The impact of mobile colloids on the transport of phosphorus in the subsurface environment is not well understood. We hypothesized that interactions between metals, organic matter, and P control the dynamics of mobile colloidal P species in excessively fertilized sandy soils. The effect of UV irradiation and additions of 32P, orthophosphate, Fe, Al, and NaF on the concentration of colloidal P was examined using gel filtration chromatography. In addition, molybdate unreactive P (MUP) was characterized using phosphomonoesterase assays. The high molecular mass reactive P (HMMRP) fraction did not react to orthophosphate additions, increased upon Al and Fe additions and decreased upon NaF addition and UV irradiation. These results support the hypothesis that HMMRP is present as organic matter-metal-orthophosphate complexes. The concentration of high molecular mass unreactive P (HMMUP) decreased upon UV irradiation. The MUP concentration slightly decreased upon incubation with phytase and acid phosphatase. These observations fitted well to the "protection" hypothesis, where hydrolyzable P bonds are protected from monoesterase attack through occlusion in colloidal material. Taken together, this study indicates the high potential for subsurface P loss by colloidal particles in soils excessively fertilized with animal manure.