North of Mexico City, in the Mezquital Valley, 85,000 ha are irrigated with mostly untreated
wastewater produced in Mexico City. Wastewater allows agricultural development in an area with
550 mm rainfall and soils with low organic matter and nutrient content. Farmers are therefore
against wastewater treatment that could remove the fertilizing matter and promote the reuse of
water within Mexico City. Wastewater contributes to the soils 2400 kg of organic matter, 195 kg
of nitrogen and 81 kg of phosphorus per hectare per year. After 80 years of irrigation, phosphorus
content in the soils has increased from 6 to 20 g/m2, nitrogen from 0.2 to 0.8 kg/m2 and organic
matter from 2 to 5%.