Maize and soybean response to phosphorus fertilization with blends of struvite and monoammonium phosphate

Aims: Struvite (MgNH₄PO₄·6H₂O), a low water solubility (<3%) mineral that is increasingly recovered from wastewater treatment plants, has potential to be used as a slow release ammonium phosphate fertilizer, especially when blended with highly water soluble phosphorus (P) fertilizers such as monoammonium phosphate (MAP). Methods: Maize and soybean were fertilized using a gradient of struvite substitution for MAP, entailing five struvite: MAP blends in a factorial combination with struvite granule size (1.5, 3.0 mm diameter) and fertilizer placement (incorporation, banding). Crop biomass, and P and N uptake (total, concentration) were used to evaluate crop response, and post-harvest soil Mehlich-3 P was measured to assess soluble P loss risk. Results: Maize biomass response was similar using up to 50% struvite and similar in soybean using up to 25% struvite. Total P uptake by maize was similar across 0–75% struvite blends, but significantly lower for 100% struvite. Maize apparent fertilizer P uptake and apparent fertilizer P uptake efficiency was greatest for 100% MAP. Despite differences in biomass, soybean apparent fertilizer P uptake and apparent P use efficiency were similar across struvite blends. Soybean P uptake was significantly greater when fertilized with 100% struvite than with 25 and 50% struvite. Inverse correlation of plant P and N concentrations with biomass indicated a biomass dilution effect. Residual soil Mehlich-3 P decreased with increasing struvite substitution of MAP. Conclusions: Struvite:MAP blends (25–50% struvite) appear to lower soluble P loss risk compared to MAP without restricting early season (vegetative) growth of maize and soybean, and this can differ by crop species.