Phosphorus Mobility in Gypsum-Amended Soils in Relation to Soil Type and Timing of P Fertilizer Application

Gypsum (CaSO4 center dot 2H(2)O) amendment can reduce phosphorus (P) export from agricultural fields and protect water quality. Past studies have examined the impact of gypsum application rate, but limited research has been conducted to assess the effect of the time interval between gypsum application and P fertilization on treatment effectiveness and P mobility. To address this question, a mesocosm-scale experiment was conducted using 36 soil columns packed with either Brookston clay-loam (total phosphorus, TP: 0.46 g kg(-1)) or Fincastle silt-loam soil (TP: 4.63 g kg(-1)). For each soil type, 9 columns were treated with gypsum (5 Mg ha(-1)) and an equal number served as untreated controls. At different time intervals (2, 30, and 58 days after gypsum application), one-third of the columns received P fertilizer (34 kg P ha(-1)). Rainwater was added to generate leachate that was collected over a 3-week period and analyzed for pH, sulfate (SO4-2), dissolved reactive P (DRP), and TP. At the end of each leaching period, soil columns were sliced (2-5 cm layers), and the amount of water-extractable P (WEP) and Olsen-P was quantified. The gypsum treatment had generally no effect on Olsen-P, but it consistently resulted in lower WEP level (1.8-fold) compared to controls. Likewise, during the later stage of the experiment (> 30 days), the gypsum treatment resulted in DRP concentration reduction (28-52%) in leachate from the Fincastle soil columns (but the effect was marginal in the Brookston columns likely due to limited distribution of surface applied gypsum in this fine-textured soil). These results indicated that the effect of gypsum on DRP leaching varies with the timing of P application. Depending on soil type, an interval of 1-2 months (shorter interval for coarse-textured soils; longer interval if soils are fine-textured) should be adopted before applying P fertilizer in order to optimize the effectiveness of the gypsum treatment.