TL;DR your comment for 1. applies to 3. All organic matters are not all made equal. I have potassium in mind : mined potassium is essential to the high yields

TL;DR your comment for 1. applies to 3. All organic matters are not all made equal. I have potassium in mind : mined potassium is essential to the high yields of industrial agriculture. You don't strictly need it and you can rely on locally sourced organic matters, but it'll never beat mining fertilizer and spraying it. Besides that, if your soil is poor to start with, you'll need to concentrate organic matter from a large area into a small spot that will become arable. For example, my ancestors were living in a swampy marsh, aka Aquitaine coast 150 years ago. They were raising large herds of sheeps feeding on low nutrients shrubbery. Then all those sheep's would rest on one specific plot, where they would eventually poop. On that plot, barley would be grown, as thanks to the sheeps poop, the ground was fertile enough. Compare that to area where the soil is very fertile (say, Ukraine or Moldavia)