ABSTRACT Water treatment residuals (WTR) are a waste product from municipal water treatment facilities. Biosolids, another by-product of waste water treatment, are inherently high in P. Both products can be applied to land as an alternative to landfill disposal. When applied separately, WTR can cause a reduction in plant available P while biosolids can cause an over-abundance of P. This study examined both the long term effects of a single co-application and the short-term impacts of a repeated co-application of WTR-biosolids on soil phosphorus dynamics. In 1991, 15 m x 15 m test plots were co-applied with a single rate of 10 Mg ha^-1 biosolids and 5,10, or 21 Mg ha^-1 WTR. In 2001 the plots were split with one half receiving a second co-application. Soil samples were taken in 2003 and 2004. Soils were analyzed for organic and inorganic P using a P fractionation method. Results from the P fractionation for both years showed that P is more commonly sorbed to Fe and Ca, contradicting the hypothesis that we would see the highest amounts of P in the Al-bound fraction. PSI on all soils was < 1, indicating that the soils are not overloaded with P; contrary, these soils can act as a P sink. Co-application of biosolids and WTR appears to decrease the amount of available P in soils both in the long and short term. Phosphorus movement within the environment should be minimized with co-application.