Fertilizer spreaders provide an effective means of accurately applying the proper amounts of fertilizers and pesticides to turf areas. But they must first be calibrated to be sure that the right amount of material is being released to the right area. This requires a little bit of math and some trial and error to get things right.

There are two types of spreaders used on turf areas. They are drop spreaders and broadcast spreaders. The drop spreaders release the material from the bottom of the hopper directly on the surface of the turf. While broadcast spreaders have a spinner that disperses a fan of material over the area (Figure 1). We will begin with the calibration of drop spreaders in this article and go on to broadcast spreaders in the second part. For more detailed information on this subject, see THE MATHEMATICS OF TURFGRASS MAINTENANCE by Christians and Agnew http://www.amazon.com/Nick-Edward-Christians/e/B001H6SB7A/ref=ntt_dp_epwbk_0.

Spreaders have various mechanisms for setting the flow rate of material from the hopper (Figure 2). The higher the setting, the more material will be released and the lower the setting the less is released. The only way to calibrate them is through trial and error and it can take some time and effort to complete. Once you have the calibration number set for a particular material, write it down for next time. Some products will recommend a fertilizer spreader setting for their product on the bag. These can be inaccurate and they should always be checked. You will need to calibrate for each different fertilizer and pesticide that you use on your turf area.

The concept of calibration is the same for drop and broadcast spreaders. You must first calculate how much fertilizer needs to be released from the spreader when it is properly calibrated. You then need to in some way determine how much is being released on a test area. You then by trial and error adjust the setting on the spreader until you have achieved the desired rate. There are various ways to determine how much product is being released. One is to run the spreader over a test area, sweep up the product and weigh it. You can also lay down a plastic sheet on the area which makes it easier to collect all of the released product. There are also catch trays that can be attached to the base of drop spreaders to catch the product so that you can weigh it. The best way, if you have a scale large enough to do it, is to weigh the spreader with fertilizer in the hopper. Run it over a test area and weigh it again. By subtraction, you can determine how much product was released over the test area.

Drop spreaders will vary in width from 18 inches to 48 inches wide. There are even larger tractor drawn models for professional use. For this example, we are going to calibrate a 30-inch-wide drop spreader with the fertilizer that we did the calculations for earlier in this series in the posting titled “Developing a fertilizer program for turf”. In that example we calculated that to apply 1 lb. N/1000 sq. ft. using a 25-5-15 fertilizer, we would need to apply 4 lbs. of fertilizer/1000 sq. ft.

We have a 10,000 sq. ft. lawn. At 4 lbs. fertilizer/1000 sq. ft. we will need 4 x 10 = 40 lbs. of fertilizer applied uniformly to the area to treat the entire lawn at a rate of 1 lb. N/1000 sq. ft.

In this case, the spreader will be operated over a plastic sheet.  The test area on the garage floor will be 20 ft. long, so the sheet should be at least 22 to 25 ft in length.  You will also need a scale that will weigh accurately to the nearest 1/10th ounce (or to the nearest gram if a metric scale is to be used). Tape the edges of the sheet to the floor and mark the 20 ft test strip.

To make this work, we are going to have to calculate how much fertilizer will be released on a test strip 36 inches wide and 20 feet long. If 4 lbs. of 25-5-15 fertilizer are applied/1,000 ft², how much will be released on the test strip when the proper calibration has been achieved. 

The test strip over which the spreader will be operated is 20 ft long and the spreader is 36 in wide (3 ft.).

20  feet

 36 in

(3 ft.)

So, if the test strip is 3 ft wide and 20 ft. long, there are 60 ft² in the test strip:

(3 ft.) (20 ft.)  =  60 ft²

If 4 lbs. of 25-5-15 fertilizer are to be applied to 1,000 ft² to achieve a rate of 1 lb. N/1,000 ft², how much should be released by the spreader on a 60 ft² test area when it is properly set? To calculate this, set up a proportion. A proportion is a logical question. In this case the question is, “if I put 4 lbs. of fertilizer on 1000 sq. ft., then how much (X) will I need to cover 60 ft. It is written mathematically as shown  below and it is solved   by cross multiplying  and dividing as shown:

        4  lb    ₌     X lb

1,000 ft²         60 ft²

(X) (1,000)   =    (4) (60)

          X     =     240

                        1,000

X   =   .24 lb.

In this case, if I need 4 lbs. on 1000 sq. ft., I will need 0.24 lbs. for 60 sq. ft. One lb. contains 16 ounces, which is:

(16)  (.24)  =  3.8 oz of 25-5-15 fertilizer

When the spreader is properly set, 3.8 oz. of fertilizer will be collected from the 60 ft². test area.

Now choose a starting point for the calibration. If the spreader has a setting bar that is marked off A to Z, I would start with a setting of K or L.  If it is 1 to 10, I would start with a 4 or 5 setting. Now you weigh the product from a test strip. If it is more than the desired amount, bump the setting down a little and if it is less, bump it up a little and try it again. When you consistently get the right amount released on the strip, write down the product name and the setting for future reference.

Speed of operation is important. You must walk at near the same speed for the test as you will walk when applying the fertilizer to the lawn. It is hard to achieve uniformity with a single pass. Unless you are an expert and you may leave skips and misses. A good way to overcome this, if you have the time and energy, is to calibrate at a half rate, in this case 1.9 oz on the test strip (2 lbs. product/1000 sq. ft.) and then go over the area twice at right angles to each other to achieve the desired rate of 4 lbs. product/1000 sq. ft. It takes more time, but the results are generally worth the effort.

There is a final step once you have calibrated the spreader. In this example we were using a 25-5-15 fertilizer and we calculated that it would require 4 lbs. product/1000 sq. ft. The lawn to be treated is 10.000 sq. ft., therefore 40 lbs. product will be needed to complete the job. The final test of your calibration will come when you treat the lawn. If you treat the area and you use only 35 lbs. product, you need to bump up the setting slightly. If you need more than 40 lbs., you need to bump it down slightly. Write down the setting and the next time you fertilize check it again until you get it just right.

 

 

Figure 1. Drop spreader above and broadcast spreader below.

 

Figure 2. Setting apparatus for drop spreader above and broadcast spreader below.

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. 5^th ed  http://www.wiley.com/WileyCDA/WileyTitle/productCd-1119204631.html

Web site:  https://www.hort.iastate.edu/directory/nick-christians/

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