Clay
Footnotes:
[1] Christina Grabach Siebe, “Human Impact on Geospheric Processes in the Critical Zone Exemplified by the Regional Water Exchange Between the Mexico City Metropolitan Area and the Mezquital Valley,” Text, Franklin Humanities Institute (blog), November 27, 2018, https://humanitiesfutures.org/papers/human-impact-on-geospheric-processes-in-the-critical-zone-exemplified-by-the-regional-water-exchange-between-the-mexico-city-metropolitan-area-and-the-mezquital-valley/
[2] Christina Siebe, “Nutrient Inputs to Soils and Their Uptake by Alfalfa through Long-Term Irrigation with Untreated Sewage Effluent in Mexico,” Soil Use and Management 14, no. 2 (1998): 119–22, https://doi.org/10.1111/j.1475-2743.1998.tb00628.x.alfal
The widespread prevalence of clay-based soils are critical to the longstanding functionality of the Mezquital’s wastewater agriculture system. Vertisols are among the most common throughout the wastewater irrigation districts. The silty, dense texture of the area’s clayey soils, along with their neutral to alkaline pH and relatively large content of organic matter combines to form an effective filter. Pollutants are absorbed by clay and organic matter particles to a high degree. However, the soils have a threshold, and scientific predictions put a cap at around 150 years for the safe consumption of cadmium concentrations in maize grain. [1] There are equally urgent thresholds related to rising concentrations of other heavy metals in wastewater crops long considered safe for consumption. The relationship between soil nutrients and structure is complex and precarious. Research has shown that enhanced nitrogen saturation in clayey soils can cause clay to disperse and lead to surficial crust formation that lessens water infiltration. [2]
Footnotes:
[1] Christina Grabach Siebe, “Human Impact on Geospheric Processes in the Critical Zone Exemplified by the Regional Water Exchange Between the Mexico City Metropolitan Area and the Mezquital Valley,” Text, Franklin Humanities Institute (blog), November 27, 2018, https://humanitiesfutures.org/papers/human-impact-on-geospheric-processes-in-the-critical-zone-exemplified-by-the-regional-water-exchange-between-the-mexico-city-metropolitan-area-and-the-mezquital-valley/
[2] Christina Siebe, “Nutrient Inputs to Soils and Their Uptake by Alfalfa through Long-Term Irrigation with Untreated Sewage Effluent in Mexico,” Soil Use and Management 14, no. 2 (1998): 119–22, https://doi.org/10.1111/j.1475-2743.1998.tb00628.x.alfal
