Calibrating a seed drill means measuring what the machine actually puts on the ground, not trusting the chart. Set it to the chart setting for your seed, run it until the meter is primed, then catch everything the drill drops over a measured distance and weigh it. On a three-foot drill, 145 feet is one hundredth of an acre. Weigh your catch in grams, multiply by 0.22, and you have pounds per acre. Adjust, run it again, and repeat until you are within about 10 percent of your target. Do it in the field you are planting, at the speed you will drive, with the seed lot you actually bought. Published rate charts miss real output by 25 to 50 percent. Native seed is too expensive to learn that in June.
Key Takeaways
- Rate charts are a starting point. NRCS says so in its own drill calibration guidance, and published settings can miss actual output by 25 to 50 percent.
- On a three-foot drill, 145 feet is one hundredth of an acre. Catch the seed, weigh in grams, multiply by 0.22.
- Prime the meter before you start measuring, or your first run reads low.
- Calibrate every box separately. Recalibrate for every seed lot, even the same species from the same supplier.
- For fluffy native seed, calibrate by driving the machine rather than turning the wheel in the shop.
If there is a rate chart on the machine, why calibrate at all?
Because the chart gives you a ratio and you need a rate. The setting tells the meter how fast to turn relative to the ground wheel. What actually falls through at that setting depends on the seed, and seed changes every year.
Mississippi State extension lists what moves the number: ground speed, seed cleaning method, seed size, shape, density, weight, purity, and any coating. Their estimate of how far a chart setting can land off is 25 to 50 percent. They also say to recalibrate for the same variety you ran last year, because the lot changed even if the bag says the same thing.
NRCS says it too, in its guidance for the Truax drill most conservation districts rent out. The charts in the seed boxes and the manual are a starting point for calibration, not the end of it. That is a manufacturer-neutral agency saying it about every drill on the market, ours included.
Whether it is worth an afternoon comes down to what is in the box. Nobody calibrates carefully for oats. A diverse native mix runs several hundred dollars an acre, and 30 percent of that is real money on ground you will plant once. Mississippi State makes the same point. Calibration matters more as seed gets expensive.
How do you calibrate a seed drill in the field?
Seven steps, an hour, a kitchen gram scale.
- Set the drill to the chart setting for your seed, or for the seed on the chart closest in size if yours is not listed.
- Fill the box so there is four to six inches of seed sitting over every opening. A nearly empty box meters differently than a full one.
- Prime the meter. Drive 100 to 200 feet with the drive engaged before you measure anything. University of Georgia extension calls starting with a primed meter crucial, and skipping it is the most common way a first run reads low.
- Rig your catch. Bag or cup every outlet, or lay a tarp where the seed drops. Check that nothing is obstructed before you start.
- Flag your distance from the table below, on ground like what you will plant.
- Drive it at the speed you will actually plant. Do not creep through the test and then plant at twice that.
- Weigh and do the math. Adjust, then run it again until you are within about 10 percent of your target.
| Test distance on a 3-foot drill | Fraction of an acre | Convert your catch |
|---|---|---|
| 32 feet | about 1/454 | grams caught equals pounds per acre |
| 145 feet | 1/100 | multiply grams by 0.22 |
| 290 feet | 1/50 | multiply grams by 0.11 |
The 32-foot version is a nice shop trick because the arithmetic disappears. A short run magnifies every small error, though. Use 145 feet for anything you care about, and 290 feet when you are metering a low rate and want more seed on the scale. For any other width or distance, the general formula from Mississippi State is area in acres equals width times distance divided by 43,560.
Can you calibrate in the shop instead?
Yes. In September that is the sensible version. You are not going to want to be flagging 145 feet in a November wind.
Jack the drive wheel clear, engage the drive, and turn it by hand a set number of times. First you need your wheel circumference, and no spec sheet is required. Mark the tire, roll the machine one full revolution across the floor, measure what it covered. Revolutions equals your test distance divided by that number. NRCS uses the same approach in its Truax procedure.
Two honest limits. There is no wheel slip on concrete and no bouncing, so a machine set in the shed meters a little differently on a side hill. University of Georgia calls the stationary method usually satisfactory, with the caveat that bumpy or hilly ground should be confirmed under real conditions.
The second limit is worth knowing. NRCS says outright that a fluffy seed box is often more accurately calibrated by driving a known distance than by turning the wheel in the shop. Running big bluestem, Indiangrass, or a warm-season mix? Go drive it.
How do you calibrate a drill with two seed boxes?
Separately. Every time. University of Georgia and NRCS both say so, and it is the step people skip because it doubles the work.
