Measurement of phosphorus desorption from a spodic horizon using two different desorption methods and pH control

Abstract

Phosphorus (P) desorption is a key process in understanding the P dynamics in the subsoil of poorly drained Spodosols of the lower coastal plain of the United States. Phosphorus desorption is the major mechanism which defines the labile pool of spodic horizon inorganic P. The objective of this study was to compare methods for determining P desorption isotherms. A sequential extraction method was compared to a dilution method, and the need for pH control within each method was evaluated. Results showed that the sequential extraction method released more P than the dilution method and that pH control during either method did not significantly alter the results. The estimated partition coefficients (K_d) from the sequential method were significantly lower than the dilution method, especially at the lower solution P concentrations. At these low soil solution P concentrations, the variability in estimated K_d values would have a significant influence on the uptake predicted by mechanistic computer models.

Keywords:

• Phosphorus
• Desorption
• Variable pH
• pH control
• Sequential extraction
• Dilution method

Acknowledgments

This research was funded by the National Science Foundation (Grant No. BSR-9019788). Technical and logistical assistance by Mary McLeod is greatly appreciated. This paper is Florida Exp. Station Journal Series No. R-08109.