April 1999 - Volume VI, Issue 4

Calibrating a Drop Spreader for Home Lawns

David M. Kopec, Extension Turfgrass Specialist

In the previous issues (January - March, 1999) fertilizer terms, costs, types of fertilizers and the universal fertilizer equation were discussed.

From that information you could determine the type of fertilizer to select for the lawn, the price per unit of fertilizer nutrient and how much fertilizer by weight you need for a particular application. Now it's time to figure on how to apply the fertilizer evenly. It's time to calibrate a fertilizer spreader.

If you do not calibrate the spreader, it is increasingly difficult to apply the product uniformly across the lawn. You can apply small amounts of fertilizer across the lawn and walk the spreader in many directions until all the fertilizer is gone. If you walk more than twice, you=re probably tempted to open the hopper and release the fertilizer out at a faster rate.

Described below is the easy way to use a fertilizer drop-type lawn spreader. These are the box-type units in which the fertilizer drops out of the bottom of the hopper and falls directly below the unit. The fertilizer is not thrown away from side to side, but falls directly below the spreader only. Most drop-type spreaders are usually either 2 feet, 2.5 feet or 3 feet wide. This is the inside width of the hopper and is not the distance from wheel to wheel.

When applying a granular fertilizer, make sure you overlap from hopper width to hopper width! If you overlap on the wheel marks you will leave a 4-5 inch strip of unapplied area each time you turn around! The lawn will have small stripes in it as a result.

1. Measure the hopper width (not the wheel width) of the drop spreader.
Example: O.M. Scotts 30 inch drop spreader = 2.5 feet(w)

2. Measure a convenient length of travel (make a travel path) of known length.
Example: Mark a straight line 50 feet long(l).

3. 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.

4. Determine the amount of fertilizer applied.

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

b). Weigh out a known amount of 21-7-14 fertilizer. Use a gram scale, or a scale which can measure to within 2 of a lb. (8 ounces). Note that 16 ounces equals 1 lb. Example: Weigh out 4 lbs. (64 ounces) of 21-7-14 fertilizer.

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

d). 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).

e). 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 3.125 lbs. of product in 500 square feet.

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

50 ounces x 2 = 100 ounces
= 100 ounces/16 oz lb. = 6.25 lbs. product applied/1000 square feet

g). 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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