Granular Applicator Calibration & kg/ha from a Catch Test

Granular insecticides and fertilisers must go on at the labelled rate — too little fails to control the pest or feed the crop, too much wastes product, damages plants and risks residues. Spreaders and applicators drift off-rate as granules, settings, ground speed and wear change. A catch test measures what you are actually applying so you can set it right before treating the whole field.

Run the applicator over a measured test strip — a known length of one or more rows, or a set area — catching the granules in a tray, pan or cloth instead of letting them hit the ground. Weigh what you collect, and the tool scales that catch from the test area up to kilograms per hectare. That figure is your actual application rate.

Actual rate = caught weight ÷ test area, scaled to a hectare. The tool takes the grams or kilograms you collected over the strip's area (length × working width, or rows × spacing × length) and converts to kg/ha. It then compares that to your target rate and shows the deviation as a percentage so you know which way and how far to adjust.

Aim to be within about ±5% of the target rate. A small deviation is normal and tolerable, but more than 5% means the gate, metering or speed needs adjusting. The tool flags whether you are over, under or on-rate so you can correct it before committing product to the whole field — far cheaper than discovering it after application.

If you are applying too much, close the gate or metering opening a step or drive faster; if too little, open the gate or slow down. Rate scales inversely with ground speed and roughly with gate opening, so a small change in either moves the rate. Re-run the catch test after each adjustment until you land within ±5% of target.

Very much. Granular applicators meter out granules over distance, so the faster you drive, the more ground each kilogram covers and the lower the rate per hectare — and the reverse when you slow down. Calibrate at the exact speed you will treat at, keep that speed steady in the field, and re-check if you change gear or terrain forces a different pace.

Yes. Granule size, density, flowability and moisture all change how fast product meters out of the same setting, so a calibration done for one product or batch may not hold for another. Damp or humid conditions can clog or slow flow. Recalibrate whenever you switch product, open a new batch, or conditions change noticeably.

Weighing the output over a whole hectare is impractical, so you catch over a small, measured strip and scale it. As long as the strip is representative — steady speed, normal gate, even ground — the kg/ha figure is reliable. Catch over a longer strip or repeat the test and average to reduce the error from a one-off short run.

Yes for the rate-per-hectare maths, but spinner broadcasters also need their spread pattern checked across the working width with a tray test, because granules don't land evenly edge to edge. Use this calculator for the overall application rate; check the distribution pattern separately so both the amount and the evenness are right.

They're as precise as your catch test. Accuracy depends on measuring the test area correctly, weighing the catch cleanly, and running at a steady, representative speed. Repeat the test, average the catches, and recalibrate when product, batch or conditions change — calibration is a routine check, not a one-time job.