Different crops vary widely, however, in the type of labor required-that is, in their power- and control-intensity. Harvesting of root crops is probably the most power-intensive, although it still requires significan control. At the other extreme, cotton, fruit, and vegetables require intensive control: in the case of apples, the threat of damage is so large that their harvesting has still not been successfully mechanized.
For example, the growth of tractors in the United States was spread over a fifty-year period with occasional spurts, but once tractors became available, they took over primary tillage within a much shorter time. Their further growth involved shifting extra operations from horses to tractors. Today few farms in the Indian Punjab plow land with animals, thresh wheat by hand, or use Persian wheels. This is only fifteen years after tractors, threshers, and pumps became an important factor in Punjab farming.