Ancient anthropogenic soil beneath Phoenix, Arizona, USA.

• Anthropogenic soils produced by centuries of Hohokam (450–1450 CE) canal irrigation underlie the city of Phoenix. • Age and origin of a hypothesized buried anthropogenic soil tested with morphological, micromorphological, pollen, and chronometric analyses. • Results indicate that Indigenous irrigation agriculture produced a 70+ cm cumulic soil in ∼500 years in our study area. • Evidence suggests application of organic materials to help maintain soil productivity. • We hypothesize that farming ceased in our study area ∼1200 CE due to reduced soil quality. Over 1000 years of irrigation agriculture by the Hohokam (450–1450 CE) left an indelible mark on soils of the lower Salt River Valley in Arizona. Defining the nature, extent, and formation of these anthropogenic soils, named Salt River Adobe during the valley's first soil survey, is important for understanding human impacts to the environment and agricultural history of arid lands. Towards that objective, we describe an undisturbed, buried example of the Salt River Adobe from the Salt River floodplain and present evidence for its anthrogenesis based on archaeological context, age, physicochemical properties, micromorphology, and pollen content. We determined that the Salt River Adobe at this location represents approximately 500 years of irrigation sedimentation and pedogenesis resulting in a 70+ cm thick cumulic soil. Cessation of irrigation within the project area occurred in the 1200s CE at a time of peak Hohokam population. Our evidence suggests fairly continuous multi-century irrigation farming by the Hohokam at one location with soil productivity maintained by irrigation sedimentation and possible fertilization. We hypothesize that cessation of farming at this location during a time of high food-demand was due in part to gradually reduced soil permeability and tilth. More research is needed to better understand the geographic extent and genesis of Salt River Adobe and the role it played in Hohokam farming and cultural history. [ABSTRACT FROM AUTHOR]