Calibrating Your Fertilizer Spreader

Most granular fertilizer products have a chart on the back of the bag in which they give settings instructions for fertilizer spreaders, to ensure proper amounts of the plant food are applied.

But, what do you do if your spreader isn’t listed, and if there is no online or phone support for your problem?

I have two ways that I cope with this issue. Both begin with determining the square feet to be covered. That’s a simple length X width measurement and then the multiplication to put it into square feet.

First option:
Blow a section of your driveway clean. Put a known amount of the fertilizer in your spreader – say 2 pounds.

As you start walking across the drive, open the spreader and let the plant food distribute across the concrete.

See how many square feet you can cover with two pounds, then figure that you can get 10X that coverage with a 20-pound bag. (Use a different multiplier for bags of different weights.) If you multiply the area you’ve covered on the driveway by that same 10X (or whatever figure you end up with) factor, you’ll see how far your spreader would have gone at that setting. That will tell you whether you need to open it up or close it down (or, if you’re lucky, leave it alone).

See how closely that 10X figure compares to the coverage statement on the fertilizer bag, for example: “One bag covers 2,000 square feet.”).

Adjust the settings accordingly and try one more time to be sure you’re fairly close. Fertilizer rates are not a precise and exact science.

Finish by sweeping the fertilizer you’ve spread on the concrete back into a pile. If it’s “clean,” you can run it through the spreader and onto your lawn, or you can sprinkle it by hand over shrub and perennial beds.

Blow the drive a second time, to get rid of any remaining granules that might contain iron. Iron can stain concrete, so don’t try this calibration technique when the driveway is moist.

Second Option:
(My prime choice because it’s easier)

The worst thing you can do is to run out of fertilizer (or any other granular product) halfway through your first pass across the lawn. That means that half of your grass will remain untreated, while the other half has twice as much as it should.

Therefore, intentionally start with the spreader letting just a little of the product out as you walk.

Your goal will be to walk the lawn at least two times, the first going north-south, and the second going east-west.

If you have to make a third pass over the lawn to put out the correct amount for that size of area, so much the better. This will be the most accurate application of all.

At some point in this trial-and-error, you’ll stumble across the setting that is, as Goldilocks said, “just right.” Make a note of it, and hang that note in the garage. Better yet, use a Sharpie and just mark it on the spreader.

Blow any granules that scatter on the patio or walk to keep them from staining.

Note About Spreaders
Most of us who have used “drop” fertilizer spreaders would be happy if we never had to use them again. It is so very difficult to apply fertilizers and other products precisely. It seems that many of us ALWAYS end up with stripes of yellowed grass alongside stripes of double-fed turf.

If you’re doing precise research, and if you need to know exactly how much of a product is being applied to precisely how many square feet, a drop spreader is your ticket.

However, for most of us mere mortals, a rotary spreader is like a gift from Heaven. They’re quicker. They’re easier to push. They’re dependable. But, most of all, no stripes!

It’s ultimately your call, of course. But, if you’re trying to cut the time you spend doing routine things like fertilizing (so you can be planting flowers instead), a rotary spreader will really be your pal.

Posted by Neil Sperry
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