How it works
Seeding guides give rates in pure live seed (PLS), the pounds of seed that will actually germinate. A seed lot is never all seed or all viable, so a tag lists purity (the share that is the target species) and germination (the share of that seed that is alive). PLS is purity times germination.
To turn a PLS target into what you buy and spread, divide by the PLS fraction. If your target is a number of seeds per square foot, first convert it to pounds using the seeds per pound for the species.
Worked example
A 3 lb PLS per acre target with 90% purity and 80% germination over 5 acres:
- PLS % = 90% × 80% ÷ 100 = 72%
- PLS per acre = 3 lb (your target)
- Bulk per acre = 3 ÷ 0.72 = 4.17 lb
- Whole area = 4.17 × 5 ac = 20.8 lb
| Input | Value |
|---|---|
| My target is | PLS pounds per acre |
| Target PLS rate | 3 lb/ac |
| Target seeds per square foot | 25 seeds/ft² |
| Seeds per pound | 1000000 seeds/lb |
| Purity (from the seed tag) | 90 % |
| Germination, including dormant or hard seed (from the tag) | 80 % |
| Area to seed | 5 ac |
| Result | Value |
|---|---|
| Bulk seed per acre | 4.17 lb/ac |
| Bulk seed for the whole area | 20.8 lb |
| PLS per acre | 3 lb/ac |
| Percent pure live seed | 72 % |
Assumptions and limits
- No species seeding rates or seed counts are built in; you take them from your seeding guide or the seed tag.
- Uses the tag’s germination and purity as printed; the test date matters, since germination drops with seed age.
- Drilled and broadcast seeding often call for different rates; use the figure your guide gives for your method.
Common questions
What is pure live seed?
PLS is the share of a seed lot that is both the target species and alive: purity times germination. Seeding guides give rates on a PLS basis.
How do I find the bulk rate?
Divide the PLS rate by the PLS fraction. For 3 PLS lb/ac at 70% PLS, bulk is 4.3 lb/ac.
How do I use seeds per square foot?
Multiply by 43,560 and divide by the seeds per pound for the species to get PLS pounds per acre, then correct for PLS.
Is dormant or hard seed included?
Many tags list total viable germination including dormant or hard seed; use the number your guide says to use, since it can change the PLS.
Sources
- USDA NRCS: Understanding Pure Live Seed; bulk seeding rate = PLS rate ÷ PLS fraction, with the Sherman big bluegrass example (3 PLS lb/ac at 70% PLS = 4.3 lb/ac bulk).
- Purdue Extension and NRCS: PLS = purity × germination ÷ 100.
- Read them: NRCS: Understanding Pure Live Seed (PLS) · Purdue Extension: Pure Live Seed
Updated 2026-09-30