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Calibrate spreader for precision fertilizer applications

University of Georgia uga-extension-spreader-calibration-a.jpg
One silver lining in the high fertilizer prices is the increased interest among growers in using precision soil sampling and technologies such as variable-rate application.

One of the common grower questions lately has been “How do I know if my fertilizer spreader is applying the correct rate?” The reason behind this question early this season is that growers know any misapplications this year are going to hurt their pocket’s a lot more than it would have last year.

One silver lining in the high fertilizer prices is the increased interest among growers in using precision soil sampling and technologies such as variable-rate application to be more efficient as well as consideration to their equipment’s application accuracy.

It is encouraging to see many growers interested in making sure whether they are applying the correct fertilizer rates, but it is also important to emphasize that when it comes to spinner-disc spreaders, verifying application rate by catching product from the conveyor chain for a certain time or using other similar method is usually not sufficient and can result in some major application issues in the field.

When it comes to broadcast spreaders, consideration to application uniformity is equally important as application rate and helps in ensuring that fertilizer is being applied at the target rate, uniformly across the swath and consequently in the whole field.

The two graphs below shows application rate and uniformity for a single fertilizer product (potash; effective spread width 50 ft.) applied with a spinner-disc spreader before and after calibration. If we look at the mean application rate in both graphs (317 and 310 lbs/ac), they are very comparable and close (95-97%) to the target rate of 300 lbs/ac.

However, if we pay attention to the fertilizer distribution (solid red line) and the corresponding uniformity values (49.5% and 12.5% for uncalibrated and calibrated, respectively) in each graph, it’s not hard to miss the stark difference in uniformity between the two applications. While the mean rate is similar in both applications, fertilizer application is highly non-uniform across the swath before calibration whereas it is much more uniform across the spread width after calibration.



Graphs illustrating difference in application uniformity for an uncalibrated (top) and a calibrated spreader (bottom). Credit: University of Georgia.

The purpose of showing this data is to emphasize application issues related to product rate and/or uniformity are very common with spinner-disc spreaders, and proper spreader calibration can help in achieving both accurate application rate and uniformity.

A standard pan testing is recommended to calibrate broadcast spreaders  and attain desired application rate (±95% of target rate) and spread uniformity (CV≤15%).

Here are few other considerations to keep in mind during calibration of spinner-disc spreaders:

  • Each calibration is specific for the type of product applied, application rate and spread width. Any changes in material type (single or blended), properties (shape, size or density), application rate and/or spread width may require an additional calibration.
  • Before starting the calibration process, refer to the operator’s manual for recommended settings for the specific product type and density. It is always a good practice to start with these recommended settings and then make adjustments there after based on the testing results.
  • During calibration, do not change more than one spreader setting (flow divider position, spinner-disc speed, gate height, etc.) at a time between the test passes to evaluate its influence on application rate or uniformity.
  • Make sure to record all information for each test pass i.e. spreader settings, material density, target rate, spread width, etc. during calibration. This information will be helpful in the future during field application and calibration, especially when using similar material types or blends.
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