The most expensive mistake on a fertilizer spreader is invisible: the machine runs, the discs spin, the fertilizer flies — and the rate landing per hectare is 20% off target. Too much buries money in the soil and puts unabsorbed nitrogen at risk; too little quietly steals yield. And unlike spraying, you don't see the result immediately; the difference speaks months later, in the colour of the crop.
The right rate has three ingredients: flow (kg/min), effective working width (m) and real speed (km/h). All three are measurable — let's take them in order.
Why the manufacturer's chart is not enough
Spreader setting charts are built for a specific fertilizer density and moisture level. The fertilizer in your yard varies by granule size, humidity and brand; the same gate opening delivers two different flows with two different products. The chart is your starting point — the real number comes from a scale.
Step 1 — Know your effective working width
On a twin-disc spreader, fertilizer is thrown in a wide pattern whose edges are sparse. That is why working width ≠ throwing width: even distribution relies on neighbouring passes overlapping at the pattern edges. Use the effective width your manufacturer publishes for your fertilizer type; if in doubt, a one-time tray test (containers laid out across the pass) reveals your true pattern — and decides the distribution quality of the whole season.
Step 2 — The weigh test: measure kg/min
Spreader calibration rests on a measurement as simple as the sprayer's bucket test:
- Disengage the discs or divert the flow into a collection container (via your model's calibration chute/kit)
- Run for exactly 1 minute at the application opening
- Weigh what came out → that is your kg/min
Repeat this whenever the fertilizer batch changes — the same setting flows differently at different moisture.
Step 3 — Measure your real speed
Spreader rate is tied to speed one-to-one: drive 10% faster and you apply 10% less per hectare. Trust measurement, not the tractor dial — the GPS Speed Meter shows true speed on your phone.
Step 4 — Turn the target into a number
Required flow (kg/min) = Target (kg/ha) × Speed (km/h) × Working width (m) ÷ 600
Example: a 250 kg/ha target at 10 km/h with an 18 m effective width → 250 × 10 × 18 ÷ 600 = 75 kg/min. If your weigh test lands noticeably above or below 75, adjust the opening and repeat.
Step 5 — Verify in the field
The most honest check is the field itself: fertilizer needed ≈ area (ha) × target (kg/ha). Covering 20 hectares at 250 kg/ha should consume 5,000 kg. If the hopper runs out clearly early or leaves product over, look at the three suspects: speed signal, working width, flow calibration. The fertilizer flows in the Nova Support wizard walk you through that elimination step by step.
For planning how much fertilizer you need by nutrient, the Fertilizer Calculator is at hand.
From manual setting to automatic rate control
Everything above assumes constant speed — but speed changes in the field, and on a conventional spreader the rate swings with it. Variable rate control closes that gap: it reads speed continuously, adjusts flow on the fly and holds kg/ha steady across the pass. Our device for exactly this job on fertilizer spreaders is the Nova 130 — with weight calibration and twin-disc support, coming soon. Keep an eye on the product page for the launch.
Frequently asked questions
Do I have to repeat the weigh test for every fertilizer?
If the batch, brand or moisture changed — yes. The same gate opening flows very differently with damp and dry product.
Can't I stretch the working width and make fewer passes?
The pattern edges are sparse; exaggerating the width creates strip-by-strip under-application (visible later as wave-like colour bands in the crop).
What about field borders?
Use the border-spreading setting or deflector to cut the throw on the boundary side — fertilizer landing in the neighbour's parcel or in waterways is both a loss and a liability.
For speed measurement and nutrient planning, the GPS Speed Meter and Fertilizer Calculator live in the Field Hub — free, no sign-up.