Large seed in the large box, small seed in the small box, and each gets its own run and its own number. A mix split across two boxes cannot be calibrated as one figure. The meters are set independently and neither knows what the other is doing.
Two things go wrong after you think you are done. Small seed settles to the bottom of a box as the machine bounces, so the mix coming out at hour three is not what you weighed at hour zero. University of Georgia suggests remixing periodically, or only loading what you will plant in about an hour and a half. On the ground a compact drill works, take the shorter interval.
The other one is bridging. Fluffy warm-season seed packs and arches over the opening, and a box that metered perfectly on your test run can meter air across half a plot while you keep driving. Open the lid and look partway through, not just at the start.
What if your rate falls between two settings?
Some drills have an infinitely variable gearbox and you can dial in any number you want. Most ground-driven, chain-driven machines give you a fixed set of sprocket combinations instead. You land near your target, not on it. That is a real tradeoff. A chain and sprocket drive has almost nothing in it to fail. You pay for that with less precision.
The fix is a second pass. Set the drill to the step just under your target and cover the ground twice, at right angles or on an offset. The Tallgrass Prairie Center recommends the double pass for uniform coverage regardless, and notes that it leaves shallow channels that hold seed and slow water across the surface.
If you cannot make a second pass and have to pick a side, go slightly over rather than under. Extra seed costs you money once. A thin stand costs you a growing season. On natives, three of them.
How do you check seeding depth while you are at it?
Do both in the same session. Rate and depth are the two things that decide whether a planting works, and NRCS names planting too deep and poor weed control as the two main reasons grass seedings fail.
The hard part is that native seed is small and brown and so is dirt. University of Georgia has the answer. Spray a quart bag of your seed lightly with orange turf paint, let it dry, and load that above the opening before you pour the rest. Now your first rows are visible and you can set depth against something you can actually see.
Shallow is what you want. Native seed should end up no deeper than a quarter inch, and NRCS gives a check that needs no tape measure. If you cannot see a few seeds on the soil surface, you are too deep. The same applies when you are dormant seeding in November, when harder ground makes it easy to go deeper than you meant to.
Where the Dew Drop Drill fits
Ground-driven metering is the design decision that matters most here, and it is worth being precise about what it buys you. Both boxes on the Dew Drop Drill are driven off the wheel through a chain, so your rate does not change when your speed does. On a side hill you speed up and slow down constantly without meaning to. A machine metering off anything but ground travel changes your rate every time you do. Speed comes off the list of variables. Seed size, cleaning, coating, and lot do not. That is why you still calibrate.
The large box runs five picker wheels with five agitators and five rates set by quick-change sprockets. The small box meters through five fluted cups with nine rates and runs on its own or alongside the large one. Two boxes, two calibration runs. Five and nine discrete settings means you will sometimes land a step off target. That is what the second pass above is for.
The sprocket chart is mounted inside the cover. That was deliberate. It lives on the machine instead of in a manual in a drawer. Treat it the way NRCS treats every rate chart: a good starting point. At three feet wide your test run is 145 feet, which you can flag along a farm lane in ten minutes. The drill sells for $9,395 plus shipping, direct from the shop in Spencer.
FAQ
How often should you calibrate a seed drill?
Every seed lot. Mississippi State extension recommends recalibrating even for the same variety, because the lot and the year changed. Coated seed is the clearest case, since coatings can add 20 to 50 percent to the weight and volume going through the meter.
Should you calibrate with the actual seed you are planting?
Yes, and with the actual lot. Calibrating with last year’s leftover switchgrass tells you how last year’s switchgrass flowed. Seed size and cleaning vary enough between lots to move your rate meaningfully. That is the whole reason the exercise exists.
Do you calibrate to the bulk rate or the pure live seed rate?
The drill meters bulk seed, so your calibration target is a bulk number. Convert first. PLS is purity times total germination, and total germination includes hard and dormant seed. Divide your PLS target by that percentage for the bulk pounds per acre you are calibrating to. Worked math is in the guide to choosing a seed drill for small acreage.
Can you skip calibration on a ground-driven drill?
No. Ground drive fixes your rate against travel speed, which is one variable out of several. It has no idea what is in the box. A setting that put down 12 pounds an acre of a clean, slick seed lot will not put down 12 pounds of a fluffy, chaffy one.
Not sure where to start?
Running a rented drill? Ask your conservation district for their calibration sheet. Most have one, and the staff will usually walk you through it. That help is worth as much as the machine. If you are running one of ours and want a hand setting it for a mix that is not on the chart, send us a note with your seed tags and target rate.