See the diagram on page 668. This shows where the potential sources of arable lands. We have no idea what the long term effects of forest destruction are, but they don't look positive. We put six times more energy into farming than we get out. More energy efficient methods of agriculture will almost certainly have to be developed.
Some crops require much less energy input than others.
This has recently led to reduced tillage agriculture, which has its own set of problems. There are also problems with over use of pesticides and herbicides. Many of these will have to be resolved in the future. Naturally occurring compounds may provide a partial solution. Biological control will almost certainly become more important.