Magnesium (Mg) is ubiquitous in soils and water, as derived from indigenous and anthropogenic sources. It has the potential to affect environmental risks associated with phosphorus (P) loss from manures, chemical fertilizers, etc. by inhibiting precipitation of stable, well-crystalline calcium phosphate minerals. Recent results of experiments with dairy manure extracts and well-defined chemical systems show that Mg can affect P stability not only by inhibiting calcium phosphate stabilization but also by preemptive formation of Mg-P or Ca-Mg-P phases. These phases would remain sparingly soluble in soils and hence serve as long-term P source. Energy-dispersive x-ray elemental spectra verified that Mg as well as Ca is associated with P in dairy manure. Also, there is x-ray diffraction evidence for the presence of Ca-Mg-P in dairy manure. Magnesium enables, rather than inhibits, Ca phosphate precipitation at high pH (>9) in the presence of high dissolved calcium carbonate, a phenomenon that can enable P recovery from wastewater. We propose that the enabling effect is the result of aqueous Mg carbonate formation which minimizes inhibitory effects of both Mg and carbonate.