SOIL DEVELOPMENT AND LANDSCAPE EVOLUTION ON LAVA FLOWS : HOW CHEMICAL AND PHYSICAL PROPERTIES OF SOIL INFLUENCED PREHISTORIC AGRICULTURE IN THE SAN FRANCISCO VOLCANIC FIELD

Sunset Crater is the youngest of about 600 cinder cones in the San Francisco Volcanic Field of Northern Arizona. The eruption of Sunset Crater (ca. AD 1075) displaced an agrarian population, estimated to be about 1800 people, to low elevations previously too dry for agriculture. However, hundreds of large habitation sites in arid environments near Wupatki National Monument attest to the fact that increasing populations settled in these areas after the eruption. These habitation sites are associated with thousands of agricultural features dominated by rock alignments, arrangements of rocks set in place to prevent strong winds from damaging young plants, that range from 2-75 meters in length and are often found on lava flows. The growing populations were able to flourish in this area due to the tephra cover from the Sunset Crater eruption which acted as a mulch by increasing soil moisture and releasing nutrients stored in the soil. This study was developed to determine which nutrients accumulated in the soil, which nutrients were removed by prehistoric agricultural practices, and differentiate physical and chemical properties of cultivated and uncultivated soil. The rock alignments in this study are located on the North Mesa lava flow (ca 0.85 Ma), an area that has not been inhabited or used for agricultural purposes in approximately 800 years. An additional purpose of this study is to show that cultivated soils are sandier, have better infiltration rates, and have less available P, N-NO₃, and NH₄ than uncultivated areas. Results also show that nutrient loss in cultivated soil is still measurable approximately 800 years after the habitation sites were abandoned and agricultural use ceased. These results confirm that soil fertility in some arid environments is slowly renewed primarily by eolian dust deposition. In light of global climate change, sustainability studies on soils in arid environments can be particularly instructive in the context of prehistoric subsistence agriculture. Methods, such as chemical and physical analysis of the soil, were also useful in evaluating whether rock alignments are in fact cultural.