Fig. 5. Relationships between soil pH (1:1 soil to water ratio) and water-extractable Ca (top) and Fe (bottom) (mg kg^–1 dry soil) for the Portneuf soil plots, highlighting the Portneuf topsoil plots that appeared elevated in the relationship between Olsen P and filterable reactive P in surface runoff (Fig. 4). Water-extractable elements were determined by extraction with deionized water for 1 h in a 1:10 soil to solution ratio.