Spreader Uniformity & CV from Tray Catches
Reveals streaks
Lay trays across the swath, spread, and weigh each one — then enter the catches to get the mean, the CV% and a uniformity rating. Good is below 15%; above 25% shows as stripes.
Test your spread pattern
Enter the amount caught in each of 5 collection trays across the swath (any consistent unit — grams or mL).
Next: CV is 6.1% — under 15%, so the pattern is good. Keep this overlap and disc setting.
CV = standard deviation ÷ mean × 100. <15% good, 15–25% fair, >25% poor — striping appears in the crop above ~25%.
Spreader uniformity — key facts
- Metric
- CV% = SD ÷ mean × 100
- Good
- below 15%
- Fair
- 15–25%
- Poor
- above 25% (stripes)
- Method
- catch trays across the swath
- Fix levers
- vanes, disc, bout width, speed
- Units
- any consistent (unit-free CV)
- Privacy
- Runs in your browser; nothing uploaded
An even pattern is worth more than an exact rate
You can hit the right average rate and still lose yield if the fertiliser lands unevenly. A row of catch trays across the swath samples the real spread pattern, and the coefficient of variation boils that down to one number: how much each tray strays from the mean. Below 15% is even, 15–25% is workable, and above 25% turns into the pale-and-green stripes you can see from the road — because the crop yields to the starved strips, not the heavy ones.
This tool gives the mean catch, the CV% and a uniformity rating straight from the tray weights you enter. Use it to calibrate spinner vanes, choose the right bout width, check a new product, and prove the fix worked by re-testing. Pair it with the Sulphur Nutrient, Exchangeable Acidity Lime and Fertigation tools for a full nutrient-management plan.
See the pattern
Turn tray weights into one clear CV% number.
Kill the stripes
Catch a poor pattern before it shows in the crop.
Set the bout width
Check that overlap evens out the spread between passes.
Prove the fix
Re-test after adjusting vanes, disc or speed.
Frequently Asked Questions
How is the coefficient of variation calculated?+
CV% is the standard deviation of the tray catches divided by their mean, times 100. The tool takes every catch weight you enter, finds the mean, computes the spread around it, and reports CV% along with the mean catch and a uniformity rating. A lower CV means a flatter, more even spread pattern across the swath.
What is a good CV for fertiliser spreading?+
As a rule of thumb, below 15% is good, 15–25% is fair, and above 25% is poor and likely to show as stripes in the crop. Granular nitrogen on visible crops is judged more strictly than lime or P and K, where slightly higher CVs are tolerated. The tool flags the band your result falls in.
How do I run the tray test?+
Lay a row of identical collection trays at even spacing across the full working width, drive the spreader over them at your normal speed and rate, then weigh the contents of each tray. Enter the catch weights in order. The trays sample the spread pattern, so spacing and consistency matter — use the same trays and a level surface.
How many trays should I use?+
More trays give a truer picture; many test kits use a tray every half metre or metre across the swath, giving roughly 15–30 trays for a wide spreader. At a minimum sample enough points to capture the shape of the pattern from one edge, through the centre, to the other edge — too few trays can hide a streak.
Why does uniformity matter?+
Uneven spreading puts too much fertiliser in some strips and too little in others. The over-fertilised strips can lodge, burn or waste nutrient, while the starved strips yield less — and the eye sees the difference as green and pale stripes. Because the crop responds to the low strips, poor uniformity quietly caps the whole field's yield.
What causes a high CV?+
Common culprits are worn or wrongly set spinner vanes, the wrong disc or impeller for the product, an off-centre or wrong drop point onto the disc, segregated or damp fertiliser, incorrect bout (swath) width, and driving too fast. The tray test tells you the pattern is bad; adjusting these one at a time and re-testing tells you what fixed it.
Does bout width affect uniformity?+
Yes — broadcast spreaders rely on overlap between passes to even out a pattern that is heaviest in the centre and tapers at the edges. Run too wide a bout and the tapered edges don't overlap enough, leaving low strips between passes. The right bout width for your machine and product is what turns an individual pass into an even field.
What units should I use for catches?+
Any consistent unit — grams, or even tray counts — works, because CV% is a ratio of spread to mean and is unit-free. Just weigh every tray in the same unit and enter them in order across the swath. The mean catch is reported in whatever unit you entered.
Are the figures precise?+
The CV and mean are exact for the catches you enter, but they only describe that one test run. Pattern can shift with product, humidity, fill level and wear, so re-test after changing product or settings and ideally a few times a season. Use the result to set up and verify the spreader, not as a one-off certificate.