Forced precipitation of struvite (magnesium ammonium phosphate hexahydrate) in wastewater treatment has recently received increased attention as a method of phosphorus (P) recycling. While both struvite and phosphate rock can be used as a P fertilizer on acid soils, phosphate rock is ineffective in calcareous soils. Magnesium (Mg) phosphates such as struvite, on the other hand, could be a more useful fertilizer in alkaline soils, especially in organic production where conventional P fertilizers are not an option. A new organic treatment method, based on an existing process using a cone-shaped fluidized-bed reactor, was developed and field-tested using dairy wastewater. The amorphous or semi-crystalline product of this treatment process, as well as other recovered Mg phosphates including crystalline struvite and dittmarite (magnesium ammonium phosphate hydrate) were evaluated in greenhouse trials. The recovered Mg phosphates were tested against a control group, commercial phosphate rock fertilizer, and commercial triple superphosphate to compare the various Mg phosphates' effectiveness as a P fertilizer at acidic and alkaline soil pH. If plants show an increased or comparable response to Mg phosphates as compared to other fertilizers, recovered phosphates could have potential as an organic P fertilizer in calcareous soils.