Investigation of the effects of microbial fertilization containing phosphorus-solubilizing bacteria combined with reduced fertilization on the biological properties of calcareous soil and eggplant (Solanum melongena L.) growth in a greenhouse

The aim of this study was to investigate the effects of a powdered commercial microbial fertilizer containing Bacillus megaterium, Pantoea agglomerans, and Pseudomonas fluorescens on soil biological properties and plant development characteristics under greenhouse conditions with calcareous soils during eggplant (Solanum melongena L.) cultivation. Throughout the plant growth period, 70% of the phosphorus (P) demand required to achieve the target yield was supplied through fertigation, while the remaining 30% was provided via organomineral basal fertilization. In this context, organomineral phosphorus fertilization was reduced by 0%, 25%, 50%, and 100%. To monitor the effects of microbial fertilization under reduced P input, soil samples were collected at five key intervals (before seedling transplantation, and at 1, 3, 7 months after transplantation, as well as after harvest). In these soil samples, physical (texture), chemical (pH, EC, lime content, organic matter, total nitrogen, available phosphorus, exchangeable potassium, calcium, magnesium, sodium), and biological (urease, alkaline phosphatase, β-glucosidase enzyme activities, total bacterial count) analyses were conducted. Additionally, plant and fruit samples collected during the 7th-month sampling period were evaluated for pomological (fruit diameter, length, peduncle length) and physiological properties (soluble solids content in fruit, total chlorophyll in leaves and peduncles, and fruit color metrics). At the end of the experiment, phosphorus use efficiency and total yield were also determined. According to the findings, microbial fertilizer treatments had statistically significant positive effects on soil biological properties: urease activity was enhanced under the 100+ treatment (100% P + microbial), alkaline phosphatase under 100+, 0+, and 100, β-glucosidase under 0+, and total bacterial count under 100+. With respect to chemical properties, soil pH improved with 0+ and 75, EC increased with 50+, 100, and 100+, while organic matter, total nitrogen, and available phosphorus levels showed notable improvements particularly under the 100+ treatment. Regarding plant development parameters, fruit length responded positively to 100, peduncle length to 50, plant dry weight to 100+, soluble solids content (SSC) to 75, leaf chlorophyll content to 50 and 50+, peduncle chlorophyll to 100, phosphorus use efficiency to 100+, and total yield to 100. Overall, this study comprehensively demonstrates that microbial fertilizers, particularly when used in combination with full-dose phosphorus fertilization, can significantly enhance soil biological activity, improve nutrient availability, and positively influence plant growth and yield under calcareous soil conditions. The combination of 100% P with microbial inoculants yielded the most substantial increases in enzyme activities, bacterial population, and nutrient concentrations. Furthermore, certain reduced P treatments supplemented with microbial inputs also led to notable improvements in physiological and pomological traits. Notably, the highest phosphorus use efficiency and total yield were achieved with the 100+ treatment, highlighting the synergistic potential of integrated microbial and mineral fertilization strategies in sustainable eggplant production systems. This study demonstrates that, contrary to common belief, phosphorus-solubilizing microbial fertilizers are not adversely affected by chemical phosphorus fertilization; on the contrary, their combined application enhances crop yield. The findings indicate that the synergistic interaction between these two fertilization strategies improves the bioavailability of phosphorus to plants. Microorganisms facilitate the solubilization of phosphorus that would otherwise become immobilized in the soil, thereby increasing the effectiveness of chemical fertilizers and improving overall nutrient use efficiency. Consequently, the integrated use of microbial and chemical phosphorus fertilizers emerges as a rational and sustainable strategy for effective phosphorus management.