My theory of the Mayan collapse is that the style of farming used by the Mayans may have been ineffective. The Mayans probably planted the same crops, in the same place, year after year. Doing this would eventually ruin the soil in that area. For example, if they had learned to plant corn one year and beans the next year the soil might not have lost its nutrients. Farmers at that time also may not have known about leaving fields to recover with no crops for a period of time. If every Mayan farmer used their land for the same crops annually, after time, Mayan land would have been ruined, and crops would fail. This would be a problem because soon after, there would be no food left and no way to get food, therefore causing people to starve or leave the area, leading to the Mayan collapse.
