Roberto N. Barbosa, Parish, Richard L.
When you buy a bag of fertilizer or granular pesticide to spread on your lawn, it will usually have a label on the back of the bag with settings for several common fertilizer spreaders. Do you know where those settings come from, how they were developed or how reliable they are?
Where Do the Settings Come From?
You might be surprised to learn that the settings on many products come from the LSU AgCenter. The Hammond Research Station for many years operated a granular applicator research facility that is well-known among manufacturers. This work is now being done in the Department of Biological and Agricultural Engineering. Over the past few years, at least12 manufacturers have contracted with the LSU AgCenter to have spreader settings developed. Other manufacturers have their own excellent test facilities and develop their own settings.
Unfortunately, some manufacturers (of spreaders and granular products) either guess at settings, carry obsolete settings for years or estimate settings using unreliable methods.
How Are the Settings Developed?
Development of settings for drop spreaders is a simple process. A laboratory test stand (Figure 1) is used to rotate the drive wheel of the sample spreader at the recommended speed (2.75 mph for homeowner spreaders).
The spreader is turned on for a given number of revolutions chosen on the basis of the spreader width to equal a test run of 1,000 square feet or some other known area. The output for this area is caught and weighed. Test runs are made at different settings until the desired delivery rates are obtained.
An extra preliminary step is needed for rotary spreaders. Since the effective width of rotary spreaders varies with each product, it is necessary to make pattern test runs to determine the effective swath width before the spreader can be put on the test stand for rate determination.
Pattern tests consist of pushing the spreader past a row of shallow collection trays that are perpendicular to the line of travel (Figure 2). The material caught in each tray is weighed, and a computer program evaluates the effect of different swath widths on the overlapped pattern. Choose the widest effective swath width that will give an acceptable overlapped pattern. Once this width is developed, it is used to determine the number of revolutions needed for rate tests on the test stand.
Testing of professional spreaders and products is a little more complicated because most professional spreaders have a means of adjusting pattern. With professional spreaders, it is necessary to make multiple pattern tests to develop a centered pattern before selecting the effective swath width.
How Reliable Are the Settings?
Reliability of the settings is tied closely to spreader quality. If a particular model of spreader is inconsistent and varies from one sample to another, the recommended settings will not be very meaningful.
Even if someone does a good job of developing settings for a specific sample of that spreader model, your spreader of the same model might deliver much more or much less at the same setting, but you have no way of knowing that.
Another common problem is that new spreader models and granular product formulations are introduced frequently; it is impossible to keep a list of settings up to date. The setting on a product label might be for last year’s model of spreader.
A further problem is that a few manufacturers don’t really develop good settings; they guess at settings, use obsolete settings or stretch width recommendations to make their product sound better.
What Should You Do About Settings?
In general, you should not rely on a list of settings provided with your spreader. Those settings are almost always obsolete and meaningless. The settings on the product bag are more likely to be correct, but that is not guaranteed. As a homeowner, your best solution is to buy a reputable spreader and brand-name products, then use the settings on the product bag. Keep track of how much area the bag actually covers, and adjust your spreader, if needed, next time.