Mechanical engineering ingenuity found solutions for even more problematic crops—the worst of which was probably cotton. In the long history of cotton's cultivation, no one had come up with a better way to harvest this scraggly tenacious plant than the labor-intensive process of plucking it by hand. The cotton gin, invented in 1794 by Eli Whitney, mechanized the post-harvest process of extracting the cotton fibers from the seedpod, or boll, but no really successful efforts at mechanizing the picking of cotton occurred until the 1930s. In that decade, brothers John and Mack Rust of Texas demonstrated several different versions of a spindle picker, a device consisting of moistened rotating spindles that grabbed the cotton fibers from open bolls, leaving the rest of the plant intact; the fibers were then blown into hoppers. Spindle pickers produced cotton that was as clean as or cleaner than handpicked cotton; soon they replaced earlier stripper pickers, which stripped opened and unopened bolls alike, leaving a lot of trash in with the fibers. The Rust brothers' designs had one shortcoming: They couldn't be mass produced on an assembly line. Thus credit goes to International Harvester for developing the first commercially viable spindle picker in 1943, known affectionately as Old Red.

Whatever their nature, one thing all crops need is water, and here again the effect of mechanization has been profound. At the beginning of the 20th century, only about 16 million acres of land in the United States were irrigated, typically by intricate networks of gated channels that fed water down crop rows. Most farmers still depended almost exclusively on rain falling directly on their fields. Then in the 1940s a tenant farmer and sometime inventor from eastern Colorado named Frank Zybach devised something better—a system that consists of sprinklers attached to a pipe that runs from a hub out to a motorized tower on wheels. As the tower moves, the sprinkler pipe rotates around the hub, irrigating the field in a grand circular sweep. Now known as center pivot irrigation, Zybach's system was patented in 1952 as the Self-Propelled Sprinkling Irrigating Apparatus. Along with other mechanized systems, it has almost quadrupled irrigated acreage in the United States and has also been used to apply both fertilizers and pesticides.

Mechanization has come to the aid of another critical aspect of agriculture—namely, soil conservation. An approach known as conservation tillage has greatly reduced, or even eliminated, traditional plowing, which can cause soil erosion and loss of nutrients and precious moisture. Conservation tillage includes the use of sweep plows, which undercut wheat stubble but leave it in place above ground to help restrict soil erosion by wind and to conserve moisture. The till plant system is another conservation-oriented approach. Corn stalks are left in place to reduce erosion and loss of moisture, and at planting time the next year the row is opened up, the seeds are planted, and the stalks are turned over beside the row, to be covered up by cultivation. This helps conserve farmland by feeding nutrients back into the soil.

As the century unfolded, everything about farming was changing—not the least its fundamental demographics. In 1900 farmers made up 38 percent of the U.S. labor force; by the end of the century they represented less than 3 percent. With machines doing most of the work, millions of farmers and farm laborers had to look elsewhere for a living—a displacement that helped fuel booms in the manufacturing and service industries, especially after World War II. It also fueled a dramatic shift in the entire culture, as metropolitan and suburban America began to replace the rural way of life. Although some may lament the passing of the agrarian way of life, in much of the developing world these transformations represent hope. The many ways in which agriculture has been changed by engineering—from new methods for land and resource management to more efficient planting and harvesting to the development of better crop varieties—offer the potential solution to the endemic problems of food shortage and economic stagnation.