How to Calibrate a Rotary Spreader by Weight Over a Measured Area

Why this matters

The setting chart on the bag and the numbers on the spreader dial are a starting point, not a rate. They were worked out on a spreader in good condition, walked at the maker's assumed speed, with product from one production run. Your spreader has a worn shutoff plate, your tech walks a little fast, and this lot of granules flows differently from the last. The only way to know what a setting puts down is to weigh what leaves the hopper over an area you measured. A spreader that is 15 percent hot puts 15 percent extra nitrogen on every lawn on the route, all season, and the product math on the work order never shows it.

This card owns spreader output for the lawn shelf: one setting, one product, one tech's pace, proven by weight. Whether the spread is even across the swath is the next card, 'How to Check a Rotary Spreader Pattern With Collection Pans', and it comes first in practice, because the effective swath from that test is the width of the strip you calibrate on here.

What you need before you start

• The target rate in pounds of product per 1,000 square feet, from 'How to Calculate Fertilizer Product From the Bag Analysis'. For a 24-0-6 at 0.75 lb N, that is 3.125 lb, carded as 3.1.
• The effective swath from the pattern test, in feet. Calibrating on the edge-of-throw width instead overstates the area covered and makes a hot spreader look right.
• A scale that reads to 0.1 lb and holds the loaded spreader, or a bucket and a scale that reads the hopper contents. Use the same scale for both weighings in a run.
• A test strip on turf you are treating or on a tarp. Never on pavement: granules on a drive or street wash into the storm drain, and many fertilizer ordinances prohibit leaving fertilizer on impervious surfaces. Blow anything that lands on hardscape back onto turf before you leave.
• The product you will apply, from the bag you will apply. Not a similar product, not last season's leftovers.

PPE is whatever the product label and SDS call for. For a plain fertilizer that is usually gloves and eye protection against dust; for a combination product with a pesticide in it, the label sets the PPE and whether a licensed applicator must handle it, and the calibration is done with that product under those rules.

Step 1: Size the strip so the scale can see the difference

Make the strip long enough that the product used is large against the scale's resolution. On a 1,000 square foot strip at 3.1 lb per 1,000, the spreader uses about 3.1 lb, and a scale reading to 0.1 lb carries a reading error of about 3 percent before you start. Double the strip to 2,000 square feet and the same scale is reading about 6.2 lb, so its resolution is under 2 percent of the result.

Why this matters: you are about to adjust the setting by a step or two based on the difference between runs. If the scale cannot resolve the difference between runs, you are adjusting on noise. A 10 ft effective swath by 200 ft long is 2,000 square feet and fits most back lawns.

Flag both ends of the strip so every run starts and stops at the same line.

Step 2: Time the walk at working speed

Walk the strip once with the spreader closed, timing it. At about 3 mph a tech covers about 4.4 ft per second, so 200 ft takes about 45 seconds. The speed is not the point; consistency is. Write down the time and hold it within a few seconds on every run.

Why the walk is part of the calibration: at a fixed setting, a rotary spreader meters roughly a fixed amount per second through the opening. Walk faster and the same pounds per second spread over more ground, so pounds per 1,000 goes down; walk slower and it goes up. A calibration done at a stroll does not describe the route pace. This is also why a spreader is calibrated per tech: two techs with different paces on the same setting put down different rates.

Step 3: Weigh, run, weigh

Load the hopper with enough product to finish the strip with plenty left over, so the flow does not change as it runs low. Weigh the loaded spreader. Start walking before the start flag, open the gate at the flag, walk the strip at the timed pace, close the gate at the end flag, and weigh again.

Pounds used divided by strip area in thousands is pounds per 1,000. Why weigh the whole spreader and not the hopper contents: emptying and re-filling the hopper spills product and loses granules in the gate housing. Weighing the loaded unit before and after keeps everything in the hopper, and because the spreader frame is on the scale both times, its weight and any fixed offset in the scale drop out of the difference.

Step 4: Adjust and repeat until on target

Compare the result with the target. A common shop tolerance is within 5 percent of target, which for 3.125 lb per 1,000 is about 3.0 to 3.3 lb. Tune that to your products and your program.

If the first run is off, interpolate between two settings to choose the next one, then prove it with a run. Why not just trust the interpolation: dial scales on many rotary spreaders are not linear, and the gate opening near the low end changes flow faster than the numbers suggest. Interpolation tells you where to look. Only a weighed run tells you what you found.

