January 1997 - Volume IV, Issue 1

Fertilizer Product Amounts and Drop Spreader Calibration

David M. Kopec, Extension Turfgrass Specialist

Most lawn fertilizers and a few weed-n-feed products are applied as granular (G) formulations. In order to apply the proper amount of active ingredient, one must be able to properly calibrate a lawn spreader. This issue will deal with the "easy way" to calibrate a drop spreader.

Before any application is made, one must first figure out how much product you will need at a minimum to apply to an area of known (which you will measure) lawn size.

A. Lawn Size:Equations
Square = length x width = square feet
Rectangle = length x width = square feet
Triangle = length x altitude/2 = square feet
Right triangle = length x width/2 = square feet
Circle = (center radius)2 x 3.14 = square feet

Example: rectangle lawn 10 (l) x 63 (w) = 630 square feet

B. Fertilizer Grade Used = 21-7-14 = (.21-.07-.14)
Target Amount Desired = 0.75 lbs. -N-/1000 ft2

C. Minimum Amount of Fertilizer Needed:
Use the following equation,
lbs. nutrient, target amount lawn size
per 1000 ft2 X (square feet) = Fertilizer Product Amount Needed
nutrient percentage 1000

From the information in A and B sections above ...

0.75 lb. x 630 ft2 = 473 = 2.25 lbs. of

0.21 1000 ft2 210 Product

We know that we need 2.25 lbs. of our 21-7-14 fertilizer to apply 0.25 lbs. of nitrogen to the 630 square foot lawn.

We still need to distribute equally to the lawn.

The unsure and risky way to do this is to put 2.25 lbs. of fertilizer in the drop spreader, open the hopper a small amount, and walk several directions until all the material is gone. However, what happens if you walk over the entire lawn once, start another direction and only have enough fertilizer left to cover one-half of the remaining lawn area? You're in trouble! You have a 50% error rate in the application, even though you did everything correctly in steps A-B-C. Steps A-B-C just told you how much material you have to buy! The better way to calibrate is as follows.

A. Measure the hopper width (not the wheel width) of the drop spreader.

Example: O.M. Scotts 30 inch drop spreader = 2.5 feet(w)

B. Measure a convenient length of travel (make a travel path) of known length.

Example: Mark a straight line 50 feet long(l).

C. Compute the amount of area covered by this drop spreader each time you travel a 50 foot path.

Spreader coverage = (l) x (w)

= 50 feet (line) 2.5 feet (spreader)

= 125 square feet, each pass

For convenience, we will want to travel a 50 foot pass, four separate times, which will give us a ground surface area covered by the spreader of 500 square feet.

D. Determine the amount of fertilizer applied.

1) Choose a low setting opening on the hopper (example: setting #5).

2) Weigh out a known amount of 21-7-14 fertilizer. Use a gram scale, or a scale which can measure to within ½ of a lb. (8 ounces). Note that 16 ounces equals 1 lb.

Example: Weigh out 4 lbs. (64 ounces) of 21-7-14 fertilizer.

3) Place the pre-weighed fertilizer into the hopper and spread it evenly along the bottom.

4) Walking at a comfortable ground speed, open the hopper (with the opening at setting #5) and walk 50 feet. Shut the hopper. Turn around and repeat walking this path three more times. This gives us 500 feet square of ground coverage (four passes at 125 square foot, each).

5) Now dump out the remaining fertilizer on a large cardboard, or garage floor (or open up the hopper setting all the way) and re-measure the amount left.

Example: 14 ounces are left, so we applied (64-14) = 50 ounces, or (50/16) or 3.125 lbs. of product in 500 square feet.

6) How much product is this per 1000 square feet? Simply multiply this amount x 2. Remember, our 50 ounces covered 500 square feet.

50 ounces x 2 = 100 ounces

= 100 ounces/16 oz lb. = 625 lbs. product applied/1000 square feet

7)The fertilizer is 27-7-14 analysis or grade. How much nitrogen did we apply from 6.25 lbs. of product per 1000 square feet? 6.25 lbs. product x 0.27 -N- content = 1.7 lbs. of -N-

If 1.7 lbs. of -N- is too high of a application rate, then repeat the process at settings #4 and #3. The amount released is not linear with the setting number. In other words, setting #6 will not put out twice as much material as setting #3. You must recalibrate! It's best to arrive at one-half of the desired target amount and then walk over the turf completely in two directions. This will assure better coverage.

Write down on a sheet of paper the setting #, the amount of product, and amount -N- per 1000 square feet. A calibration worksheet should look like the following. 

Joes 21-7-14 Fertilizer

2.5 foot Drop Spreader

Spreader Setting Amount Product/1000 sq. feet Amount -N- 1000 sq. feet

3 2.65 0.7

5 6.25 1.7

7 10.50 2.2  

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