Conclusions

• Phosphorus desorption from suspended soil particles is relatively rapid, with the majority occurring in the first few minutes of soil-solution contact. As described previously by others, net desorption kinetics follow logarithmic relationships with contact time.

• As known, increasing ionic strength and divalent cations decrease P desorption. Additional studies are needed to separate inorganic-ion dissolution from colloidal and/or particulate desorption interferences. Initial irrigation water quality and up-stream effects on down-stream water quality parameters will affect subsequent P sorption/desorption.

• Several soil P tests estimate DRP concentrations at near equilibrium, as well as initial concentrations for a range of solution characteristics. Some are well established agronomic tests.

• There were excellent relationships between dilution, time and soil test P concentrations. This suggests that P loading rates could be estimated for given suspended sediment and DRP concentrations in the runoff water.

• Additional studies are needed to integrate desorption equations with transport and/or transfer processes in surface irrigated furrows/fields to estimate soluble P losses to receiving water bodies.

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