Confined animal feeding dairy operations have generated excess amounts of manure, creating a need to identify alternative uses for this plant nutrient source. The objectives in this study were to (i) study the effect of composted dairy manure on Windthorst soil (fine, mixed, active, thermic Udic Paleustalfs), (ii) evaluate two soil testing methods for measuring P when composted dairy manure is applied, and (iii) determine tall wheatgrass [Thinopyrum ponticum (Podp.) Barkworth & Dewey 'Jose'] yield response to six rates of composted dairy manure and two rates of inorganic N fertilizer. A randomized complete block design experiment arranged in a split-plot with four replications was initiated in 2001. Main plots received a single application of composted dairy manure ai raies of 0, 11.2, 22.4, 44.8, 89.6, and 179.2 Mg ha-1, which were incorporated before planting tall wheatgrass at the rate of 17 kg ha-1. Subplots received annual split applications of inorganic N at 224 or 336 kg ha-1. Composted dairy manure averaged across the 2002-2003 and 2003-2004 growing seasons increased soil organic matter (OM) 54%, pH 55%, infiltration rate 550%, P 480%, and K 84% in this soil. The improved soil properties increased dry matter (DM) yields each growing season (2002-2003 and 2003-2004) up to 96 and 58%, respectively. Tall wheatgrass had similar crude protein (CP) (158-231 g kg-1), DM yields (3858-9536 kg ha -1), P concentrations (1.4-2.8 g P kg-1), and P removal rates (5.4-26.7 kg ha-1) compared to other cool-season perennial grasses. American Society of Agronomy.