MORPHOLOGY, GENESIS, AND AGE OF PREHISTORIC ANTHROPOGENIC SOILS BENEATH METROPOLITAN PHOENIX, ARIZONA

At the turn of the 20^th century, Thomas Means conducted the first soil survey of the lower Salt River Valley, Arizona, and recognized fine-grained soils that he called Salt River adobe. Commonly found within the Salt River floodplain, these soils spatially correlated with the traces of prehistoric canals built by the Hohokam (AD 450–1450), and Means hypothesized that the soils were the result of prolonged irrigation sedimentation rather than floodplain deposition. The Salt River adobe is no longer recognized as an official soil type but it is clear that clay-enriched irragric anthrosols produced by Hohokam farming underlie many parts of metropolitan Phoenix. Few physical and chemical analyses have been conducted specifically on these soils, and questions remain as to what degree they are anthropogenic, how long they take to form, and what impacts they had on soil quality and Hohokam food production. Here we present a recent investigation of the Salt River adobe next to Sky Harbor Airport in Phoenix that involved field descriptions, micromorphological and microbotanical analyses, and ¹⁴C dating. We determined that at this location the soil experienced ~500 years of humification and irrigation sedimentation resulting in an ~70 cm thick, anthropogenic cumulic soil. We hypothesize that soil quality would have diminished through time at this location, leading to the need for increased fallow time and field rotation for maintaining food production. As a biophysical record of ancient human impact on the environment, more research is needed to define the full extent, age, and genesis of the Salt River adobe.