METHODS

• Duplicate soil cores, 45-mm (1¾”) in diameter, were taken to a depth of 1 meter in Fall 2002 and Spring 2003 from five cropping systems at the WICST plots. Cores were stored at 4°C until processing.
• Soil cores were sectioned at 1-cm intervals to 40-cm depth and at 5­cm intervals for the remainder. Samples were then air-dried and ground for analysis.
• Bray P was extracted using dilute hydrofluoric acid (Bray and Kurtz, 1945; Kuo, 1996).
• Phosphorus sorption isotherm data was obtained using a modification of the method of Graetz and Nair, 2000 (Avila-Segura et al., 2004) and fit to a Langmuir from using nonlinear parameter optimization.
• Phosphorus in extracts and digests was determined by the ascorbic acid method using microplate methodology (Avila-Segura et al., 2004).