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Low precipitation can limit plant distributions. Soil variation might interact with precipitation gradients to define species borders. Analyzing the eastern species border of the California annual Clarkia xantiana ssp. xantiana, we assessed the following: (1) the geography of plant water status, precipitation, and soil; (2) soil control of plant water status; and (3) water status control of plant performance. Plant water potential declined toward the border in consecutive years. Precipitation declined in parallel and was lower in the year of lower water potential. Many border soils are derived from metasedimentary rock, while igneous rock dominates the species range to the west and unoccupied areas to the east. Compared with igneous soils, metasedimentary soils were found to have greater penetration resistance and finer texture, and they exhibited greater water storage capacity but also stronger restrictions on water release. Within populations, plant water status was inferior on shallow slopes with hard, fine soils. Among populations, plant size and fruit production increased with water potential, though seedling-to-adult mortality was independent. Low precipitation and an abrupt change in bedrock likely contribute to the species border of C. xantiana ssp. xantiana. Understanding adaptation and distribution shifts under climate change requires a consideration of both climate and soils.