Fig. 1 Langmuir adsorption isotherms for P sorption of the surface layers of selected pedons (a) Pedon 3 (b) Pedon 4 (c) Pedon 10 and (d) Pedon 13 representing the mountains, the upper footslopes, the lower footslopes and the alluvial terraces, respectively; Pa is the amount of P sorbed by unit weight of soil and C is the concentration of P remaining in solution