The beneficial reuse of water treatment plant residuals (WTR) and biosolids via land co-application is of concern since the WTR is postulated to greatly reduce plant phosphorus (P) availability and, along with biosolids, possibly provide an additional source of trace metals to soil. Potential plant Al toxicity with increasing WTR rates, because of the Al content of WTR [Al2(SO4)3·14H2O], has also been speculated. 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 [Agropyron smithii (Rydb.) A. Love]. Co-application rates were a factorial combination of 0, 2.5, 5, 7.5, and 10 g kg of WTR and 0, 2.5, 5, 7.5, and 10 g kg of biosolids. Increasing WTR rate, averaged over biosolids rate, resulted in a decrease (p