the concentration of P compared with Mg-treated soil. The results of this study demonstrated the potential of gypsum to decrease transport of total N and total P in runoff and the importance of Ca in the soil solution to decrease P solubility.