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DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Predicting Phosphorus Desorption from Mid-Atlantic Coastal Plain Soils

^a USDA, ARS, ANRI, AMBL, B-163F Rm. 5, BARC-East, 10300 Baltimore Ave., Beltsville, MD 20705
^b Dep. of Plant and Soil Sciences, 152 Townsend Hall, University of Delaware, Newark, DE 19717

* Corresponding author (pvadas{at}anri.barc.usda.gov)

Pollution of surface waters by P from agricultural areas is a water quality issue in Delaware. The FHANTM 2.0 computer model can help identify areas with a high potential for P loss, but the model's representation of P desorption from soils to runoff waters needs re-evaluation. The equation, P_d = K P_o t W^ß, has been proposed to predict such P desorption, but equations originally proposed to predict values for the constants K, , and ß from the ratio of soil clay content/soil organic C content may not be accurate for Delaware soils. Therefore, we measured P desorption for 23 sandy Delaware soils for times of 5 to 180 min, water/soil ratios of 10 to 1000 L kg^-1, and three initial levels of soil desorbable P. Values for the constants K, , and ß were calculated and related to soil properties. We found that K, , and ß values were not well related to clay/OC, but were better related to the ratio of oxalate-extractable Fe/OC content () or the sum of oxalate extractable Fe and Al (ß and K). These results can be used to help refine the FHANTM 2.0 model in predicting P loss from agricultural areas in Delaware and similar landscapes in the Mid-Atlantic Coastal Plain.

Abbreviations: Al_ox, acid ammonium oxalate-extractable Al • AWS, amount of rainfall needed to saturate the topsoil layer in the FHANTM 2.0 model • B, extraction coefficient used in the FHANTM 2.0 model • (C_av)_p, quantity of P in the topsoil available for runoff used in the FHANTM 2.0 model • CPLAB, quantity of desorbable P in the topsoil used in the FHANTM 2.0 model • (C_w)_p, concentration of P in runoff used in the FHANTM 2.0 model • Fe_ox, acid ammonium oxalate-extractable Fe • FHANTM, Field Hydrologic and Nutrient Transport Model • OC, organic C • OM, organic matter • UDSTP, University of Delaware Soil Testing Program • K, empirical constant in soil P desorption equation • K_d, partitioning coefficient used in the FHANTM 2.0 model • P_d, amount of P desorbed from the soil • P_o, initial concentration of desorbable P in soil • POR, porosity used in the FHANTM 2.0 model • Pr, value for precipitation used in the FHANTM 2.0 model • Q, value for runoff used in the FHANTM 2.0 model • SSG, soil specific gravity used in the FHANTM 2.0 model • t, time of P desorption • TMDL, total maximum daily load • W, water/soil ratio during P desorption • , empirical constant in soil P desorption equation • ß, empirical constant in soil P desorption equation

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