The sheet from one calibration

One spreader, one product, one tech, three runs on a 2,000 square foot strip. Weights are the loaded spreader; the hopper was topped up between runs.

SPREADER CALIBRATION SHEET
Spreader     Truck 3, push rotary, unit R-2
Product      24-0-6 granular, lot 1182
Target       3.1 lb per 1,000 (OK 3.0 to 3.3)
Swath        10 ft effective, from pan test
Strip        10 ft x 200 ft = 2,000 sq ft
Pace         200 ft in 45 s, about 3 mph
Run  Setting  Before  After  Used   Per 1,000
1    12       48.4    43.2   5.2    2.6  low
2    14       47.9    41.1   6.8    3.4  high
3    13       47.6    41.4   6.2    3.1  OK
Locked at 13. Tech JR. Dry, 62 F.

Run 1 at setting 12 used 48.4 - 43.2 = 5.2 lb over 2,000 square feet, or 2.6 lb per 1,000, about 17 percent under the 3.125 target (2.6 / 3.125 = 0.83). Too low.

Run 2 at setting 14 used 47.9 - 41.1 = 6.8 lb, or 3.4 lb per 1,000, about 9 percent over (3.4 / 3.125 = 1.09). Too high.

Interpolating: the two runs moved 0.8 lb per 1,000 across two dial steps. The target sits 0.525 above run 1, so straight-line interpolation points at about 13.3. The dial clicks in whole numbers, so the next run went at 13.

Run 3 at setting 13 used 47.6 - 41.4 = 6.2 lb, or 3.1 lb per 1,000, under 1 percent off target (3.1 / 3.125 = 0.99) and inside the 3.0 to 3.3 band. The interpolation predicted 3.0 at setting 13; the run read 3.1. That gap is the reason the run was made.

Setting 13 goes on the spreader card for this spreader, this product and this tech, and onto the shop's calibration log. See related: 'The Spreader and Sprayer Calibration Log SOP'.

Why every product gets its own calibration

Two products at the same nominal rate can need settings several steps apart on the same spreader. What changes the flow is the granule, not the analysis: particle size, density, shape and how easily the granules slide past each other. A small, dense, round granule pours through a given opening faster than a large, light or irregular one. A product that has picked up humidity in an open bag flows slower and clumps at the gate.

So the setting found here belongs to this product. When the shop switches to a different fertilizer, a lime, a seed or a combination product, the card for that product gets its own calibration, even if the bag chart says the setting is the same. A product from a new production lot that looks different in the hand is worth a check run too.

What changes the answer on the day

• A different tech on the spreader. Re-time the strip at their pace before trusting the card. If their 200 ft takes 38 seconds instead of 45, they are walking about 18 percent faster (45 / 38 = 1.18), so at setting 13 they put down roughly 3.1 / 1.18 = 2.6 lb per 1,000, assuming the spreader meters a fixed amount per second. Either they calibrate their own setting or they walk the card's pace.
• Wet or heavy turf. Push spreaders slow down and wheels slip on long, wet grass, which raises the rate. Calibrate in the conditions the route runs in.
• A worn gate or shutoff plate. If last month's setting now gives a different rate on the same product, the opening has worn or bent. Fix the hardware before resetting the number, or the card drifts every month.
• A pattern problem. If the pan test showed a lopsided throw, fix that first. Calibrating a lopsided spreader to the right average still puts the wrong amount on both sides.

How to know the calibration is holding

The field check is the bags. On a route figured from measured turf, the product used at the end of the day should match the route total on the work order within a few pounds. A route that needed 125 lb and used 108 is about 14 percent light (108 / 125 = 0.86); a route that used 145 is 16 percent hot (145 / 125 = 1.16). Either one means the spreader, the pace or the turf figures are off, and the spreader gets a check run before the next route.

References

• The product label and SDS for PPE, and any applicator licensing a combination product requires.
• The local fertilizer ordinance where the property sits, for application to impervious surfaces.
• See related: 'How to Check a Rotary Spreader Pattern With Collection Pans'
• See related: 'How to Calculate Fertilizer Product From the Bag Analysis'
• See related: 'Rotary Spreader Parts and What Each Setting Changes'