Soil organic matter effects on plant available and water soluble phosphorus

The degree of phosphorus saturation (DPS) has been shown to be a suitable initial indicator of P loss potential from agricultural soils to surface waters. In addition, several agronomic soil tests have been evaluated as environmental predictive tools. The objectives of this study were: (1) to evaluate the modified Morgan soil test, used on acid, high-aluminum soils of the northeastern United States, as a predictor of water-soluble P and (2) to determine the effect of soil organic matter (SOM) on the ability of both DPS and soil test P to predict water-soluble P. The soils were divided into three SOM classifications depending on their loss-on-ignition contents and analyzed for water-soluble P, modified Morgan soil test P, and oxalate-extractable P, Al, and Fe. The relationship between DPS and water-soluble P showed a change point at about 15% DPS, as did the relationship between DPS and modified Morgan soil test P. A DPS of 15% corresponded to about 14 mg kg−1, a threshold above which water-soluble P could be expected to increase more rapidly with additional P loading. The slopes of the regression lines of water-soluble P as a function of soil test P were 0.050, 0.036, and 0.021 (mg water-soluble P kg−1 soil/mg soil test P kg−1) for the low, medium, and high SOM classification groups, respectively. SOM level had a less significant effect on the relationship between DPS and water-soluble P. Higher levels of SOM were associated with higher levels of oxalate-extractable Fe and Al and, therefore, higher P sorption capacities and lower DPS values, resulting in less P in solution at all soil test levels.