Fig. 3. Spatial distribution of (a) percent clay, (c) oxalate-extractable aluminum (Al_ox), and (e) the phosphorus sorption index (PSI) at Rowel Branch (RoBr), and (b) percent clay, (d) Al_ox, and (f) PSI at Grindle Creek (GrCr). Contour maps were made by ordinary point kriging using spherical semivariogram models for each individual soil property.