Soil variability is a key driver of plant diversity and endemism, yet the ecological roles of specific soil types remain poorly understood. In the Khorassan-Kopet Dagh floristic province of northeastern Iran and southern Turkmenistan, the coexistence of gypsum, calcareous, and serpentine soils provides a unique setting to explore these relationships. We carried out extensive field surveys and soil analyses across nine habitats to refine the classification of edaphic endemic plants and reassess their conservation status. Using elemental analyses and standard soil tests, we characterized chemical properties such as carbonates, organic matter, gypsum, pH, and conductivity. Our results documented 71 plant taxa from 30 families, including 35 regional endemics and six Iranian endemics. Based on soil affinity, species were grouped as gypsophiles (3%), gypsovags (20%), obligate calcicoles (25%), and facultative calcicoles (52%). Several species required reclassification from gypsum to calcareous specialists. Hemicryptophytes (45%) and chamaephytes (28%) were the dominant groups, especially among calcicoles, with the highest species richness found at elevations between 1,500 and 2,300 m. Updated distribution data revealed that facultative calcicoles are largely secure, while obligates remain vulnerable. This study offers the first integrated account of edaphic endemism in the region, providing essential insights for biodiversity monitoring and conservation under rising anthropogenic and climatic pressures.

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