Substrate pH and Fertilizer Rate Differentially Modulate Petunia × hybrida Responses to Biostimulants

Biostimulants can promote healthy plant growth under reduced fertilization and abiotic stress. This study investigated how the efficacy of three commercial biostimulants was influenced by initial substrate pH and fertilizer rate. Biostimulants included Micromate (humic and fulvic acids), Cease (Bacillus subtilis QST 713), and Lalrise Vita (Bacillus velezensis). Petunia × hybrida (petunia) ‘Picobella Blue’ was grown in a peat-based soilless substrate with an initial pH of 5.5, 6.2, or 7.0 and fertilized with either 50 mg·L−1 N (low rate) or 150 mg·L−1 N (recommended rate). Growth and plant health were assessed at flowering using digital phenotyping. The effect of biostimulants on plant growth depended on initial substrate pH and fertilization rate, whereas the vegetation indices were influenced only by fertilization rate. Lalrise Vita increased plant size across all substrate pH levels and fertilizer rates, while improving canopy health indices across fertilization rates. Micromate resulted in the greatest increases in growth, flowering, and canopy health across all pH and fertilizer treatments, with especially strong responses under recommended fertility. Our findings demonstrate that biostimulant efficacy in petunia ‘Picobella Blue’ is context-dependent, varying with product, substrate pH, and fertility rate, demonstrating the need for species- and environment-specific evaluations before widespread greenhouse adoption.