Drought stress coupled with calcareous soil constraints severely limits seed quality, yield, and essential oil profiles of medicinal crops such as Trachyspermum copticum L. (ajwain). This two-year (2022-2023) split-plot field study in semi-arid Iran evaluated irrigation regimes (25%, 50%, 75% allowable depletion of available soil water) and eight fertilizer treatments, testing the hypothesis that integrating 50% sulfur with Acidithiobacillus spp. (sulfur-oxidizing bacteria) and Funneliformis mosseae (arbuscular mycorrhizal fungus) termed the triple combination, would synergistically outperform individual or dual amendments by enhancing nutrient mobilization, osmotic adjustment, and terpene biosynthesis under drought. Under severe drought (75% depletion), the triple treatment most effectively preserved membrane stability (22-23% lower electrolyte leakage), boosted germination vigor (44-46% higher index), sustained seed yield at 147.8 +/- 4.2 g m-2 (18-20% above control), and enriched essential oil content by 20% (1.82 +/- 0.05% w/w) with thymol elevated to 48.4 +/- 0.6%, reflecting prioritized secondary metabolism. These improvements, consistent across years, stemmed from rhizosphere acidification, extended hyphal networks, and enhanced sulfate-mediated antioxidant and osmotic protection - mechanisms that generated true synergism beyond additive effects. This work provides the first multi-year field validation of reduced-sulfur-rate sulfur + SOB + AMF synergy in a shallow-rooted medicinal Apiaceae under concurrent calcareous and drought stresses, offering a low-input, climate-resilient strategy that simultaneously reduces chemical fertilizers by 50% and secures both agronomic productivity and pharmacological quality in arid medicinal farming systems.