I don't know how to tell this without being condescending, but do you guys even understand how farming works?
The fertile soil is a result of a long process of bringing organic material and ash and depositing it on the field. Exactly the opposite of what we are doing today
When they lived in the rainforest, they would gather whatever leftovers from eating, possibly their feces, decaying matter, and deposit it all on the field. They would keep hunting, they would keep gathering, and all that hunted and gathered material would end up on the field. Maybe they would compost it separately and then drop it on the field, maybe they would drop it on the field and the field would be in effect the composting area. Over hundreds or thousands of years this would create those amazingly fertile fields.
It is very easy to see how this developed. They probably have noticed that if they keep throwing their leftovers or feces in a particular area, everything starts growing better there. The same if you throw out ash. Which they had lots of beacuse they were burning a lot of wood every day, living in the rainforest and not facing energy price hikes. If you have a village and you hunt and gather every day, if you constantly keep burning wood fire and spread it all around your village for thousands of years -- guess what -- whether you planned it or not you will create those fields because I bet nobody would be trucking the garbage far from the village.
The problem is we do not live in the rainforest and we do not have so much organic matter to cover our fields. And the leftovers from what we eat we dump mostly into landfills. Our feces are also escaping the system and are not providing matter back to our fields.