Alum [A12(SO4)314H20] is commonly used in the municipal water treatment process to destabilize colloids for subsequent flocculation and water clarification. Water treatment residuals (WTR) can classified as a waste material from these treatment plants. Concerns over land application of WTR are due to its postulated reduction of plant available P and potential plant AI toxicity with increasing WTR rates. Co-application of WTR with biosolids may benefit municipali-ties with biosolids inherently high in P concentrations and in terms of a cost savings by landfill avoidance. In a greenhouse study, we investigated the efficacy of co-application of WTR and biosolids to the native shortgrass prairie species blue grama (Bouteloua gracilis H.B.K. Lag) and western wheatgrass [Pascopyrum smithii (Rydb.) A. Love]. Our objectives were to quantify co-application effects on plant P and AI concentrations and uptake, biomass production, and WTR P adsorbing capacity. With blue grama, we observed a positive linear relationship between increasing WTR rate and yield and a ' negative linear relationship with increasing WTR rate and shoot P and AI concentration (P < 0.10). With western wheatgrass, increasing WTR rate produced a negative quadratic effect on shoot AI concentra-tion (P < 0.10). Some investigators have observed P deficiency symp-toms associated with WTR application; however, we did not. Our adsorption study indicated that co-mixing of the City of Fort Collins, CO, WTR and biosolids at ratios of 8:1 will adsorb all soluble biosolids P. Beyond this ratio the WTR could adsorb all biosolids available P and possibly some soil-borne P.