Evaluating agronomic soil phosphorus tests for soils amended with struvite

Abstract

Struvite is an emerging, recycled phosphorus (P) fertilizer of low water solubility (<5%). As a first step towards wide-scale integration of struvite into agricultural systems, distinct interpretation of soil test phosphorus (STP) values for soils amended with struvite may be needed due to the persistence of struvite for months after its application (i.e., residual struvite). However, STP methods were developed for soils amended with highly water-soluble P fertilizers and may not necessarily translate to soils with residual struvite prior to soil testing for P recommendation. We evaluated the potential effects of STP method and edaphic properties (pH, clay content) on STP values for soils with residual struvite. To mimic residual struvite, struvite granules were added to a quartz control and to six soils encompassing a range of pH (4.3, 6.0, 8.1) representative of agricultural soils and with contrasting clay content. The mixtures were then extracted by common STP methods (Mehlich-3, Bray-1, Olsen), Resin, and Haney 3A-2. In the quartz control, dissolution of struvite granules in STP extraction solutions ranged from 59% in Resin to 10% in H3A-2. In soil treatments, apparent dissolution of struvite among STP methods was 19–401% higher for acidic soils with low versus high clay contents. Adsorption experiments confirmed that the disparity in the apparent dissolution of struvite in soils was caused by adsorption of dissolved P on clay minerals. Additionally, for acidic soils with high clay content, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy identified surface deposition of Al and Si on the struvite granule surface during STP extractions, which may have decreased struvite dissolution. Depending on STP method, residual struvite dissolution may overestimate STP concentrations by 20–3900%. Results demonstrate the need to account for the impacts of residual struvite on measured STP values. As a next step, quantifying in situ struvite dissolution rates across soil and cropping conditions is needed to evaluate the extent to which residual struvite may require adjustment of yield-based calibration of STP values.