Purity is the percentage of actual seed by weight. The rest of the material in a seed lot may be made up of inert material, weed seed, or other crop seed. Total Viable is the percentage of seed that will germinate. This can be determined by adding the germination percentage plus the dormancy percentage or from a tetrazolium (TZ) test. By multiplying these two factors together, you can determine the percentage of material that is pure live seed versus the percentage of material that is essentially worthless.
To determine the bulk lb of seed that are needed, divide the PLS lb required by the PLS percentage.
A Switchgrass seed lot has a germination of 92.00%, hard seed of 5.00%, and a purity of 98.00%. The PLS % for the seed lot is 95.06% ((0.92 + 0.05) x 0.98). If the Switchgrass seeding rate is 3.00 PLS lb per acre, 3.16 bulk lb should be planted per acre (3.00 ÷ .9506).
A Butterfly Milkweed seed lot has a TZ of 85.00% and a purity of 95.00%. The PLS % for the seed is 80.75% (0.85 x 0.95). If the seeding rate is 0.12 PLS lb per acre, 0.15 bulk lb should be planted per acre (0.12 ÷ .8075).
State seed law requires that all native grass and wildflower seed distributed must include a seed analysis tag for every species. The tag should include the following:
- Lot Number
- Species Name
- Test Date
- Purity, Inert Matter, Other Crop, and Weed Seed Percentages
- Germination and Hard Seed Percentages