ZINC SORPTION BY B-HORIZON SOILS AS A FUNCTION OF PH

Better information is needed to predict how soil acidity affects exchangeability of heavy metals in soils dominated by Fe and Mn oxides with pH-dependent charge. Our laboratory study sought to separate exchangeable and nonexchangeable Zn sorbed by four B horizon soil materials with varied mineralogy and sesquioxide contents. A Myersville silt loam (fine-loamy, mixed, mesic Ultic Hapludalf), a Jackland sandy loam (fine, mixed, mesic Aquic Hapludalf), a Christiana sandy loam (clayey, kaolinitic, mesic Typic Paleudult), and an Evesboro loamy sand (mesic, coated Typic Quartzipsamment) were compared for the exchangeability of Zn as a function of pH. Surface charge was measured across the pH range of 3.5 to 7.6, a 175-mu-mol ZnCl2/kg soil treatment was applied, and sorbed Zn was exchanged with K. Cation-exchange capacity (CEC) was positively correlated with nonexchangeable but not with exchangeable Zn forms. All four soils retained nearly the same amount of Zn, even though CEC ranged from < 10 to > 80 mmol5/kg. An abrupt transition from exchangeable to nonexchangeable Zn occurred in the pH range 5.6 to 5.9 of the Myersville, Evesboro, and Christiana soils. Differences in oxide mineralogy and proton buffering intensity appeared more important than CEC as controls on the exchangeability of Zn in these soils. This result is germane to predicting retention of heavy metals in sewage-sludge-amended agricultural soils.