In this paper I focus on three factors that strongly influence maize productivity: water availability, pH, and salinity. A recent paper by Tankersley et al. (2016) has argued that the elevated salinity values of Chaco Canyon valley-floor sediments are due to high concentrations of dissolved sulfate, which favor the growth of maize. In the following I show that any dissolved solid added to soil water, including sulfate decreases the ability of soil water to enter a plant via its root system. As salinity increases, the rate of water diffusion into maize decreases until the wilting point is reached. The present paper demonstrates that valley-floor soils in Chaco Canyon and along the Chaco River between the west end of the Canyon and the Great Bend of the Chaco River are too saline to grow maize or beans and that side-valley areas in the canyon are too small in area to sustain more than a few hundred people. To support a larger population, maize must have been imported.

I suggest that the substantial amount of winter precipitation in the Chuska Mountains combined with surface-water features (washes) that concentrate and convey spring snowmelt to the eastern Chuska Slope allowed the diversion of surface water to large irrigated fields that at some point sustained over 10,000 residents of the slope. Given the documented trade between Chaco and the Chuskas, I suggest that the Chuska Slope may have provided the subsistence base for many of those residing in Chaco Canyon during its 1050–1130 CE expansion.