A better understanding of the effectiveness of placed phosphorus fertilisation with struvite for silage maize : A pot experiment

Struvite is emerging as a circular, slow-release phosphorus (P) fertiliser. However, its effectiveness for crops and its impact on soil P tests still remain largely unclear. We analysed the effectiveness of struvite as a row fertiliser in a pot experiment with maize in a sandy soil and a loamy soil, both exhibiting a low agronomic soil P status. Maize was seeded in both soils, amended with either granular struvite or water-soluble di-ammonium phosphate (DAP) as row fertilisers. A positive control received an excess of sodium phosphate, mixed homogeneously through both soils; a negative control did not receive additional P. We measured the agronomic maize performance at the end of the vegetative growth stage (V10) and assessed the suitability of two established agronomic soil P tests (i.e. P-CaCl2 and P-AL) to predict plant-available P in the soils amended with struvite. Furthermore, we performed an additional batch experiment to better understand the dissolution dynamics of struvite during a prolonged period of extraction of soil in 0.01 M CaCl2. Placed struvite application led to a lower performance of maize in comparison to DAP in terms of biomass production as well as P uptake (53 and 71 % lower, respectively). Yet the agronomic performance of struvite in general was higher than the performance of the negative control, confirming the potential of struvite as an emerging P fertiliser. Surprisingly, both soils fertilised with struvite showed a significantly higher soil P test than any other P treatment in our pot experiment. This was an artefact due to the dissolution of residual struvite granules during the soil extraction procedures. These results call for a reconsideration of how to interpret P-CaCl2 and P-AL as a basis for P fertiliser recommendations for soils receiving struvite as a P fertiliser.