Soil Properties of an Ancient Dryland Agricultural Complex in Central Arizona.

Prehistoric dryland agricultural soils were investigated near Queen Creek in central Arizona as part of the U.S. 60 Archaeological Project. This study aimed to: (1) determine the long-term effects of cultivation and agricultural management practices on soil quality; (2) measure the effects of rock mulch on unsaturated hydraulic conductivity; and (3) compare the results with previous soil studies of ancient agricultural systems in the region. A suite of physical and chemical soil tests were used to compare the soil properties associated with terraces, rock alignments, and rock piles, and uncultivated control soils from similar soils and landscape positions. Justification for this type of study is provided by the following factors: (1) soil formation is slow in semiarid climates, so cultivation effects persist for centuries to millennia; (2) many ancient fields have remained uncultivated since they were abandoned, so long-term soil properties reflect ancient agricultural use; (3) agricultural features provide clues for identifying and sampling ancient cultivated and control soils; and (4) ancient farming systems are relevant to modern studies of agricultural sustainability. Results of this study indicate that cultivation and management practices improved soil quality in agricultural contexts relative to the controls. Rock mulch soils tend to have elevated organic carbon, nitrogen, and total and available phosphorus levels, with reduced or no changes in soil pH. Bulk density tests showed that cultivation has not caused soil compaction. Salinity is elevated in some agricultural contexts, but not at levels detrimental to crop growth. Rock mulch soils have higher unsaturated hydraulic conductivities than uncultivated controls. Overall soil fertility and productivity is improved in the cultivated soils, which is similar to previous studies of ancient agricultural studies in the middle Gila and lower Verde River valleys of Arizona.