Sustainable, agriculture must be low-input - and this can only achieved in diverse, tightly integrated agro-ecological systems. The real future of farming lies in complex polycultures ... in short, mixed farming.
To work its wonders nature employs three main tricks. It is extremely diverse, tightly integrated, and makes do on low inputs - basically on what is around.
Certainly, it makes no use of fossil fuels. The sun and geothermal heat (with tidal power around the edges) provide all the energy that's needed.