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Assessment of the potential for soil carbon sequestration based on soil type, land use, and climate scenarios is crucial for determining which agricultural regions can be used to help mitigate increasing atmospheric CO2 concentrations. In semi-arid and Mediterranean-type environments, soil organic carbon (SOC) storage capacity is rarely achieved under dryland agricultural systems. We aimed to assess both actual (measured) and attainable (modelled) SOC stock values for the dryland agricultural production zone of Western Australia. We measured actual SOC storage (0–0.3 m) and known constraints to plant growth for a range of soils types (3–27% clay) and land uses (continuous cropping, mixed cropping, annual and perennial pastures) on the Albany sand plain in Western Australia (n = 261 sites), spanning a rainfall gradient of 421–747 mm. Average actual SOC stocks for land use–soil type combinations ranged from 33 to 128 t C/ha (0–0.3 m). Up to 89% of the variability in actual SOC stock was explained by soil depth, rainfall, land use, and soil type. The scenarios modelled with Roth-C predicted that attainable SOC values of 59–140 t C/ha (0–0.3 m) could be achieved within 100 years. This indicated an additional storage capacity of 5–45% (7–27 t C/ha) depending on the specific land use–soil type combination. However, actual SOC in the surface 0–0.1 m was 95 to >100% of modelled attainable SOC values, suggesting this soil depth was ‘saturated’. Our findings highlight that additional SOC storage capacity in this region is limited to the subsoil below 0.1 m. This has implications for management strategies to increase SOC sequestration in dryland agricultural systems, as current practices tend to concentrate organic matter near the soil surface.