Lines of Distribution
If you spread many granular chemicals, you already know that it's important to keep your walk-behind spreader properly calibrated. Many calibration instructions place the emphasis on achieving the correct application rate, which is important. But just as important is achieving uniform distribution of the granules. If the distribution pattern isn't fairly uniform, you'll end up with application flaws despite having a correct application rate.
Walk-behind spreaders are offered in two types: drop and broadcast (rotary). Each has its distinct advantages and disadvantages for application and distribution uniformity.
- Drop spreaders
These spreaders typically are designed with small, adjustable holes underneath the hopper that allow the product to “drop” through and fall onto turfgrass. Advantages: Drop spreaders offer a more uniform pattern and more consistency, naturally. They demonstrate low drift potential and more precise control. Disadvantages: They sometimes skip or overlap and provide non-uniform spread when you're turning corners. Ground clearance is also a potential problem
- Rotary spreaders
These spreaders generally feature a hopper that includes an impeller underneath that spins to throw particles in a semi-circle arc. The speed of the impeller is directly correlated to the speed you push the spreader. Advantages: Rotary spreaders can cover a much wider area than drop spreaders, saving you a lot of time. They also are better on uneven terrain. Disadvantages: Many outside factors can influence their distribution uniformity, including the speed and direction of the wind and the characteristics of the granules you're applying (different sizes and weights do not spread uniformly).
Techniques for ensuring a uniform distribution pattern depend on the type of spreader you have. Because drop spreaders offer fairly consistent uniformity, the best way to maintain that uniformity is to maintain the spreader. Keep the metering holes in the bottom of the hopper clean and unobstructed. Also to ensure even distribution, check your application methods. The particles fall within the wheelbase, so you should slightly overlap each pass with the spreader.
For rotary spreaders, ensuring a uniform distribution is more difficult. The method for checking distribution uniformity is very much like checking for application rate, except that you don't weigh the product for results. To measure distribution, follow these steps:
Lay out a row of collection pans or boxes at regular intervals (about every 1 or 2 feet) on each side, perpendicular to direction of spreader travel. How you space the pans depends on the overall width you expect from the spreader, but be sure the pans cover the entire throwing width. The collection trays should be the same size and shape, at least 1 square foot and a few inches deep. Note: Granules will sometimes bounce out of trays, affecting results; James Robbins at the University of Arkansas recommends placing a piece of artificial turf on the bottom of collection trays to minimize this.
Select the manufacturer-suggested setting for calibration, and load the spreader with the material you'll be applying. If you don't have manufacturer's suggestions, choose a setting as a starting point and work from there.
The key to achieving the most accurate distribution measurement is walking at a uniform speed. About three miles an hour is a good average, but as long as you walk at the same rate you'll later use for application, your reading should be accurate.
Make several passes (at least three), traveling in the same direction.
Visually access the pans. The distribution should be relatively similar on both sides. If it's not, adjust the controls and try again until it is.
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