Seed rate follows plant population, not the other way round. Buying a round number of kilogrammes and planting until it runs out is how a field ends up half overcrowded and half gapped. Work back from the stand you want and the order falls out of the arithmetic.
The three steps
One: the population you want standing. That comes from your spacing, and the sum is in how to calculate plant population per acre.
Two: seeds to sow = target population divided by the expected germination rate. At 85 per cent germination, 18,000 standing plants needs 18,000 divided by 0.85, which is about 21,200 seeds.
Three: convert seeds to weight using the seed count per kilogramme. Weight needed = seeds to sow divided by seeds per kilogramme.
Worked through on maize
Target 18,000 plants per acre. Germination tested at 85 per cent, so 21,200 seeds to sow.
If the seed runs at about 3,000 seeds per kilogramme, that is 21,200 divided by 3,000, which is about 7 kg per acre.
Seed size varies enough between varieties to matter. A larger grain at 2,500 seeds per kilogramme needs 8.5 kg for the same stand; a smaller one at 3,500 needs 6 kg. Buying by weight without knowing the count is why the same order plants a different area each season.
The crop-specific figures are in maize seed rate per acre and bean spacing and seed rate per acre.
Where to get the germination rate
Certified seed carries a germination percentage on the pack, and for fresh seed from a reputable supplier that figure is usually sound. Two situations call for testing it yourself: seed held over from last season, and seed bought loose.
The test is a hundred seeds on damp cloth or in damp sand, kept warm, counted after the days that crop normally takes. The method and the counting rules are in how to test seed germination, and what the percentage means for your order is in understanding seed germination rates.
Germination is not the same as emergence. Seed that sprouts on cloth in a warm room may not push through a crusted seedbed, so field emergence usually runs below the laboratory figure. If your own fields consistently come up short of what the pack promised, use your observed figure rather than the printed one.
Add for gapping, not for luck
A small allowance above the calculated figure is reasonable, because some seed will be taken by birds or termites and some stations will need filling. Five to ten per cent covers it.
A large allowance is not insurance, it is overcrowding. Sowing double and never thinning gives a stand that competes with itself for the same water and the same fertiliser, and the usual result is a thin cob on every plant rather than a full cob on half of them.
When seed rate is quoted per hectare
Most published rates are per hectare. One hectare is about 2.47 acres, so an acre needs about 0.405 of the hectare rate. A rate of 25 kg per hectare is about 10 kg per acre. The conversions are in acre to hectare conversion for farmers.
Frequently Asked Questions
How much maize seed do I need for one acre? Work it back from the stand you want. At 18,000 plants per acre, 85 per cent germination and roughly 3,000 seeds per kilogramme, the arithmetic gives about 7 kg, plus a small allowance for gapping. Change any of those three inputs and the answer moves, which is why a single remembered number is a poor substitute for the sum.
Does seed rate change with spacing? Yes, directly. Seed rate exists to deliver a population, and the population comes from the spacing. Closer spacing means more plants, which means more seed. If you change the spacing without redoing the seed order, you will run out or have seed left over, and the stand will be wrong either way.
Should I use the germination rate printed on the pack? For fresh certified seed from a supplier you trust, yes. For seed carried over from last season or bought loose, test it: germination falls with time and with poor storage, and the printed figure describes the seed when it was packed, not the seed in your hand now.
Is it safer to plant extra seed? A little, yes, and a lot, no. Five to ten per cent above the calculated figure covers losses and gapping. Much beyond that produces a crop competing with itself, and unless you thin it back the stand will carry the cost all season.
Why is seed sold by weight if what matters is the number of seeds? Because weight is what a shop can measure. The link between the two is the seed count per kilogramme, which varies by crop and by variety. Ask for it, or count out a hundred seeds and weigh them, and the conversion is yours for that variety.