Effects of Phosphorus, Sulfur, and Sulfur-Oxidizing Bacterial Inoculants on Yield and Yield Components of Wheat

Background and Objectives
The presence of calcareous parent materials, combined with low precipitation, has led to the development and formation of calcareous and alkaline soils across most regions of Iran. Evaluation of the nutrient status in these soils indicates that despite the abundance of certain essential nutrients (such as phosphorus, iron, and zinc), their soluble and bioavailable forms remain below the threshold required for optimal plant growth and development. Consequently, nutrient deficiency constitutes a major limiting factor for crop production in these soil types. The application of sulfur and organic matter alters soil chemical properties, particularly by reducing soil pH, which subsequently enhances the availability and uptake of nutrients required for wheat (Triticum aestivum). Under such conditions, utilizing elemental sulfur combined with an inoculum of sulfur-oxidizing bacteria can serve as an effective strategy to improve plant nutrition and increase yield. In calcareous soils, phosphorus is rapidly fixed, reducing its availability to plant roots. Therefore, any factor capable of decreasing the localized rhizosphere pH can significantly enhance phosphorus use efficiency.
Materials and Methods
This study was conducted to evaluate the effects of sulfur application, combined with a sulfur-oxidizing bacterial inoculant, and varying rates of triple superphosphate (TSP) fertilizer on the yield and yield components of wheat (cv. Alvand) in Osku county, East Azerbaijan province. Wheat seeds were sown on November 14, 2021. Each experimental plot covered an area of 30 m² (10 m in length) with a row spacing of 25 cm. Within each plot, the four central beds were seeded, while the outermost beds (the first and sixth) were left unplanted to serve as guard rows. The experiment was laid out as a factorial based on a randomized complete blocks design (RCBD) with three replications. The factors consisted of four levels of sulfur combined with Thiobacillus bacteria [S0: 0 (control), S1: 500 kg ha⁻¹ sulfur + 10 kg ha⁻¹ Thiobacillus, S2: 1000 kg ha⁻¹ sulfur + 20 kg ha⁻¹ Thiobacillus, and S3: 2000 kg ha⁻¹ sulfur + 40 kg ha⁻¹ Thiobacillus] and three levels of TSP fertilizer [P0: 0% (control), P1: 65%, and P2: 100% of the phosphorus requirement based on soil test results].
Results
The results indicated that effects of sulfur and phosphorus on wheat production parameters (grain yield, biological yield, number of spikes per plant, 1000-grain weight, number of tillers per plant, and number of grains per spike) were significant. The main effects of both sulfur and phosphorus significantly influenced several production traits; specifically, phosphorus notably affected grain and biological yields, while sulfur significantly impacted grain yield and certain yield components. The sulfur and phosphorus interaction (S × P) was significant for some traits and non-significant for others. Overall, increasing sulfur application rate enhanced biological yield and the number of spikes, with the highest biological yield observed in the treatment of S3 (2000 kg ha⁻¹ sulfur + 40 kg ha⁻¹ Thiobacillus). However, for 1000-grain weight and tiller number, the optimal response was predominantly recorded at intermediate levels S1 (500 kg ha⁻¹ sulfur + 10 kg ha⁻¹ Thiobacillus) or S2 (1000 kg ha⁻¹ sulfur + 20 kg ha⁻¹ Thiobacillus). Similarly, phosphorus application generally improved grain yield and its associated components, although the maximum effect was frequently observed at the P1 rate (65% phosphorus requirement) rather than the full requirement level (P2).
Conclusion
Based on the findings of this study, it can be concluded that the combined application of sulfur with sulfur-oxidizing bacteria, alongside balanced phosphorus fertilization, presents an effective strategy for improving wheat growth and yield in nutrient-limited calcareous soils. By reducing the localized rhizosphere pH, these treatments increased the solubility and availability of nutrients. Consequently, the enhanced uptake of phosphorus, iron, zinc, and other micronutrients provided highly favorable conditions for plant growth. Furthermore, the observed increases in grain yield, straw yield, and yield components demonstrated that improving the crop's nutritional status directly enhanced productivity. However, the results also indicated that excessive sulfur application does not necessarily yield further agronomic benefits; in several instances, intermediate treatment levels produced more favorable outcomes compared to the highest application rates. This highlights that the efficacy of this approach heavily relies on determining the optimal sulfur rate, the appropriate type and dosage of the microbial inoculant, as well as the initial soil conditions. Consequently, the implementation of this strategy must be targeted and guided by soil testing and integrated nutrient management. This will not only maximize yield but also prevent the excessive application of fertilizers and mitigate unnecessary economic costs.