Characterizing the behaviour and efficacy of struvite fertilizer for organically managed crops in Manitoba soils

Abstract

Recycling phosphorus (P) from wastewater for use in fertilizers can contribute to an improved circular economy for P. Struvite recovered from municipal wastewater is a sparingly soluble fertilizer that has been recommended as a suitable P source for organic production, where soil P deficiencies can be particularly severe. Optimizing the use of struvite fertilizer in such systems requires greater understanding of its effects on crop performance, soil P dynamics, and soil organisms such as arbuscular mycorrhizal fungi (AMF). Field studies on an alkaline clay soil demonstrated increases in yield and P uptake for organic spring wheat (Triticum aestivum L.) and alfalfa (Medicago sativa L.)–grass forage, whereas flax (Linum usitatissimum L.) showed no response. Forage response to struvite increased in the second and third years after struvite application, demonstrating the potential of struvite as a multi-year P source. A soil incubation using the same alkaline clay and a neutral-pH sandy clay loam showed that P transformation processes were delayed in soil amended with struvite rather than monoammonium phosphate (MAP), and that P concentrations in labile P pools were also affected by soil type and application rate. However, water-extractable P was unexpectedly high after incubating struvite-amended soils only a few days, possibly due to sample grinding. A pot experiment on alfalfa in the same soils showed no differences between struvite and MAP on alfalfa performance or labile soil P pools. Plant P uptake increased over the unfertilized control only in the neutral-pH soil at a high P application rate. At this application rate, struvite showed a smaller inhibitory effect than MAP on root colonization with mycorrhizal arbuscular fungi. Overall, this research shows that struvite can be an effective P fertilizer, even in alkaline soils, but that struvite dissolution and transformation processes and the resulting effects on plants and soil organisms vary with application rate and soil type. Further research is needed to determine appropriate methods for assessing P availability in struvite-amended soils.