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31. Apply Fertilizer With A Global Positioning System

Global Positioning System

Global positioning systems, or GPS, can be used to determine the precise part of the field that a tractor is crossing. This allows producers to map fertility levels and adjust fertilizer application rates to specific areas of a field. The ability to vary application rates on the go is called "variable rate technology" or other such names: site-specific application, prescription farming, or precision farming, to list a few.

With global positioning systems, or GPS, satellites send signals to a ground-based station, as well as to the receiver on a tractor. In turn, the ground-based station sends a "correction signal" to the tractor. Then the tractor's on-board receiver puts all of this information together to pinpoint the area of the field that the equipment is passing over.

Farmer using a GPS in his tractor cab Most farmers using GPS connect their equipment to standard systems that can locate their position in the field within a diameter of about 6 to 20 feet. But a newer system, known as "kinematic GPS," can locate your position within a 1-foot-diameter circle.
farmer using a GPS in his tractor cab So far, systems based on GPS have been primarily used to do soil testing, apply fertilizer, and monitor yields. Using variable rate technology to adjust herbicide rates is more limited than with fertilizer application. But it is being used to adjust the rates of some herbicides according to particular soil characteristics.
airel view of a watershed It's generally assumed that more precise fertilizer applications translate into less nutrients moving into groundwater and surface water. However, there has been little research on the environmental impact of variable rate technologies that use GPS. If variable rate technology becomes more practical for nitrogen and pesticides, the environmental benefits could potentially be great.
field fertility levels But does GPS pay? University of Illinois research has shown that using variable rate technology to apply phosphorus and potassium became more economical as the average fertility level and the fertility variability in the field increased.
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