Imagine there is a small island, like Malta, and the ecosystem has no external inputs (no fertilizer delivery, etc).

What does a sustainable plant based agriculture system look like? Are there already existing examples?

I'm very interested in how top soil is maintained without external fertilizer inputs?

One area of concern, I have, with modern monocrop farming techniques is the depletion of top soil, with something like 60 crop cycles left before we are current system falls over.