Make sure that they are well fed. And fence your garden with floppy fence, followed by a chicken undesirable crop. A hard wire placed at a strategic height before the floppy fence helps. Hot or not. Provide some good perching at least 25 ft from said fence. Will sit on junk tree instead of their insatiable need to sit high as possible on your fence. I say at least 25 ft away, and higher. Highest will satisfy them, but too far away to glide over that scary floppy fence. Get heavy (body mass) layers for free range. All said, a full chix is a happy chix.
Oh yeah, you wanted them to turn soil. Eh, don't let them. If it's a small garden, you can do it yourself. Let them turn and fertilize a
compost pile instead. I see that being more productive than finding a way around to make them turn garden soil.