Seeding Rate from Target Plant Population

A crop is established in plants, and seed is bought in kilograms. This equation is the bridge, and it has exactly four terms because there are exactly four things standing between the two: how many plants you want, how much one seed weighs, how many of those seeds are alive, and how many of the live ones will actually reach daylight.

Work a spring wheat crop through it. The target is 300 plants per square metre, the seed lot has a thousand-kernel weight of 40 g, the tag says 95% germination, and past experience on that field in that seedbed says 85% emergence. Establishment is $0.95 \times 0.85 = 0.8075$, so you have to sow $300 / 0.8075 = 371.5$ seeds for every square metre. Each of those weighs $40/1000 = 0.040$ g, so the rate is $371.5 \times 0.040 = 14.86$ g/m². A gram per square metre is exactly ten kilograms per hectare, so the answer is 148.6 kg/ha — and the same crop on a cold, cloddy seedbed at 65% emergence needs 194 kg/ha to reach the identical stand.

The mistake that costs the most is treating the tag germination as the whole story. Germination is measured on moist paper at a controlled temperature by a laboratory that is trying to be fair to the seed. Field emergence is measured by the seed, in your soil, at your seeding depth, against whatever the weather did in the ten days after the drill went through. The gap between them is never zero and is frequently thirty points, and a rate calculated from germination alone under-seeds by exactly that.

The second mistake is carrying last year's rate forward when the seed lot changed. Thousand-kernel weight is the term with the widest swing in the whole equation: canola runs 3 to 6 g, wheat 30 to 50, corn 250 to 350, and field beans past 500. Two wheat lots at 33 g and 47 g differ by 42% in weight for the identical plant stand, so a grower who sows "the usual 150 kg/ha" gets a thin crop one year and a thick one the next and blames the weather. Weigh a thousand seeds — it takes ten minutes and a counting board, and it is the cheapest measurement on the farm.