I can't speak to farming, but I know the going day labor rate, usually construction, for random (illegal) immigrants you can pick up at Home Depot is $20/hr minimum around here. So you'll certainly have to do better than that to get natives. Again, if $20/hr isn't enough, the market will adjust until it is enough.
Who cares whether unemployed people are capable of hard physical labor? Plenty of people in the country are capable, so farm wages will just have to compete against other sectors. And when it does, it will raise the wages of EVERYONE in those other sectors because people will leave for the higher wage farm work. This is the mechanism I keep bringing you back to about how it helps all the poor, not just the ones who take the farm labor jobs.
