Field-aged phosphate impacts on glyphosate and phosphorus sorption patterns in two prairie soils
This research measured field-aged phosphate concentrations in sandy clay-loam, clay-loam soils and utilized samples in batch equilibrium studies to quantify adsorption maximum of phosphate and glyphosate retention. Soil samples were collected in 2013 from two research sites that had received annual applications of mono ammonium phosphate at different rates from 2002 to 2009. The effect of fresh phosphate addition on glyphosate sorption in soil was also determined. Measured parameters were the glyphosate sorption distribution constant, Kd, as well as the Langmuir adsorption constants, b (phosphorus adsorption maximum), and k (affinity constant). Glyphosate Kd values significantly decreased with increasing phosphate level in soils, regardless of the background liquid (CaCl2 and KCl) used in the batch equilibrium experiment. Applications of potassium dihydrogen phosphate in the laboratory with glyphosate reduced the available sorption sites to retain glyphosate in soil. Field-aged phosphate did not significantly influence the adsorption maximum (b), but b was significantly higher with CaCl2 than KCl as background liquid. The affinity constant was significantly greater in soils with lower field-aged phosphate concentrations, suggesting that these soils had more readily-available sorption sites for phosphate than soils with higher phosphate concentrations.
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