What percent of Earth's surface can be used to grow crops?

Answer 1

I seem to recall reading not so long back that about 12% of the land mass can sustain crops of one sort or another. The article was talking about genetic engineering and suggesting that modified seed could be grown in some areas where it's not presently possible to grow anything.

12% of the land mass is the same as 3% of Earth's surface. The reason it's a small figure is because huge swathes of land are desert of one form or another (ice, snow,sand etc), there are extensive mountainous regions, much land is urbanised and in some places climate and soil conditions are not conducive to growing anything.

Answer 2

Much moor than what we are presently using.