It is known that south of there, in the Amazon basin, as far back as 2500 BC, the natives were making “Terra preta”, artificially created, self-sustaining agricultural soil.
Rain forests normally have very poor soil, with most of the nutrients in the plants above it, and water tending to leech the nutrients out of the soil. But the natives figures out that a combination of low temperature charcoal, likely soaked in urine for a while, as well as smashed bone, feces, and baked pottery shards, when mixed in soil from 1 foot to 1 yard in depth, not only made it far better for plants, but tended to revitalize itself, continuing to keep itself fertile, even after hundreds of years.
Since flat, wet land is conducive to grasses, it is no surprise that after the harvest of a good year, those grains preserved for their quality to be used as seed might, in a relatively few generations, make some big jumps.
I’m too tired and lazy to build proper links for these related topics:
The “terra preta” (black earth) of the Amazon basin is a fascinating topic, but I am not sure how this relates to agriculture in southern Mexico.