Most grounds managers know that spreaders require periodic calibration and maintenance, and they know that if they don't calibrate, application rates may be incorrect. Most of us have read articles and heard lectures over the years describing how to calibrate a broadcast spreader, but rarely do we ever discuss why it is important beyond simply achieving the correct application rate.
The distribution pattern is another important reason to keep your spreader accurately adjusted. After all, if the pattern isn't fairly uniform, then you'll inadvertently end up applying too much to some areas and not enough to others, even if the overall amount you apply is correct for the area you're treating. Here's a different look at calibrating spreaders that will provide a better understanding of this routine practice and why it matters in more ways than you might think.
This article focuses mainly on walk-behind broadcast spreaders, but the same logic works for larger tractor- or truck-mounted broadcast spreaders. The basic function of a spreader is to drop particles of fertilizer or lime (or granular pesticide) onto a spinning impeller that throws the particles out from the spreader. Factors such as the forward speed of the spreader, the rate of spin of the impeller, the speed and direction of the wind and the characteristics of the particles (moisture, density, size) can influence the ultimate distribution rate and pattern. Here, I'll focus on the physical properties of particles and how they impact distribution patterns.
Not all particles are created equal
The most important physical properties of particles are density and “stickiness.” (The table on page Contractor 36 gives some examples of bulk densities for common materials.)
For this discussion, think of limestone as a bowling ball and urea as a volleyball. Both particles are about the same size but limestone weighs more. If you drop both the urea and limestone on the same spinning impeller wheel of a fertilizer spreader, you'll find that the distribution pattern coming off the blades is different. The limestone (bowling ball) particles will drop off the impeller blades first, before the lighter-weight urea particles: The lighter particles stay on for a longer ride on the spinning “merry-go round.”
However, particle weight is not the only factor that influences the length of the ride on the spinner. Particles that are wet, whether heavy or light, are “stickier” and will tend to stay on the spinner for a longer time than dry particles.
Knowing that not all materials behave alike in spreaders, how do spreader manufacturers make it possible for us to make adjustments in the distribution pattern of products with different physical properties? It's actually fairly simple. Particle distribution is influenced by where the particles drop onto the spinner. Particles that tend to stay on the impeller longer should be dropped onto it closer to the outer edge, offsetting the particles' slow exit from the spreader. Conversely, materials that tend to leave the impeller quickly can be dropped onto it closer to the middle, forcing them to travel further to leave the impeller.
Manufacturers commonly use two different approaches to solve this problem.
The first approach is to use holes in the bottom of the fertilizer hopper that the user can adjust to different positions over the spinner.
The second is to use a single hole that drops particles on a variable cone that distributes particles to different spots on the impeller wheel. The cone can be rotated by the user to achieve the desired distribution.
To visualize how the latter design works, consider a dense fertilizer particle like potassium chloride. When potassium chloride is dropped toward the outside of the spinner, it typically causes a shift to the right in the distribution pattern because the particles leave the impeller sooner (see Figure 1, above). To compensate for this, a shift of the drop area toward the center of the impeller achieves a more uniform distribution pattern (see Figure 2, page 34). A correction for a lighter (less dense) particle, such as urea, can be achieved in similar fashion.
In many cases, you are not applying a straight fertilizer or lime product. However, the same steps help correct the distribution pattern for a blended fertilizer with particles of different densities. Adjusting the drop position influences the pattern for a particular blend (see Figure 3, above).
Measuring distribution patterns
Fertilizer manufacturers frequently provide label information about appropriate settings for their products when used in common spreader models. These settings assume certain conditions, including a “normal” walking speed. However, just as you should calibrate spreaders periodically to check for application rate, you also should check the distribution pattern. Use the manufacturer settings as a starting point.
The method for checking distribution generally is the same for calibrating application rate. However, in this case, we can make conclusions without actually weighing the product.
The most common method involves placing uniformly spaced trays or collection tubes at right angles to the forward direction of the spreader (see Figure 4, at left). I typically use inexpensive aluminum trays with a piece of artificial turf or a similar soft material in the bottom of them to minimize the loss of particles that bounce out. It is best to set the trays so that the spreader will straddle them as you walk forward.
Similar to requirements for calibrating for rate, you need to make sure your walking speed is constant and representative of your usual speed of travel while operating a spreader. You will get a more accurate representation if winds are calm and are moving in the same direction as the spreader. I typically use three to five passes, all going in the same direction. Once you have made your passes, simply look in the trays or collection jars and visually evaluate whether the distribution is reasonably similar on both sides. Be sure to evaluate the total amount of material, as well as the mix of particles (if you're using a blended product).
If the distribution is discernibly non-uniform, adjust the spreader and try again. It shouldn't be too hard to find the right setting. Remember that calibration for one material does not apply to a different material.
Dr. James Robbins is an extension specialist with the University of Arkansas (Little Rock, Ark.).
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