Anthropogenic Effects on Soil Properties of the Las Capas Irrigation System of Tucson, Arizona.

A soil study was conducted at the Las Capas archaeological site in the northern Tucson Basin of southeast Arizona in order to document and evaluate soil productivity and hydraulic soil properties of this ancient agricultural irrigation complex. This site presented an unprecedented opportunity to study the complete configuration and evolution of the oldest irrigation system documented in the United States to date, at more than three millennia old. The Las Capas site is significant archaeologically for a number of reasons, including: (1) the antiquity (~575–1225 B.C.) of the Early Agricultural period irrigation systems at the site, (2) the fact that irrigation systems dated to different times are separated stratigraphically within the site, and (3) the fact that extensive and well-preserved gridded irrigation features were identified in the field by mechanical stripping and then sampled. The stratigraphic separation and abundant cultivated irrigation plots facilitated soil sampling so that field, border, and uncultivated control samples could be compared in order to measure the anthropogenic effects of agriculture on soil quality in the irragric soils. Long-term indicators of agricultural soil quality such as organic carbon, nutrient content, and hydraulic soil water properties indicate that anthropogenic changes were favorable for agricultural production and that the Las Capas irrigation system was sustainable. Canals regularly supplied water to the fields, but they also supplied nutrient-rich sediments that continually renewed soil fertility, enough to counter nutrient losses resulting from crop uptake, volatilization, leaching, and oxidation. Fields have significantly elevated organic carbon, nitrogen and available phosphorus levels relative to the borders, at levels that are slightly below but comparable to the control soils. Sodium and sodium adsorption ratios, though elevated in the Las Capas fields, are far below levels that could have had a serious detrimental effect on crop production.