Response of onions to phosphorus, pH-modifying fertilisers, and biostimulants: implications for productivity and soil P availability

Optimising phosphorus (P) fertiliser formulations and application rate is crucial for efficient P use in onion (Allium cepa L.). Biostimulants and pH-modifying fertilisers have been proposed to enhance soil P availability, remain poorly quantified. A greenhouse pot experiment with spring onion was conducted to evaluate the effects of (i) P fertiliser types (monoammonium phosphate (MAP), triple superphosphate (TSP), and ammonium polyphosphate (APP)) at 0–40 kg P.ha⁻¹, (ii) three pH-modifying fertilisers (ammonium sulphate, elemental sulphur, calcium nitrate), and (iii) a commercial biostimulant (IPE) on soil nutrients, pH, physiological response, biomass and yield. APP was most effective at increasing plant growth and soil P content. Biomass yield response curve steadily increased with increasing rates of APP and MAP, without reaching a plateau. P uptake and soil P increased with rising P rates across P fertilisers, while phosphorus use efficiency declined. Soil pH was unaltered by P fertiliser types, rates, and pH-modifying fertilisers. The biostimulant had no significant impact on plant or soil parameters. These results indicate that fertiliser formulation strongly influences P availability, onion growth and P uptake, while pH-modifying fertilisers and biostimulant application did not show significant effect, underscoring the need for further assessment before integration into onion P-management strategies.