Estimating soil phosphorus sorption saturation from Mehlich-3 data

Soil phosphorus sorption saturation (Plat) measures the degree to which soil phosphorus (P) sorption sites have been filled and has been found to be a good indicator of P availability to runoff and leachate. At present, analytical methods required to estimate P-sat are generally not offered by soil testing laboratories. This study evaluated the use of Mehlich-3 data in estimating P-sat in a wide range of soils. In acidic soils (pH = 4.1-5.9), P-sat estimated from Mehlich-3 P, iron (Fe), and aluminum (Al) was highly correlated with P-sat estimated from ammonium oxalate data (r = 0.94) as well as with a reference P-sat estimated from bicarbonate P and the Langmuir sorption maximum (r = 0.89). In alkaline soils (pH = 7.3-8.4), P-sat estimated with Mehlich-3 P and calcium (Ca) was highly correlated with the reference P-sat (r = 0.84), and the strength of that correlation improved only slightly by factoring in soil clay content (r = 0.86). Results indicate that P-sat may be effectively estimated from Mehlich-3 data across a wide range of soils. This study confirms that P-sat may be readily estimated by soil testing laboratories that routinely measure Mehlich-3 P, Al, Fe, and Ca.