We assessed the phosphate-buffering capacity of soils from the slope of an adsorption or desorption isotherm relating P quantity to P intensity. Although adsorption isotherms are readily assessed through solution P changes from P additions with a single soil, P desorption isotherms are not as conventionally determined. We conducted this research to compare P adsorption and desorption isotherms using various laboratory procedures with anion exchange resins. Resin desorptions were assessed on an Ascalon sandy loam and on an Apishapa silty clay loam. Desorption procedures involved (1) keeping the amount of resin constant while varying the equilibrating time, (2) keeping time and resin constant while varying soil quantity (surface P), and (3) keeping time constant and varying the amount of resin added to the soil.
The results show that adsorption and desorption curves did not coincide under the experimental conditions. Phosphate was less soluble when subjected to desorption treatments. Slope values varied ninefold. Because the slope is generally used as a measure of the P diffusive characteristics, some consistent and standardized way to measure this parameter is needed.
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