Field-scale evaluation of struvite phosphorus and nitrogen leaching relative to monoammonium phosphate

Struvite (MgNH₄PO₄·6H₂O) is a wastewater-derived phosphorus (P) fertilizer with potential to reduce P as well as nitrogen (N) losses due to its low water solubility. To test hypothesized lower P and N losses from struvite relative to monoammonium phosphate (MAP), field experiments with a randomized-complete block design were conducted in central (Urbana) Illinois on an Endoaquoll-Argiudoll complex and in southern (Ewing) Illinois on a Fragiudalf-Hapludalf complex. Fertilizer was broadcast applied in the fall prior to spring planting of soybean (Glycine max L.) at a maintenance rate of 29.5 kg P ha⁻¹ (Urbana) and 22.0 kg P ha⁻¹ (Ewing). In the spring, soil extractable N and Mehlich 3-P at 0- to 15-cm and 15- to 35-cm depths were determined, and leached N and P were estimated using fall-installed ion-exchangeable resin (IER) lysimeters. At Urbana, soil extractable nitrate-N was higher under MAP than struvite at 0- to 15-cm depth. At Ewing, soil Mehlich 3-P under struvite was lower than MAP at both depths. At Urbana, leached P was 10-fold lower, and leached N was twofold lower under struvite than MAP. Soybean yields were similar between MAP and struvite at Urbana (4.1–4.3 Mg ha⁻¹) and Ewing (3.2–3.5 Mg ha⁻¹), but at Ewing yields were 23% higher under struvite compared to the P-unfertilized control. Off-season yield-scaled P and N losses under struvite were lower than MAP by 51% at Urbana and by 10% at Ewing. Our results support the hypothesized potential of struvite to reduce nutrient losses while meeting crop P needs. Additionally, we identify disproportionally greater reductions in N leaching and yield-scaled N losses by substituting struvite for MAP in fall applications, indicating that struvite can offer greater relative benefits for N loss reduction than P loss reduction.