A gradient in plant species diversity was investigated to determine how soil chemical properties limit the distribution of desert plants. Species-poor communities (lacking succulents) growing on basalt-derived soils were compared with richer communities on soils that developed on basalt or quartz diorite. Species richness and density were determined in three 100-m2quadrats at five sites. Soil samples were collected from A (0–10 cm) and B (10–30 cm) layers near the center of each quadrat. Leaves of species growing in or near quandrats were collected as indicator tissue. Elemental concentrations were measured in soil saturation extracts and leaf tissue digests. Surface soils were non-saline and had near-neutral pH. Soil saturation extracts were high in NaCl and adequate in Ca but low in N, P, B, K, Mg, S, and Fe. Leaves of indicator plants (Viguieraspp.) were high in K, Ca, and Na, medium in B, and low in Mg, P, and some micronutrients. Analysis of variance revealed that the Ca: Mg ratio in the soil solution differed significantly between sites and was lowest at species-rich sites. Simple regression revealed a moderate positive dependence of species richness on Mg in the soil solution and a negative dependence of species richness on soil solution Ca:Mg. But a marked dependence was found when species diversity (Shannon-WeinerH‘ Index) was regressed on Ca: Mg in the soil solution. These results indicate that the geographic ranges of desert species (and of succulents in particular) can be restricted by even moderate Ca in the rooting medium when Mg is critically low. Magnesium concentration in leaves of species studied in Baja California was among the lowest, and Ca: Mg ratios among the highest ever reported in desert plants.