Seed drill calibration (worked)
Calibrating a seed drill step by step: width, wheel turns, test area, weighing seed, wheel skid, row uniformity and fertiliser drills, worked.
On this page
The seed rate printed in a recommendation is only a number until a seed drill actually delivers it. Seed lots differ in size and flow, metering mechanisms wear, and settings on the drill's chart are only approximate. A drill sowing 20 per cent too little gives a thin, weedy crop; 20 per cent too much wastes expensive seed and crowds the plants. Calibration adjusts the drill so it sows the intended rate, and takes less than an hour.
Seed drill calibration is a classic practical and numerical exam question in farm machinery and agronomy courses.
How a seed drill meters seed
A seed drill has a hopper, a metering mechanism for each row, seed tubes and furrow openers, and often press wheels or covering devices. The metering mechanism is usually driven by a ground wheel, so the amount sown is proportional to the distance travelled, whatever the speed. Common metering devices:
- Fluted rollers: the most common for cereals; the exposed length of the flute is adjusted to change the rate.
- Internal double-run feeds, for different seed sizes.
- Cup feeds and inclined or vertical plates with cells for larger seeds such as maize or groundnut.
- Pneumatic (air) metering in precision planters and air seeders.
Seed-cum-fertiliser drills have separate hoppers and meters for fertiliser, calibrated in the same way.
The calibration procedure
- Measure the effective width W (number of furrow openers × row spacing) and the circumference C of the drive wheel (π × diameter, or measured by rolling it one turn).
- Choose a test area A, for example 1/40 ha (250 m2) or 1/10 ha, and calculate the number of drive wheel turns: N = A ÷ (W × C).
- Jack up the drill so the drive wheel turns freely, place bags or containers under each seed tube, and turn the wheel N times at about field speed.
- Collect and weigh the seed, from each tube separately if checking uniformity.
- Calculate the rate (kg/ha = weight collected ÷ test area in ha), compare with the target, adjust the setting and repeat until within a few per cent.
Allowing for wheel skid
In the field, a ground-driven wheel skids or slips, so it turns somewhat less than the distance travelled would suggest, often by 5 to 10 per cent. To mimic this, reduce the number of test turns by the expected skid, or check the rate in the field over a measured distance.
Field check
Fill the hopper to a marked level, sow a measured area, and weigh the seed needed to refill it; or count seeds per metre of row in the field.
Calibrating a wheat drill
A drill has 9 rows at 20 cm spacing and a drive wheel of 1.2 m diameter. The target seed rate is 100 kg/ha.
- Width W = 9 × 0.20 = 1.8 m.
- Circumference C = π × 1.2 ≈ 3.77 m.
- Area per turn = 1.8 × 3.77 ≈ 6.79 m2.
- For a test area of 1/40 ha (250 m2): N = 250 ÷ 6.79 ≈ 36.8 turns. Allowing 5 per cent wheel skid: 36.8 × 0.95 ≈ 35 turns.
- Expected seed: 100 kg/ha × 1/40 ha = 2.5 kg.
After 35 turns the drill delivers 2.2 kg, so the actual rate = 2.2 ÷ 0.025 = 88 kg/ha, 12 per cent low. The operator increases the setting and repeats until the collection is close to 2.5 kg.
Row uniformity. The nine tubes delivered 240, 245, 250, 238, 260, 245, 230, 247 and 245 g (total 2,200 g, mean 244 g). The lowest tube (230 g) is about 6 per cent below the mean; a damaged flute or blocked tube is worth checking.
Seeds per metre. At 100 kg/ha and a thousand-grain weight of 40 g, the drill should sow 100,000 g ÷ 0.040 g ≈ 2.5 million seeds per hectare; with 50,000 m of row per hectare (10,000 ÷ 0.20), that is 50 seeds per metre of row.
Precision planters
Planters for maize, soybean, cotton and vegetables place single seeds at set spacings. They are calibrated by seed spacing rather than weight: count the seeds dropped per metre, check for skips (missing seeds) and doubles (two seeds together), and adjust the plate or vacuum setting. Modern planters with electric drives and seed sensors report singulation and spacing on a cab display, and can vary the rate across a field.
Common problems
- Mismatched settings: the chart in the drill manual assumes a particular seed size; always calibrate with the actual seed lot.
- Seed treatment can change flow, so calibrate with treated seed.
- Worn or cracked flutes, blocked tubes and loose drive chains.
- Hopper level: some meters deliver differently when the hopper is nearly empty.
- Ground speed: although metering is ground-driven, high speed increases bouncing and uneven placement.
Key points
- Calibration matches the drill's actual delivery to the recommended seed rate.
- Turns for a test area: N = A ÷ (W × C), with W = rows × row spacing and C = wheel circumference.
- Collect, weigh and compute kg/ha; adjust and repeat; check each tube for uniformity.
- Reduce test turns by expected wheel skid (often 5–10 per cent), and confirm in the field.
- Always calibrate with the actual seed lot and treatment.
What the exams ask
- School-leaving and entrance papers: parts of a seed drill, and why calibration is needed.
- Degree and research-fellowship entrances: metering mechanisms, the calibration procedure and calculations, and seed-cum-fertiliser drills.
- Agricultural engineering practical examinations: carrying out and reporting a calibration, row uniformity, and field checks.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
How often should a drill be calibrated?
Before each season, and whenever the crop, seed lot, seed treatment or variety changes, since seed size and flow affect the rate.
Does driving faster change the seed rate?
With ground-driven metering, the rate per hectare stays roughly the same, but very high speeds increase bouncing, uneven depth and spacing, and seed damage.
Can the same method calibrate a fertiliser drill?
Yes. Collect and weigh fertiliser from each outlet after the calculated number of turns, and compare with the target rate.
Sources
- Hunt, D. and Wilson, D. Farm Power and Machinery Management, 11th edition. Waveland Press, 2016. Chapter on planting machines.
- Kepner, R. A., Bainer, R. and Barger, E. L. Principles of Farm Machinery, 3rd edition. AVI Publishing, 1978.
- Sharma, D. N. and Mukesh, S. Farm Machinery Design: Principles and Problems. Jain Brothers, latest edition.
- Grains Research and Development Corporation. Seeder Calibration fact sheets. GRDC, Australia.
- Srivastava, A. K., Goering, C. E., Rohrbach, R. P. and Buckmaster, D. R. Engineering Principles of Agricultural Machines, 2nd edition. ASABE, 2006.
More in agricultural engineering and water
Discussion
Questions, corrections and notes from readers on this article. Every comment is checked before it appears: keep it on the topic, and do not share phone numbers, links or contact details.