Ceanothus otayensis McMinn (Rhamnaceae) was previously known only from metavolcanic-derived soils of the northern Peninsular Ranges—predominantly the San Ysidro Mountains—in San Diego County, California, and adjacent Baja California, Mexico. Recently, a new population of C. otayensis was discovered on sedimentary soils at Marine Corps Air Station Miramar, 25 km northwest of the next nearest known population. Sedimentary deposits at the new locality are thought to produce unusual soils. It is possible that the disjunct distribution of C. otayensis is a response to soil conditions, a phenomenon frequently seen in other members of Ceanothus, for instance on serpentine. The present study uses soil chemistry data for seven populations and subpopulations of C. otayensis (metavolcanic: n  =  5; sedimentary: n  =  2), as well as 22 populations of closely related Ceanothus, to determine whether soils of C. otayensis are chemically distinct from those of closely related Ceanothus, and answer the following question: are sedimentary-derived soils at the new locality chemically similar to metavolcanic-derived soils that support all other known populations of the species? Soils of C. otayensis proved to be chemically distinct from soils of closely related Ceanothus, with significantly lower levels of nitrate, sulfur, and conductivity. Sedimentary and metavolcanic soils of C. otayensis proved to be chemically indistinguishable from one another (P < 0.05), with low levels of all assayed nutrients other than Ca, suggesting a chemical similarity among the soils of C. otayensis that may help explain its disjunct distribution. Population size estimates indicate that the new disjunct locality at Marine Corps Air Station Miramar supports about 75 adult individuals.