Evaluation of struvite from source-separated urine as a phosphate fertilizer.
The potential shortage of phosphorus (P) fertilizer is a threat to food security and closing the nutrient loop through recycling human excreta, especially urine, has been considered, so as to mitigate this crisis. Struvite (magnesium, ammonium phosphate), a material derived from human urine, is a product which is gaining credence with regards to using urine as a P amendment since more than 90% of P in urine can be captured during struvite production. A study to evaluate the potential of struvite as a P amendment in three contrasting soils was conducted. The soils used were an A horizon of Inanda (Ia), A horizon Sepane (Se) and an E horizon of Cartref (Cf). Phosphate adsorption properties of the soils were studied and the Freundlich model used to derive sorption parameters. From these studies, Pmax was related to the Kf parameter of the Freundlich equation. Two sets of incubation studies were then conducted. The first ran for 122 days and the second for 22 days to examine in closer detail the early stages of dissolution of the struvite as the major P release occurred during this time period of the incubation. A pot experiment was conducted in a controlled environment so as to determine the effect of P released from struvite on maize growth. The Ia, with high content of iron and aluminum oxides, displayed high sorption and affinity for P, whereas soil texture was a principal factor in the sorption properties of the Se (clayey) and Cf (sandy). The Kf decreased in the order Ia > Se > Cf and external P requirements decreased in the order Se > Ia > Cf. In the incubation studies solution P content increased with an increase in application rate of struvite. Struvite dissolution and P release varied between the different soils and the dissolution was found to be related to the P adsorption maximum of each individual soil and soil pH. The magnesium content also increased with time. In the glasshouse study, drymatter yield after six weeks growth was improved by the addition of struvite. There were no benefits achieved by using more than the recommended application rates for each soil. Struvite was as effective as conventional single superphosphate in the Ia and Cf, while superphosphate outperformed struvite on the Se. The findings of this study suggest that struvite has the potential to release P in an available form although its effectiveness and capability to release P could depend on soil pH, exchangeable acidity and initial P levels. Further research needs to focus on the effect of pH on struvite dissolution, the effect of struvite on soil pH, as well as comparison of nutrient release patterns between struvite and rock phosphate.
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