Equipment Options: Walk-behind spreaders

Calibrating chemical-application equipment is critical. If you don't take care that you are applying product at the correct rates, you can end up applying more product than necessary, which wastes money and can cause environmental and legal concerns. Underapplying product causes a different set of problems: poor performance and, subsequently, increased client callbacks and unhappy customers.

Calibrating a walk-behind spreader to avoid these problems is a relatively simple process that you can perform easily with only a few tools: a scale, measuring tape, bucket and calculator. Spreader calibration is not a one-time process, however, but rather it is one that you should perform frequently as you use different products. Products generally have information on different spreader-model settings. But, while this information is helpful, you should only use these settings as a guide. Calibrating your specific equipment allows you to know how much product you actually are putting out for your specific location and conditions.

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When choosing the type of spreader to use-drop spreaders or broadcast/rotary spreaders-be aware of the characteristics of each type and their respective advantages and disadvantages.

*Drop spreaders. These units have small adjustable holes at the bottom of the spreader. These holes allow granulated product to drop through the openings. The larger the opening, the more product the spreader dispenses. Drop spreaders are especially effective when applying light materials, as well as products under windy conditions or in situations where you can't allow off-target deposition. Drop spreaders, however, can sometimes skip or overlap. Consequently, most experts recommend that you make split-rate applications of the product. Also, ground clearance can sometimes be a problem with drop spreaders because some models have limited clearance.

*Broadcast (rotary) spreaders. These units are better-suited to applying materials to large turf areas. Broadcast spreaders also can be effective for areas of uneven terrain or areas with taller grass where ground clearance is a problem. Broadcast spreaders generally have a bucket-type container on top and a whirling horizontal wheel or plate underneath that rotates (based on walking speed) and distributes the product as it flows through the adjustable port holes in the spreader; the larger the port holes, the more product they dispense and vice versa. Most broadcast spreaders also have a shield or deflector apparatus that you can adjust to prevent depositing product on sidewalks, in pools, etc.

Uniformity of application by broadcast spreaders is generally not as precise as drop spreaders. Therefore, when using a broadcast spreader, you need to plan for some degree of overlap. This is because broadcast spreaders distribute product more heavily in the middle of the spread pattern with a correspondingly lighter pattern on the edges of the application area. Also keep in mind that you won't distribute heterogeneous materials (granules of differing sizes and weights within a product mix) as evenly when using a broadcast spreader because the spreader will throw the heavier materials farther.

Determining application rates The actual application rate for any spreader type varies and depends on the size and weight of the product you are applying. Other variables that affect application rates include the size of the spreader's port holes, the terrain's roughness and the operator's walking speed. This last point is particularly important because everyone walks at a different speed. Thus, if different operators use different spreaders each time they make an application, those operators must calibrate each spreader each time they use one.

In general, manufacturers give recommended product rates (whether it is of fertilizers, herbicides, fungicides, lime or seed) for turf areas on the basis of 1,000 square feet. For example, 15-5-10 fertilizer may have a recommended application rate of 1 pound of nitrogen (N) per 1,000 square feet. Now, let's suppose you have a 5,000-square-foot lawn area. How many pounds of product should you apply to this lawn to achieve the desired application rate? The first step is to determine how many total pounds of N you need for the lawn as a whole. You can calculate this by multiplying the number of 1,000 square feet of turf by the amount of N recommended per 1,000 square feet.

5,000 square feet of turf at 1.0 pound of N per 1,000 square feet = 5 pounds of N

If your fertilizer has 15-5-10 analysis, this means you have a nutrient ratio of 15 percent N, 5 percent phosphorus and 10 percent potassium. The second step, then, is to divide the total pounds of N you need by the percentage of N in the product.

Total pounds of N needed / percent N in product 5 pounds / 0.15 = 33.3 pounds of product

In this case, then, to apply 1.0 pound of N per 1,000 square feet over a 5,000-square-foot lawn, you need a total of 33 pounds of product.

