Soil analysis in discussions of agricultural feasibility for ancient civilizations: A critical review and reanalysis of the data and debate from Chaco Canyon, New Mexico

<div><p>Questions about how archaeological populations obtained basic food supplies are often difficult to answer. The application of specialist techniques from non-archaeological fields typically expands our knowledge base, but can be detrimental to cultural interpretations if employed incorrectly, resulting in problematic datasets and erroneous conclusions not easily caught by the recipient archaeological community. One area where this problem has failed to find resolution is Chaco Canyon, New Mexico, the center of one of the New World’s most vibrant ancient civilizations. Discussions of agricultural feasibility and its impact on local population levels at Chaco Canyon have been heavily influenced by studies of soil salinity. A number of researchers have argued that salinized soils severely limited local agricultural production, instead suggesting food was imported from distant sources, specifically the Chuska Mountains. A careful reassessment of existing salinity data as measured by electrical conductivity reveals critical errors in data conversion and presentation that have misrepresented the character of the area’s soil and its potential impact on crops. We combine all available electrical conductivity data, including our own, and apply multiple established conversion methods in order to estimate soil salinity values and evaluate their relationship to agricultural productivity potential. Our results show that Chacoan soils display the same salinity ranges and spatial variability as soils in other documented, productive fields in semi-arid areas. Additionally, the proposed large-scale importation of food from the Chuska Mountains region has serious social implications that have not been thoroughly explored. We consider these factors and conclude that the high cost and extreme inflexibility of such a system, in combination with material evidence for local agriculture within Chaco Canyon, make this scenario highly unlikely. Both the soil salinity and archaeological data suggest that there is no justification for precluding the practice of local agriculture within Chaco Canyon.</p></div

General location of Chaco Canyon Cultural Historical Park in relation to the Chuska Mountains.

<p>Four Corners is in upper left corner of figure at intersection of black state lines. Only selected major drainages contributing to or near Chaco Wash are represented. Black triangles are the three closest pedons to Chaco Canyon that have been sampled by the USDA.</p

This table shows estimated crop yield declines at particular soil or irrigation water conductivities.

<p>EC_e is a measurement on the extract from a saturated soil paste. EC_w is the conductivity of irrigation water with yield declines based on an estimated 15–20% leaching fraction. These data are always presented as guidelines, not definitive limits, and are for modern crop varieties. Given the range of tolerance within a given crop type, see squashes, it is possible that varieties used by Chacoan farmers were less susceptible than modern varieties largely grown in wetter climates. Data, except for sunflower, is from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0198290#pone.0198290.ref081" target="_blank">81</a>]. Amaranthus, found to be part of diets at Salmon Ruin and Antelope House, is considered a tolerant plant to salinity [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0198290#pone.0198290.ref082" target="_blank">82</a>]. Chenopodium, Amaranthus, and Asteraceae were found to be significant diet contributions [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0198290#pone.0198290.ref082" target="_blank">82</a>], and each is considered a halophytic, or salt adapted, plant.</p

Estimated time and labor efforts for Chaco residents to transport maize to the canyon based on varying population sizes, carrying capacities, and travel times.

<p>Round trip lengths are based on a distance of 85 km between Chaco Canyon and the Chuska Mountains. Six-day trips were suggested by Benson [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0198290#pone.0198290.ref033" target="_blank">33</a>] based on traveling 26.67 km/day, while 9.5 and 27 days—assuming a speed of 18 km/day and 6.34 km/day respectively—were derived from travel times determined by Malville [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0198290#pone.0198290.ref137" target="_blank">137</a>] and reported by Windes and McKenna ([<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0198290#pone.0198290.ref142" target="_blank">142</a>] p136). Drennan [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0198290#pone.0198290.ref132" target="_blank">132</a>] used loads of 20 and 50 kilograms in his studies of long distance transport, both of which are paired with Benson’s assertion that the trip could be made in six days. The second two weight estimates derive from Malville [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0198290#pone.0198290.ref131" target="_blank">131</a>], as reported in Windes and McKenna ([<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0198290#pone.0198290.ref142" target="_blank">142</a>] p136) and are directly linked to the trip lengths determined in the first column. Number of trips and travel days are rounded based on the completion of an entire trip.</p

Shows SAR variation from 41 measurements from 8/6/1976 to 10/6/1983.

<p>Max SAR/fine—the highest flat horizontal line—is the maximum SAR value usable for irrigation on fine textured soils under any management practice and is the highest flat horizontal line. Mean SAR represents the value of 5.74. No SAR issues—the horizontal shaded area at the base—indicates that below a value of 3 there is no projected impact from the Na composition. The Na/SO4 Ratio is the simple ratio of the USGS data for each reported in mg/L. Shaded vertical bars indicate the seasonal period of precipitation at Chaco Canyon: July through October.</p

Comparison of variations in electrical conductivity measurement.

<p>Comparison of variations in electrical conductivity measurement.</p