Publication Details (including relevant citation information): Zhang, Wei, Faulkner, Joshua W., Giri, Shree K., Geohring, Larry D., Steenhuis, Tammo S., 2009, 174 (10)
Abstract: The phosphorus (P) sorption isotherm experiment is a widely used tool in environmental applications for assessing soil's vulnerability to P loss to runoff or drainage. The sorbed legacy P (S0) (i.e., the P retained in soils from previous P applications) participates in sorption processes but cannot readily be determined in a sorption experiment. Thus, it is important to accurately estimate S0 for P-enriched soils (e.g., the soils that heavily receive fertilizer, manure, farm wastewater, or sewage sludge). Two curve-fitting procedures (i.e., one-step method and two-step method) with Langmuir models have been used to estimate S0 and other sorption parameters, including the P sorption maxima (Smax), the bonding energy constant (k), and the zero-sorption equilibrium concentration (EPC0). This study evaluated these two methods on 16 samples of Langford, Volusia, and Mardin channery silt loam soils at surface (0-8 cm) and subsurface (61-91 cm) in New York. The results indicate that the two methods agreed well in estimating P sorption maxima, and the estimates of k were close. The S0 estimates by the two methods had a good agreement for surface soils but a poor agreement for subsurface soils, which may be of little concern because of small S0 of subsurface soils. Although the one-step method yielded greater EPC0 estimates, the EPC0 estimates by the two methods had an excellent linear correlation for P-enriched surface soils, suggesting that both methods could work equally if only the relative magnitudes of EPC0 among soils are needed. Overall, both methods are acceptable to fit the Langmuir isotherms.