"Sowell has not interviewed farmers..."
Again, this doesn't follow. Whether he has interviewed farmers about the issue is beside the point. Immigrant workers, American workers, automation...anything else(?)...all compete to get the work. The market will determine which is best.
If American workers are slow, that's just the same as their not being "willing" to do the work (or take the jobs). The point is the same.
And Dr. Sowell's point is that regardless of all this the most efficient outcome will be to let the market decide how the work gets done. It's about a "need" for labor only to the extent that government, or some other jobs, are competing for the labor and making it difficult for farmers to hire at the pay rate they prefer to pay. In absence of government interference the pay would float to the place it belongs (could be either up or down) and will be reflected in the price of food. Competition will make the concept of "need" nonsense, and we will get the food we demand at the price we can pay.