BROADCAST SPREADER CALIBRATION
, 6 min reading time
In the previous posting, drop spreader calibration was discussed. Here we will consider the calibration of broadcast spreaders. The concept is the same. First calculate how much fertilizer is needed per 1000 sq. ft. and then set up a test strip and calculate how much fertilizer should be released on the strip. This is more difficult because the broadcast spreaders throw the material much further and it is harder to catch and weigh. There are catch trays made for calibrating broadcast spreaders but they are expensive and generally used by professionals. If you are a professional, I would recommend buying one. They save a lot of time and effort if you do a lot of calibration. However, most homeowners or others that do this infrequently will likely choose another method. If you have a floor scale large enough to weigh the spreader and fertilizer together, you can operate the spreader over the test strip, reweigh, and by subtraction determine how much material was released. An alternative is to put a known weight of fertilizer in the hopper, operate the spreader over the test strip and pour the remaining material into a bucket and weigh it with a smaller scale. This works, but it is very time consuming.
The first step is to determine the effective spread width of the fan of material. This will vary with material and with walking speed. First, put some fertilizer in the hopper and set your walking speed. Run a test strip on a turf area. It is useful to have some help with this and have a couple of others mark the extent of the throw on both sides. Next measure how wide the fan is and that will be the effective spread width for that material.
You will also need to consider how you are going to apply the material. You can go from fan to fan as is shown in Figure 1 or overlap wheel track to wheel track as in Figure 2. The overlap means that you will have to calibrate the spreader at one half rate to apply the right amount to the area. If you go fan to fan, you may want to calibrate at a half rate and go over the area twice at right angles to one another to improve uniformity (Figure 3).
For this example, we are going to use the same fertilizer used in the drop spreader example. This was a 25-5-15 to be applied at 1 lb. N/1,000 sq. ft. We are going to assume fan to fan and go over the area twice (Figure 3), meaning that we will calibrate at a half rate or 0.5 lbs. N/1000 sq. ft. For the drop spreader example, we determined that 4 lbs. of fertilizer/1000 sq. ft. would be needed to achieve the 1 lb. N. We are then going to calibrate the broadcast spreader to release 2 lbs. fertilizer/1000 sq. ft., which will give us 0.5 lbs. N on each pass. The spreader has been determined to have an effective spread width of 12 ft. for this fertilizer. A 60 ft test strip is to be used. So the question is, how much fertilizer will be released from the spreader on the 60 ft x 12 ft. test strip when it is properly calibrated?
60 ft.
12 ft.
(12) (60) = 720 ft2
The test strip is 720 ft2.
If we need to release 2 lbs. fertilizer/1000 sq. ft., then how much will be applied to 720 ft2? As before we will set up a ratio and ask a logical question, “If 2 bs are needed for 1000 sq. ft., how much is needed for 720 sq. ft.? Cross multiply and divide to get the answer:
2 lb = X lb
1,000 ft2 720 ft2
(1,000) (X) = (2) (720)
X = 1440
1,000
X = 1.44 lb. of fertilizer
When properly calibrated, the spreader will release 1.44 lb. of fertilizer on the 60 ft. test strip. There are 16 oz/lb., therefore:
(16)(1.44) = 23 oz. of fertilizer will be released on the strip.
For this example, we will be using a floor scale large enough to weigh the spreader. Weigh the spreader and fertilizer together, then operate it over the 60 ft. test strip and weigh it again. By subtraction, you can determine how much material was applied on the test strip.
Begin by setting the apparatus on a medium setting. If the first trial is less than 23 oz., bump it up and if it is more than 23 oz. bump it down. Continue this process until you have achieved the proper setting.
In the drop spreader example, we calculated that 40 lbs. of fertilizer would be needed to treat the 10,000 sq. ft. lawn. As a final test, make two passes over the lawn at right angles to one another. You should use 40 lbs. of fertilizer. If not, adjust the setting accordingly. Write it down for future applications.
If you lack the equipment for calibration, there is an alternative for small areas. Calculate the amount of fertilizer needed, in this case 40 lbs. for a 10,000 sq. ft. lawn. Set the spreader at a very low setting and go over the area at right angles until you have applied the desired amount of product to the area. This is a lot of work, and for larger areas, you will definitely want to calibrate.
Figure 1. Fan to fan application. Picture from Mathematics of Turfgrass Maintenance.
Figure 2. Wheel track to wheel track overlap. Picture from Mathematics of Turfgrass Maintenance.
Figure 3. Applying material twice at right angles. Picture from Mathematics of Turfgrass Maintenance.
Nick Christians, University Professor of Horticulture, Emeritus, Iowa State University
books: http://www.amazon.com/Nick-Edward-Christians/e/B001H6SB7A/ref=ntt_dp_epwbk_0
turf blog: http://www.iaturf.blogspot.com/
Fundamentals of Turfgrass Mgt. 5th ed http://www.wiley.com/WileyCDA/WileyTitle/productCd-1119204631.html
Web site: https://www.hort.iastate.edu/directory/nick-christians/