Soil Organic Matter Effects on Phosphorus Sorption: A Path Analysis

While P sorption in mineral soils has been extensively studied, P sorption behavior in organic‐rich soils is less known. This study was conducted to determine the relationships between Langmuir P sorption maxima ( S max ) and selected physicochemical properties of soils, with particular emphasis on organic matter (OM) content. The S max values were determined for 72 soil samples from the North Carolina Coastal Plain, along with pH, clay and OM contents, oxalate‐extractable P (P ox ), Al (Al ox ), and Fe (Fe ox ), and Mehlich 3 extractable P (P M3 ), Al (Al M3 ), and Fe (Fe M3 ). Path analysis was used to examine direct and indirect effects of soil properties on S max In the oxalate path analysis, the direct effects of clay, Al ox , and Fe ox on S max were significant in the order Al ox > clay > Fe ox ( P < 0.05). The S max was highly influenced by the indirect effect of Al ox and Fe ox through OM content. A two‐piece segmented linear relationship existed between S max and OM and the regression slope in soils with OM ≤ 49 g kg −1 was 10‐fold greater than that for soils with OM > 49 g kg −1 This finding suggested that noncrystalline or organically bound Al and Fe in the soils with OM > 49 g kg −1 is less effective for P sorption than in the soils with lower OM content. In the Mehlich 3 path analysis, the direct effects of clay, OM, and Al M3 on S max were significant in the order Al M3 > OM > clay ( P < 0.05) while the direct effect of Fe M3 on S max was not significant. Oxalate may be better suited than Mehlich 3 as an extractant for predicting P sorption capacity in the Coastal Plain soils.