Once you determine which type of spreader to use and the application rate, you then can calibrate your equipment to deliver the desired amount. You must measure two variables when calibrating a spreader: the area covered and the amount of product used. The general principles of calibration are basically the same for both drop- and broadcast-type spreaders.

10-step calibration method You can break down calibration into 10 easy steps. Keep these handy (laminate them on a card to hang on the spreader handle) so you and your crew will remember what is involved. 1)Collect the materials needed. These include a scale, measuring tape, bucket, spreader, product and calculator. 2)Select an area for testing. The terrain should be similar to the area where you will distribute the product. 3)Measure off 100 feet. Mark off a length of 100 linear feet with measuring tape. Be sure to clearly mark a starting and finishing point. 4)Set the spreader. Set the spreader adjustment ports. As mentioned, some products will have suggested settings for specific spreader types. If this information is not available, begin at a low setting to avoid over-application of the product. 5)Add pre-weighed material to spreader hopper. Add a weighed amount of material to the hopper-at least 20 pounds. Be sure the spreader port holes are in the closed position. 6)Make a pass with the spreader. Beginning 10 feet before the starting line, begin pushing the spreader and open the ports as you pass over the starting line. Continue walking at a consistent pace over the pre-marked 100-foot distance. Be sure to walk at a pace typical of what you would normally use. Close the ports as you cross the finish line. 7)Determine the width of spread. The effective width of the spread for a drop-type spreader will be the width of the hopper. For a broadcast spreader, you determine the effective width from the spreader's lateral throw distance. A second person can observe this by watching how far the product is thrown to the left and right of spreader. 8)Weigh remaining material. Carefully pour the remaining product from the hopper into a bucket. Weigh the product and record the data. Subtract the weight of the remaining material from its initial starting weight to determine the weight of the material you dispensed. Some manufacturers make a collection plate (a catch can) that you can place on the bottom of drop spreaders to collect the product rather than actually applying it. If you use a catch can, then simply weigh the material and record the data. 9)Calculate the effective area covered. To determine the effective width of the area you covered, multiply the width of spread (from Step 1/4) by the length of the distance covered (100 feet in this case). 10)Calculate the rate of application. From the following formula, calculate the rate of application in pounds per 1,000 square feet:

weight of material applied (Step 8) = X ----------------------------- ------------ 100 feet x width of application 1,000 sq ft (Step 9)

A realistic example As an example, let's suppose you initially added 20 pounds of a 15-5-10 product to your broadcast spreader. After completing a 100-foot pass, you measured the remaining product left in the hopper (12 pounds). By subtracting the weight of material left in the hopper from the total initial material added, you then determined that you applied a total of 8 pounds. With the help of a second observer, you determined that your spreader had an effective lateral spread width of 12 feet. To calculate the effective area covered, you multiply the width of spread by the distance covered. To determine how many pounds of product you used per 1,000 square feet, you use the formula in Step 10.

8 pounds =X ----------------------------------- 100 feet x 12 1,000 sq ft

X = 6.67 pounds of product applied per 1,000 square feet

To determine your final output in terms of N applied per 1,000 square feet, you multiply the total pounds of product applied by the percentage of N contained in the product:

6.67 pounds of product x 15 percent = 1.0 pound of N/1,000 square feet

In this example, through the calibration process, you determined that you applied 1.0 pound of N per 1,000 square feet for a given spreader setting. If the desired rate is 0.5 pound of N per 1,000 square feet or 1.5 pounds N per 1,000 square feet, then you need to adjust the spreader setting accordingly and re-test that setting-following Steps 1 through 10 again-to ensure you deliver the desired amount.

Although the calibration process is somewhat time-consuming, it is an important step in serving your customers. By selecting the best spreader type for your situation and by calibrating it properly, you can apply the material with confidence knowing you'll achieve the desired product performance as well as happy customers and a healthy bottom line.

Chris Burrows is an assistant professor of Sports Turf Technology at Nebraska College of Technical Agriculture (Curtis, Neb.).

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