Seed rate calculator
Seed rate calculator
Seed for a field at the sowing rate for the crop, adjusted for the germination and purity printed on the lot.
How this is calculated
Seed = area x rate. Where germination and purity are entered, divide that by their product as a fraction, so a lot at 80 per cent germination and 95 per cent purity needs seed / 0.76.
- Germination and purity are multiplied, not added: the fraction of the sack that is both clean and alive is the only part that can become a plant.
- A higher sowing rate is not a yield strategy. Past the point where the crop closes its own canopy, extra seed becomes extra competition, so the rate is the one the variety was recommended at and the adjustment is only for the lot you are holding.
The seed rate on a packet is what the crop needs to raise a stand. The seed goes into the ground at a rate that has to survive the difference between the two — and germination and purity are what stand between them.
A rate is not a plant population
A seed rate is a mass per area, and it changes with the variety, the size of the seed and the method: a drill puts fewer seeds where each one can be reached than broadcasting does, and a hybrid is advised at a lower rate than a farm-saved crop of the same species. This page does not hold a table of rates for that reason — it takes the figure you are working to.
Germination and purity are multiplied, not added, because the part of the sack that can become a plant is the intersection of the two: seed that is clean but dead and seed that is alive but full of inert matter both fail to establish a crop. A lot at 80 per cent germination and 95 per cent purity has 76 per cent of its mass able to grow.
Worked through
A 1.2 ha field at 75 kg per ha is 90 kg of seed on the plain rate. The certified tag for the lot reads 80 per cent germination and 95 per cent physical purity, so the fraction of the sack that is both alive and clean is 0.80 × 0.95 = 0.76.
90 ÷ 0.76 = 118.42 kg goes into the drill — 28.42 kg more than the recommendation, and an effective 98.68 kg per ha. Without that allowance, the 90 kg sown establishes a stand the crop was never advised to be grown at.
Questions readers ask
Should I sow more seed to get a better yield?
Not past the rate the variety is recommended at. Extra seed beyond the point where the crop closes its canopy buys tillering or branching, competition for light and water, and a stand that lodges — the adjustment on this page is for the quality of the lot in your hand, not a yield strategy.
What is the difference between germination and purity?
Germination is the share of pure seed that produces a normal seedling under the test conditions; physical purity is the share of the sample that is seed of the crop at all, once inert matter, other crop seed and weed seed are taken out. A seed lot has both figures on its tag and both reduce the established stand.
My seed is treated. Does that change the answer?
It changes the mass, not the germination. Fungicide or insecticide coatings add a small percentage to the weight of a sack, so a rate expressed in kilograms of loaded seed needs the coating weight taken out before the stands agree.
Why does the page show a rate per hectare at the bottom?
Because the sowing figure and the recommended one are in different units of account. The last line converts the adjusted mass back to a rate, so you can see that 118.42 kg over 1.2 ha is 98.68 kg per ha and check it against what the packet advised.
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