Across Iowa's corn country, huge machines with anteater snouts gulp the ears off 8-ft.-high cornstalks, an instant later spit golden kernels into self-contained bins. In California, packing machines out in the fields seal freshly picked lettuce heads in plastic, drop them into cardboard boxes, then dis gorge the boxes ready for market. On farms in the Southwest, machines work the fields with surgical precision, injecting minuscule broccoli seeds one by one into the soil at measured intervals.
And on even the rockiest farmland, plows dig freely across the fields, the threat of...