Anthropogenic Soil Formation during a Millennium of Prehistoric Irrigation Agriculture in the Gila River Valley, Arizona.

Agriculture has profoundly altered soils world-wide over thousands of years, both through deliberate management, and unintentionally. Ancient agricultural soils are long-term sources of data on anthropogenic soil change and processes. One kind of agricultural soil transformation is the development of irragric soils, which result from prolonged deposition and accumulation of fine sediments from irrigation water. Ancient irragric soils centuries to millennia old occur in several world regions, especially in arid environments of Asia and the Americas. Sediment deposition associated with irrigation agriculture greatly influences soil properties and agricultural productivity. We present evidence for an ancient irragric anthrosol in the American Southwest, along the Snaketown Canal System in the middle Gila River Valley, Arizona. This soil formed during a millennium of irrigation (ca. 450 to 1450 A.D.) by prehistoric Hohokam farmers, who built the most extensive canal irrigation systems in the pre-Hispanic Americas north of Peru. The irragric soil consists of a mantle of silty-to-loamy textures with minimal pedogenic development overlying a natural argillic horizon on a Pleistocene stream terrace. A soil mapped independently by the USDA-Natural Resources Conservation Service with these horizons corresponds closely with the canal-field system. Soil within the canal-field system tends to be lower in salt (EC), sodium (SAR), and pH compared with external soils. This suggests that irragric processes improved soil for crop production through long-term leaching and additions of fresh sediments with the irrigation water. Anthropogenic sedimentation from irrigation has had a long-lasting impact on the sedimentary record and soils in this arid environment. The Snaketown irrigated soil falls within the wide range of ancient irragric soils in other global regions for most characteristics such as age, duration, extent, sedimentation rate, thickness, texture, and salinity. Knowledge about ancient and traditional irrigation systems and soils can contribute to developing and maintaining viable and sustainable irrigation systems.