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PHOSPHORUS MANAGEMENT

Effectiveness of Recovered Magnesium Phosphates as Fertilizers in Neutral and Slightly Alkaline Soils

^a Dep. of Environmental Earth System Sci., Stanford Univ., Stanford, CA 94305
^b Inst. for Livestock and the Environment, Colorado State Univ., Fort Collins, CO 80523-1170
^c USDA-ARS NWISRL, 3793 North 3600 East, Kimberly, ID, 83341
^d Dep. of Biological and Agricultural Engineering, Louisiana State Univ. AgCenter, Baton Rouge, LA 70803-4505

^* Corresponding author (Jessica.Davis{at}colostate.edu).

Magnesium phosphates such as struvite (MgNH₄PO₄ · 6H₂O) can be recovered from municipal, industrial, and agricultural wastewaters. However, limited information is available on the beneficial reuse of these recovered products; research has focused on low pH soils. This study determined whether recovered struvite and dittmarite (MgNH₄PO₄ · H₂O) were effective P fertilizers in neutral to slightly alkaline soils. In addition to commercially available triple superphosphate (TSP) and certified organic rock phosphate (RP), recovered struvite, dittmarite, and a heterogeneous recovered phosphate were evaluated in a laboratory dissolution study and as fertilizers for spring wheat (Triticum aestivum L.) in a greenhouse study. Struvite and dittmarite were much more soluble than RP, but less soluble than TSP. Laboratory dissolution kinetics were fast, with most materials nearing equilibrium within 7 to 14 d. At a soil pH of 6.5, both dittmarite and struvite increased the average plant P concentration over the control. Struvite and dittmarite performance was similar to TSP. There were no significant differences in plant dry matter (DM) production or total P uptake at pH 6.5. In the limed soil (pH 7.6), many treatments had plant P concentrations significantly lower than the control, but most fertilizers increased DM production over the control; all fertilizers generally performed similarly to one another. These findings support previous work showing recovered Mg phosphates to be effective in acidic soils, and provide evidence that they are also effective in slightly alkaline soils. Recovered Mg phosphates could become a useful alternative for P fertilization in arid and semiarid environments.

Abbreviations: DM, dry matter • RP, rock phosphate • TSP, triple superphosphate • WWTP, wastewater treatment plant

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Received for publication May 2, 2008.