Peat-Based Organomineral Fertilizers Inoculated with Bacillus spp. Improve Lettuce Growth and Nutrient Accumulation Under Contrasting Growing Conditions

This study evaluated the effects of peat-based organomineral fertilizers with different compositions and Bacillus spp. inoculation on the growth and nutrient accumulation of loose-leaf lettuce grown under summer and winter conditions. Two independent greenhouse experiments were conducted using a randomized complete block design with eight treatments: no basal fertilized control (T1); conventional mineral fertilization (T2); peat-based organomineral fertilizers containing 50% (T3), 40% (T4), or 30% peat (T5); and the corresponding formulations supplemented with Bacillus subtilis, Bacillus megaterium, and Bacillus aryabhattai (T6–T8). All fertilized treatments were standardized to supply the same P rate. Multivariate analyses revealed a strong effect of fertilization strategy on plant growth and nutritional status. In both seasons, fertilized treatments significantly outperformed the control, while organomineral fertilizers performed similarly to or better than conventional mineral fertilization. The greatest shoot fresh mass and nutrient accumulation were observed in formulations containing lower peat proportions and higher mineral nutrient density, particularly when combined with Bacillus spp. inoculation. In the summer experiment, the 40% peat formulation supplemented with Bacillus spp. (T7) produced the highest shoot fresh mass (197.57 g plant−1), whereas in the winter experiment the highest value was obtained with the 30% peat formulation supplemented with Bacillus spp. (T8; 157.86 g plant−1). These treatments also exhibited greater accumulation of macronutrients and micronutrients, particularly N, P, K, Fe, Mn, and Zn. The results indicate that the performance of peat-based organomineral fertilizers was influenced by the balance between the organic matrix and mineral fraction, as well as by seasonal growing conditions. In addition, Bacillus spp. inoculation was associated with improved performance of formulations with greater mineral nutrient density but did not compensate for less favorable fertilizer compositions. Under the conditions evaluated, peat-based organomineral fertilizers containing lower peat proportions and supplemented with Bacillus spp. performed similarly to or better than conventional mineral fertilization and promoted greater lettuce growth and nutrient accumulation than the non-fertilized control. These findings are limited to a single lettuce cultivar grown in pots under greenhouse conditions across two seasonal experiments conducted at one location.