Effect of Methodology on the Interpretation of Degree of Phosphorus Saturation.

The revised Maryland Phosphorus Site Index, now the University of Maryland-Phosphorus Management Tool (UM-PMT), includes degree of P saturation (DPS) as a source factor. However, multiple methods for estimating DPS are found in the literature. Most rely on an estimate of soil P and the soil P sorption capacity based on aluminum (Al) or iron (Fe) and Al concentrations, along with an alpha value. Our objective was to determine the effect of DPS approach on UM-PMT final score. We collected 380 soil samples from fields across Maryland and determined Mehlich 3 (M3) and ammonium oxalate (ox) extractable P, Fe, and Al. We also conducted seven-point P sorption isotherms to derive alpha values using Langmuir P sorption maxima on a subset of 34 samples. Methods compared included (i) the ratio [P_ox]to [Fe_ox]+[Al_ox] using a Langmuir-derived alpha value; (ii) the ratio of [P_ox]to [Fe_ox]+[Al_ox] with alpha value set to 0.5; (iii) ratio of [P_M3] to [Fe_M3]+[Al_M3]; (iv) ratio of [P_M3] to [Al_M3]; and (v) linear regression that converts method iii to method i. Good correlation (R² >0.7, P<0.0001) was observed when comparing method v to methods ii, iii, or iv. A weaker correlation (R² = 0.49 - 0.53, P<0.0001) was observed between method i and all other methods. These results indicate various methods of calculating P saturation can impact the final score of the UM-PMT, which could have a significant economic impact on Maryland farmers or a significant environmental impact on Maryland water quality. The uncertainty surrounding these methods should be considered when including them in models or other decision-making tools.