Eolian deposition and soil fertility in a prehistoric agricultural complex in central Arizona

Prehistoric farmers in the semi-arid American Southwest were challenged by marked spatial and temporal variation in, and overall low levels of, precipitation with which to grow their crops. One strategy they employed was to modify their landscape with rock alignments in order to concentrate surface water flow on their fields. A second challenge that has been less focused on by archaeologists is the need to maintain soil fertility by replenishing nutrients removed from the soil by agricultural crops. Numerous studies have shown that rock alignments can result in long-lasting impacts on soil properties and fertility. However, the direction and magnitude of change is highly variable. While previous work has emphasized the importance of overland flow in replenishing soil nutrient pools, none have investigated the influence of eolian deposition as a contributor of mineral-derived nutrients. This thesis explores the effects of the construction of rock alignments, agricultural harvest, and eolian deposition on soil properties and fertility on Perry Mesa within the Agua Fria National Monument. This site experienced dramatic population increase in the late 1200s and marked depopulation in the early 1400s. Since that time, although agriculture ceased, the rock alignments have remains, continuing to influence runoff and sediment deposition. In the summer of 2009, I investigated deep soil properties and mineral-derived nutrients on fields near Pueblo La Plata, one of the largest pueblos on Perry Mesa. To examine the effects of rock alignments and agricultural harvest independent of one another, I sampled soils from replicated plots behind alignments paired with nearby plots that are not bordered by an alignment in both areas of high and low prehistoric agricultural intensity. I investigated soil provenance and the influence of deposition on mineral-derived nutrients through analysis of the chemical composition of the soil, bedrock and dust. Agricultural rock alignments were significantly associated with differences in soil texture, but neither rock alignments nor agricultural history were associated with significant differences in mineral-derived nutrients. Instead, eolian deposition may explain why nutrient pools are similar across agricultural history and rock alignment presence. Eolian deposition homogenized the surface soil, reducing the spatial heterogeneity of soils. Dust is important both as a parent material to the soils on Perry Mesa, and also a source of mineral-derived nutrients. This investigation suggests that prehistoric agriculture on Perry Mesa was not likely limited by long term soil fertility, but instead could have been sustained by eolian inputs.