From target stand to seeds
Not every seed becomes a plant. Some fail to germinate, others die before or after emergence. The seeding rate corrects the target population for these losses:
Seeds to sow = target plants ÷ (germination rate × field emergence)
The germination rate is printed on the seed tag. Field emergence depends on soil temperature, moisture, crusting, pests and planting depth; 90% to 95% is common for corn and soybeans planted in good conditions.
Row spacing and seed spacing
An acre has 43,560 ft². With rows 30 inches (2.5 ft) apart, it holds 43,560 ÷ 2.5 = 17,424 ft of row. Dividing the seeds per acre by the row length gives seeds per foot, and its inverse the spacing between seeds.
Example: corn
Target 34,000 plants per acre, germination 95%, emergence 95%, rows 30 in apart:
- seeds per acre = 34,000 ÷ (0.95 × 0.95) ≈ 37,700;
- seeds per foot of row = 37,700 ÷ 17,424 ≈ 2.16;
- spacing ≈ 12 ÷ 2.16 ≈ 5.5 in;
- with 80,000-seed bags, about 0.47 bag per acre, or 47 bags for 100 acres.
Typical targets
| Crop | Usual final stand |
|---|---|
| Corn | 28,000 to 36,000 plants/acre |
| Soybeans | 100,000 to 140,000 plants/acre |
| Sunflower (oil) | 17,000 to 23,000 plants/acre |
| Cotton | 30,000 to 45,000 plants/acre |
Seed companies and extension services publish recommended populations by hybrid, soil productivity and row spacing.
Seeding by weight
Small grains and forages are usually sown by weight. Convert using the seed count per pound, or the thousand-kernel weight:
lb per acre = seeds per acre ÷ seeds per lb
Wheat, for example, has about 12,000 to 18,000 seeds per pound depending on the variety and seed size, so a fixed rate in bushels per acre can give quite different stands.
Tips
- Calibrate the planter or drill at the start of each season and whenever you change seed size.
- Increase the seeding rate when planting early into cold soil, or into heavy residue.
- Count plants in several 1/1,000-acre lengths of row a few weeks after emergence: 17 ft 5 in of row in 30 in rows is 1/1,000 acre.