Soil pH shapes rhizosphere microbial structure and diversity, influencing nutrient cycling, plant growth, and ecosystem health. However, the effects of pH on the microbiome in greenhouse-grown ornamental plants in peat-based soilless substrates are less well understood. This study examined the impact of substrate pH and plant species on rhizosphere bacterial communities to see how the interaction of these factors influenced microbial diversity. A two-factor experiment with substrate pH (4.5, 5.5, 6.2, and 7) and plant species (geranium, marigold, petunia, and tomato) was conducted in a greenhouse with six replicates per treatment (n=6) in a random complete block design. Substrate-only controls were included at each pH level to evaluate plant species influence on the bacterial community. The peat-based substrate pH was adjusted with dolomitic limestone. Plants were fertilized at each irrigation with 100 µg·g-1 N from 15-5-15-Ca-Mg fertilizer. After eight weeks of growth, the ornamental plants were fully flowering, and the tomato plants were beginning to bud. At that time, plant morphology and vegetative indices were evaluated by 3D image analysis, vegetive tissue and substrate leachate were evaluated for nutrient content, and rhizosphere samples were collected to evaluate bacterial composition and diversity by 16S amplicon sequencing. Plant species differentially modified the substrate pH from the starting levels. Geranium and marigold acidified the substrate, tomato tended to increase substrate pH, and petunia maintained pH close to the initial values. Shannon diversity indices indicated that bacterial diversity varied across the pH and plant species treatments. At pH 4.5, geranium, marigold, and tomato plants reduced the diversity relative to the no plant control. Geranium and tomato plants also reduced diversity at pH 5.5, while petunia plants at 5.5 exhibited the highest diversity across all plant species (p=0.0159). Beta diversity analysis identified pH as the dominant factor explaining 53.7% in Bray-Curtis dissimilarity and 68.5% in the weighted UniFrac distance metrics (both p
