Phosphorus Saturation Index (PSI)

Graph showing relationship between PSI and the amount of phosphorus filtered from water

The Phosphorus Saturation Index (PSI) is a method for evaluating soils for their capacity to retain phosphorus. It is a simple measurement of the amount of phosphorus in the soil compared to iron and aluminum. The premise is that phosphate ions (PO3-)—the biologically available form of phosphorus—are highly attracted to iron and aluminum. When new bonds form between these elements, phosphorus is no longer soluble in water or available for uptake by plants like algae.

This method has been used for years in agriculture to evaluate the risk of phosphorus loss from fields treated with manures and fertilizers. A high PSI value indicates that phosphorus loss may occur. In such circumstances, phosphorus can be immobilized, or retained, by amending the soil with additional iron and aluminum. This can be achieved by adding water treatment residuals to soil, which are spent iron and aluminum oxides used in the filtering of water for drinking.

While both these methods have been used successfully in agriculture, research is underway to examine how they might be applied to rain gardens and bioswales. Initial results indicate that the PSI is a good indicator of phosphorus retention in such systems.