Making Circles into Squares
One of the drawbacks of center pivot systems is that the circles of well-irrigated land they produce are usually forced to fit into a square system of land parcels and roads. When the first pivot systems were being installed in the 1950s and 60s, most farmers were working quarter-of-a-mile sections of 160 acres. The center pivot systems were able to reach only 133 acres of that "quarter," and the corners inevitably produced poor crop yields.
One way to solve the problem is to buy enough land to break the square pattern of land distribution and install the circles in a hexagonal pattern that minimizes the space between circles. This approach was used in the 1960s in Libya, but it doesn't work very well in most of the U.S. where land is already divided into square and rectangle parcels.
Some farmers tried to solve the problem by planting more drought-resistant crops on the corners. The main field would typically be planted in corn and the corners planted in milo. But those different crops have different water, chemical and harvesting needs, so it was an imperfect solution.
Pivot manufacturers tried to solve the problem by installing large sprinkler guns on the ends of the pivot systems. Under high water pressure, those guns could reach out and water at least part of the corners. But then the manufacturers had to figure out how to turn those guns on and off they wanted the guns to operate only on the corners, or else the farmer ended watering a lot of the road or a neighbor's field on the center edges of the circle.
Finally, in the 1970s, manufacturers developed corner systems that had a special arm that would swing out on the corners and then tuck back in on the edges. In this oral history video, Robert Daugherty explains how his company and others square the circle. To accomplish that the corner arms had wheels and drive systems that could turn and steer themselves. To tell the arm when to swing out, most modern systems bury a low voltage wire in a precise pattern around the outside of the field. That wire swings out on the corners and pulls back closer to the center on the edges. Then a radio signal is sent down the wire and a radio receiver is mounted on the corner arm. When the receiver senses that the wire is swinging out, it tells the steering system to follow the wire.
Written by Bill Ganzel, the Ganzel Group. First published in 2006. A partial bibliography of sources is here.