Soil-geomorphic setting and change in prehistoric agricultural terraces in the Mimbres area, New Mexico

A BSTRACT — Soil-geomorphic relationships at some prehistoric agricultural terrace sites in the Sapillo and Mimbres Valleys in southwestern New Mexico were investigated to learn about agricultural management in this semiarid mountainous region, evaluate soil productivity, and determine long-term effects of agriculture on the physical environment. The sites, farmed during the Mimbres Classic period (AD 1000 to 1130), occur within certain geomorphic settings, implying strategies to optimize local climatic and hydrologic conditions for runoff agriculture. The landscape was modified by terracing, which served to reduce runoff velocity, increase soil moisture, and thicken naturally thin soil A horizons. Comparison of these prehistoric agricultural soils with nearby uncultivated soils that are similar in geomorphic setting and degree of development show that significant differences in soil properties still exist, nearly nine centuries after farming ceased. Soil changes resulting from the prehistoric agriculture mostly involved degradation, including accelerated erosion, compaction, and reduced concentrations of nutrients such as nitrogen and phosphorus. Despite soil degradation, maize growth in these soils indicates favorable productivity with nitrogen inputs and improved soil conservation.