Cotton Agriculture and the Function of Gravel Mulch in the Northern Rio Grande
December 6, 2019
Haury Room 129
Abstract: The site of Poshu’Owingh 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 larger 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 exist for the function and mechanics of the technologies that 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 feature hypothesized to have supported cotton production across a large portion 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 strongly indicate that cotton was the main crop grown in gravel mulch fields at Poshu’Owingeh. A comparison with another well documented pollen assemblage suggests that cotton cultivation was more intensive at Poshu’Owingeh; characterized by a substantial increase in the ratio of cotton to maize and a decrease in the diversity of economic wild plant taxa. Soil analysis revealed no evidence for soil degradation associated with gravel mulch. Cation ratios and particle size distribution in soil A horizons 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 storms with a recurrence interval of several decades. Peaks in the spatial expansion of farming villages are coincident with rapid population growth and evidence for extension of exchange networks. The timing of socioeconomic expansion corresponds to an abrupt strengthening of the North American Monsoon which increased summer precipitation in the region. Given the mechanics of agricultural technology documented in this study, and hypotheses for the importance of cotton in the regional economy, I conclude that climate change and human niche construction were important drivers of socioeconomic change in the late precontact Northern Rio Grande.