One role of agricultural machines is the dispensing of materials such as seeds, fertilizers, and sprays. These machines are designed to dispense the material at a fixed, or variable rate, and in a fixed, or variable pattern. A machine that fails to dispense the material at the desired rate and pattern should not be used. An insufficient amount of material will not produce the desired results, and excessive amounts are a lost resource that may result in crop damage and/or contribute to contamination of the environment. A pattern that is not correct may cause streaking or skips. Calibration charts or tables are usually supplied with or are available from the manufacturers of the machines in the owner's manuals or from the supplier of the material being dispensed. Charts and tables become lost or damaged, leaving the operator with no or incomplete information.

Incorrect application of materials can have several causes. A machine may have been damaged or modified. Variations in the weight, size, moisture content, and cleanliness of the seed or variations in the physical condition (lumpiness, flow ability) of the fertilizer or chemical granules can all contribute to an inaccurate dispensing rate and distribution pattern. The economic penalties and potential environmental damage from incorrect application of materials warrant the time and effort required to ensure that the machine is dispensing the desired amount and that the distribution pattern is acceptable.

Checking a machines application rate and patterns of dispersal is called calibration. Although the exact procedure used to calibrate machines and other measuring devices varies from one situation to another, this chapter will illustrate the calibration of four common agricultural material dispensing machines.

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