A key factor in environmental fate of P in manure-impacted soils is P solubility. Determinants of P solubility in manure are not well understood. Objectives of this study were to evaluate effects of Ca sources and Ca:Mg ratios in diets of lactating dairy cows on (i) manure total concentrations of Ca, Mg and P and (ii) Ca, Mg, and P solubility in manures. Eight diets were fed to 24 randomly selected cows in three-21d periods. Calcium sources were CaCO3 for diets 1-4 and CaCl2 for diets 5-8; Ca:Mg ratios were 1.7, 2.9, 3.0, and 5.0 for diets 1-4 and 5-8. Manure samples were collected the last 10 d of each period; a composite sample was obtained per cow per period. Manure was analyzed for total and water soluble Ca, Mg and P. Calcium source in the diet significantly affected manure Ca concentrations. Increased Ca and Mg concentrations in the diet resulted in significantly higher concentrations of Ca and Mg in manure (P < 0.0001). No significant difference in manure P concentrations was found. Calcium concentration and source showed significant effect on cumulative P release from 10 successive water extractions (1:100 manure:water) as percent of total with higher Ca and CaCl2 diets resulting in lower P extracted. Increased Mg in the diet reduced P and Ca extractability from manures. Percent of Mg extracted was not affected by any variable. Reduced Ca in the diet increased Ca solubility in water. Dietary control of Ca and Mg can potentially reduce manure P solubility.