Phosphorus (P) is an essential macronutrient required for plant growth. In fertilizers used for soilless substrates, P is in the form of phosphate and is readily leached from containers leading to environmental contamination. The FeSO4 bridge method has been proposed to reduce P losses by forming insoluble iron-phosphate complexes that retain P within the substrate, but reduced bioavailability may limit plant growth. The aim of this research was to evaluate the FeSO4 bridge method in combination with inoculation of the substrate with phosphate-solubilizing bacteria (PSB) and siderophore-producing bacteria (SPB) to gradually release P for plant uptake. An experiment was conducted to evaluate the growth of marigolds ‘Durango Yellow’ (Tagetes patula) using a substrate composed of 80% peat and 20% perlite by volume, with substrate either amended or not amended with 3.0 kg·m-3 FeSO4·7H2O. After transplanting, plants received 100 mg·L-1 N from 15N-2.2P-12.5K-2.9Ca-1.2Mg Jack’s Professional water-soluble fertilizer as the P source for two weeks to promote the formation of iron-phosphate complexes. Additionally, two bacterial application strategies were evaluated: one starting from transplanting and another applied when P deficiency symptoms appear, with three weekly inoculations each. The bacterial treatments included: a control without bacterial inoculation; three PSB strains, Bacillus megaterium (C3F10), Pantoea rwandensis (C3A8), and Pseudomonas sp. (C6E7); one SPB strain Pseudomonas soli (C10A8); and a commercial inoculum of Bacillus velezensis (from the biostimulant LalRise Vita, Lallemand, Inc). Plants were grown under controlled greenhouse conditions, with weekly measurements of spectral variables and morphological parameters using the TraitFinder digital phenotyping system. Leachate was collected using the pour-through method and analyzed for P concentrations using ion chromatography. The results confirmed that the FeSO4 bridge method formed iron-phosphate complexes, reducing P leaching when amending the substrate with 3.0 kg·m-3 FeSO4·7H2O compared to non-amended substrate. Visual differences in plant growth were observed among treatments, with better outcomes when bacterial inoculation started at transplanting. In contrast, plants that received bacterial treatment after symptoms appeared could not recover, showing significantly reduced visual quality. Among the bacterial treatments, Pseudomonas soli (C10A8) promoted the highest biomass accumulation and spectral variables such as hue and green leaf index (GLI) indicated improved visual quality compared to the control (no bacteria) and other bacterial strains. These findings demonstrated that amending soilless substrates with FeSO4, combined with bacteria like Pseudomonas soli (C10A8), enhances P retention while promoting plant growth. This method offers growers a practical approach to reducing environmental impact while maintaining crop quality.
