Gypsum soils are unique substrates with nutritional imbalances, which occur in drylands worldwide. Nutritional imbalances limit plant life, but also favour plant diversification. Gypsum soils harbour rich endemic floras with highly specialized species. Gypsum specialists accumulate calcium, sulphur and magnesium in leaves, which are highly available in gypsum soils. However, it is still unknown why gypsum specialists have such foliar accumulation and whether it provides an ecological advantage. This article reviews the available literature on plant adaptation to gypsum soils, and concludes with a hypothesis on the role of this accumulation: gypsum specialists use sulphur to tolerate high calcium concentrations, and thus improve phosphorus nutrition, which is the most limited nutrient in gypsum soils. Gypsum soils are amongst the most widespread extreme substrates of the world, occurring in 112 countries. This type of hypercalcic substrate has a suite of extreme physical and chemical properties that make it stressful for plant establishment and growth. Extreme chemical properties include low plant-available nitrogen and phosphorus and high plant-available sulphur and calcium, which impose strong nutritional imbalances on plants. In spite of these edaphic barriers, gypsum soils harbour rich endemic floras that have evolved independently on five continents, with highly specialized species. Plants that only grow on gypsum are considered soil specialists, and they have a foliar elemental composition similar to the elemental availability of gypsum soils, with high calcium, sulphur and magnesium accumulation. However, the physiological and ecological role of the unique foliar elemental composition of gypsum specialists remains poorly understood, and it is unknown whether it provides an ecological advantage over other generalist species on gypsum soils. This article reviews available literature on the impact of gypsum soil features on plant life and the mechanisms underlying plant adaptation to gypsum environments. We conclude with a hypothesis on the potential role of the nutritional strategy underlying plant specialization to gypsum soils: Gypsum specialists primarily use SO42- as a counter anion to tolerate high Ca2+ concentrations in cells and avoid phosphorus depletion, which is one of the most limiting nutrients in gypsum soils.