Effects of sheep wool fertilizer application combined with plant growth-promoting rhizobacteria applications on some growth parameters, nutrient content of maize plants and some biological properties of the soil

The combined application of microbial and organic fertilizers holds strategic importance for developing sustainable and balanced agricultural approaches, enhancing soil fertility, and optimizing plant nutrition. To this end, a comprehensive study was conducted to investigate the synergistic effects of combinations of a specific organic fertilizer, such as sheep's wool, with various plant growth-promoting rhizobacteria (PGPR) strains on the growth of grain maize (Zea mays L.) and the soil's biological properties. The study examined the effects of increasing doses of sheep's wool fertilizer combined with five different PGPR strains on the growth parameters, nutrient element content of grain maize, and the biological properties of the soil. Four different doses of sheep's wool fertilizer (0, 30, 60, and 90 kg da⁻¹) and five different PGPR strains (Bacillus subtilis, Bacillus mojavensis, Bacillus megaterium, Pseudomonas fluorescens, and Pseudomonas putida) were applied to a clay-loam textured soil, in which maize plants were subsequently cultivated. The most favorable results were observed in combinations where maize seeds were inoculated with PGPR and 60 kg da⁻¹ of sheep's wool fertilizer was added to the soil. It was determined that Bacillus mojavensis enhanced root development, whereas Bacillus subtilis significantly increased plant height and leaf chlorophyll content (SPAD). Conversely, the 90 kg da⁻¹ fertilizer dose not only failed to increase yield but was also found to cause antagonistic effects on the uptake of certain nutrient elements. While a fertilizer-bacteria synergy was effective in the uptake of macroelements (P, K, Mg), bacterial mobilization was identified as playing a critical role in the uptake of microelements (Fe, Mn, Zn, Cu, B). The combined use of sheep's wool fertilizer and PGPR inoculations at the 60 kg da⁻¹ application rate was found to offer a sustainable and balanced approach for soil fertility and plant nutrition. PGPR applications in conjunction with sheep's wool fertilizer were found to have statistically significant effects on certain growth parameters, nutrient element content of the maize plant, and specific biological properties of the soil. Evaluations revealed that Bacillus species exhibited a greater biostimulant effect compared to Pseudomonas species on the plant's yield components, with the exception of stem diameter. Based on the data obtained, the optimal application dose of sheep's wool fertilizer for plant development was identified as 60 kg da⁻¹. The results demonstrated that even under conditions where the sheep's wool fertilizer exhibited a certain salinity potential (despite its organic origin), the applied PGPR strains were able to survive in the soil medium and establish an effective symbiotic relationship with the plant. Consequently, the collective data from the study comprehensively established that rhizobacteria of Bacillus and Pseudomonas origin, when applied in combination with sheep's wool fertilizer, play a supportive role in plant growth, nutrient uptake, and yield parameters, notwithstanding potential stress factors associated with salinity. These findings illustrate the strategic importance of microbial and organic fertilizer interactions in agricultural production and demonstrate that productivity can be enhanced while avoiding excessive fertilization.