Dairy manure applications to soils not only elevate phosphorus (P) concentrations, but also result in build-up of manure-derived components such as Mg, Ca, Si and dissolved organic carbon (DOC).  Despite high pH and abundant Ca, stable forms of Ca and P such as apatite were not detected and high levels of P continue to be released from manure-amended soils. We evaluated the inhibitory effects of Mg, Si, and manure-derived DOC on Ca-P crystallization in the presence and absence of solids (clay-sized fractions) obtained from manure-amended soils. Solutions containing Mg, Si or DOC were prepared in a medium containing average concentrations of other major chemical species found in leachates of manure-amended soils. These solutions, including a control, were incubated with and without solids for 20 weeks. Median equilibrium concentrations of Ca and P after incubation were significantly less in solutions without solids, with the exception of the DOC-solution. Control and Si solutions had lower median P concentrations in the presence of solids than did the Mg-solution. Presence and absence of solids did not affect behavior of P in the DOC-solution; however, Ca concentrations declined, possibly due to organic complexation. Formation of hydroxyapatite in both control and Si solutions and the more soluble brushite in the Mg-solution was confirmed by x-ray diffraction. Apatite formation can be inhibited by Mg and/or DOC in dairy manure-amended soils, maintaining high P release. Mg-P in manure-amended soils would maintain P solubility, and Mg in soil solution would inhibit stable forms of Ca-P. Therefore, it is important to consider Mg in addition to Ca in explaining the fate of P in manure-amended soils.