Labile phosphorus pools in struvite-amended soils over 112 days of incubation

Optimal management of the slow-release fertilizer struvite (NH4MgPO4·6H2O) requires a greater understanding of its behaviour in differing soil types and its effect on phosphorus (P) pools extracted in agronomic and environmental soil tests. The objective of this study was to quantify temporal changes in labile soil P pools in contrasting soils amended with different rates (100 and 200 mg P kg-1) of granular struvite or monoammonium phosphate (MAP) and incubated for 112 d without plants. Contrary to expectation, water-extractable P (WEP) and Olsen-P concentrations were initially greater in soils amended with struvite rather than MAP at the high application rate. Grinding of soil samples prior to extraction may have increased the extractability of P in undissolved struvite granules. For both fertilizers, WEP declined over time and converged to similar concentrations by Day 112. Olsen-P concentrations in soils amended with the high rate of struvite declined below those for the same rate of MAP by Day 70. Olsen-P concentrations were lower in a neutral-pH sandy clay loam than in an alkaline clay soil, whereas WEP concentrations were greater in the sandy clay loam, but only at the high P application rate and in the initial days of incubation. These findings demonstrate the complexity of P dynamics in struvite-amended soils, emphasizing the need to clarify the roles of specific soil properties, soil sample preparation, and P extraction methods in determining the effects of struvite application on soil P pools.