Cotton Agriculture and the Function of Gravel Mulch in the Northern Rio Grande

Abstract

The site of Poshu’Owingeh was one of several ancestral Tewa villages to experience rapid growth in the 14th and 15th centuries A.D. in northern New Mexico. Recent research has proposed that this growth was one aspect of a trend characterized by nonlinear socioeconomic change produced by increasing population size and connectivity. Agriculture, commodity production, and exchange are fundamental to this model, but direct evidence for intensification is limited and no empirical data exists for the function and mechanics of the technologies which are supposed to have supported surplus production. This research addresses the problem by examining paleobotanical and soil evidence for the function of gravel mulch, a unique agricultural technology hypothesized to have supported cotton agriculture in portions of the Northern Rio Grande. Physical soil properties and base cation ratios are used to reconstruct the irrigation effect of gravel mulch, and soil nutrient levels are measured to assess change in soil quality associated with cultivation. Fossil pollen assemblages recovered from agricultural soil layers are used to determine the mix of crops grown in gravel mulch fields. A spatial database of archaeological sites is used to reconstruct Puebloan population dynamics in the Tewa Basin. This is compared to published estimates of population growth, the timing of socioeconomic developments in the region, and climate reconstructions. Fossil pollen concentrations indicate that cotton was the main crop grown at Poshu’Owingeh. The substantial increase in the ratio of cotton to maize and a decrease in the diversity of economic wild plant taxa at Poshu’Owingeh suggest cotton cultivation was more intensive here than other documented sites in the region. Soil analysis revealed no evidence for degradation associated with gravel mulch. Cation ratios and particle size distribution in the A horizon suggest that gravel mulch continues to enhance subsurface water flux. I estimate that the runoff required to produce the sodium leaching observed in mulched profiles is generated by relatively intense precipitation generated by monsoon storms. Peak rates for the spatial expansion of farming populations responsible for the construction of the gravel mulch-cotton fields follows rapid population growth in the great Tewa Basin, is coincident with the intensification of regional exchange networks, and strengthening of the North American Monsoon. Given the mechanics of agricultural technology documented in this study, and hypotheses for the importance of cotton in the regional economy; it is concluded that population dynamics, climate change, and human niche construction interacted played a significant role in social change and economic expansion in the late precontact Northern Rio Grande.