I was reading about the collapse if Cahokia, a large city built by Native Americans of the Mississippian culture. One of the causes of the collapse is thought to be decreased agricultural yields due to soil depletion. I've read similar theories about the collapse of the Maya.
There are many cities or areas in Eurasia, including in China, Europe, and the Middle East, that have sustained large agricultural populations for thousands of years. How did these populations avoid the same soil depletion that the Native Americans suffered in many of their larger cities?
Pretty much all cities that have sustained farming for thousands of years are located on alluvial floodplains. The Tigris/Euphrates floodplain in Mesopotamia, Nile in Egypt, Yellow River in China, Indus and Ganges in India, Brahmaputra and Mekong in southeast Asia.
An alluvial floodplain is a flat stretch of land by a river that undergoes regular periodic flooding. Every time the river overflows its banks, it spreads fresh silt across the floodplain. Silt contains the minerals that were depleted from the soil. When the flood subsides, the land is fertile again.
An alluvial floodplain can produce crops indefinitely, so long as the river continues to flood.
In the absence of flooding, you need some other means to replenish the soil. Without that, virgin farmland will stop producing crops in a few years or decades, depending on how fertile it was to begin with. You can extend fertility with animal fertilizer, plant waste (letting the field lie fallow and grow weeds in some years), crop rotation (usually with a legume that can fix atmospheric nitrogen), and of course, modern fertilizers.
I don't know specifically what happened to the Maya, but probably whatever means they used to replenish the soil ultimately proved insufficient.