Pure live seed: keep biological content separate from bulk weight

Pure live seed, often shortened to PLS, is a way to connect seed purity and the accepted viability measure. It helps compare lots and calculate bulk quantity where a specification is written on a PLS basis. It is not a promise that every counted viable seed becomes a plant. Before using the calculation, confirm that the project actually specifies PLS rather than bulk seed mass.

Write the definition used by the project

The basic relationship multiplies the pure-seed fraction by the accepted germination or viability fraction. Some conservation specifications include reported dormant or hard seed in the accepted total; others need a different treatment. The NRCS seed-quality technical note describes its conservation approach, including those categories. Do not adopt that definition silently for a Canadian project.

Copy the approved definition onto the calculation sheet. If the seed analysis and specification use different terms, ask the responsible seed specialist or designer to reconcile them. Do not count the same category twice or assume an unreported value is zero.

Use actual values for the delivered lot

Obtain the lot's purity and test information, including the date and identifier. Connect it with the delivered package. A typical percentage for the species or an earlier purchase is not an acceptable substitute. If a mixture contains several species with different analyses, the calculation may need to be performed component by component rather than with a single unsupported mixture percentage.

CFIA's common-seed guidance explains that testing and information requirements vary, including for species outside Schedule I. Where the project needs analysis beyond the supplied label, request it before accepting the lot.

Convert percentages into fractions consistently

For a hypothetical lot with 96 percent purity and an accepted viability figure of 85 percent, the PLS fraction is 0.96 multiplied by 0.85, or 0.816. That is 81.6 percent PLS under the stated assumption. A 10 kilogram bulk quantity would therefore contain 8.16 kilograms PLS. These invented values demonstrate arithmetic only and describe no actual product.

Use one consistent format in a spreadsheet: either fractions throughout or a clearly documented percentage formula. Multiplying 96 by 85 without dividing by the necessary factor produces the wrong scale. Keep enough decimal places during calculation, then round only the practical final quantity using the project's accepted approach.

Calculate the bulk quantity from a defined target

Divide the required PLS mass by the lot's PLS fraction, a relationship explained in the NRCS seeding-rate technical note. If an entirely hypothetical project target were 12 kilograms PLS and the fraction were 0.816, the corresponding bulk quantity would be about 14.706 kilograms. The 12 kilogram target is not a seeding recommendation; it is an input assumed for this example. A real target must come from the accepted design or specification.

Check units before multiplying by area. A rate in kilograms PLS per hectare cannot be multiplied directly by square metres without conversion. Keep the area, rate, PLS requirement and bulk requirement in separate labelled columns so another person can reproduce the result.

Treat mixtures and coatings carefully

A component percentage by mass is not the same as the PLS percentage of that component. If a mixture target is species-specific, calculate the contribution of each component using the appropriate lot data and mixture basis. Do not assume equal bulk shares deliver equal viable seed numbers, because seed size also differs.

Coated or enhanced seed introduces additional basis questions. Determine whether the reported purity, package mass and rate refer to seed, coating or the complete product. CFIA's mixture guidance distinguishes labelling categories and discusses specialty seed. Obtain supplier and project clarification rather than applying an ordinary uncoated-seed formula blindly.

Keep PLS separate from establishment predictions

A laboratory test operates under defined conditions. Field outcomes also depend on moisture, temperature, placement, soil, competition, disturbance and care. A calculation that equalizes PLS between lots does not guarantee equal field performance. Dormant seed may contribute to an accepted viability measure without emerging during the first inspection period.

Do not increase the specified rate automatically to compensate for uncertain establishment. That is a design change, not arithmetic. Identify the suspected limitation and refer it to the responsible reviewer. More seed cannot be assumed to solve compaction, drainage or an unsuitable species choice.

Compare two hypothetical lots fairly

Assume lot A has a PLS fraction of 0.80 and lot B has a fraction of 0.90 under the project's accepted definition. For the same illustrative target of 9 kilograms PLS, lot A would require 11.25 kilograms bulk and lot B would require 10 kilograms bulk. This demonstrates why equal bulk weights do not deliver the same calculated PLS. It does not establish that one lot will produce a better field stand.

If comparing verified supplier quotations, divide each bulk unit cost by its PLS fraction to compare cost per PLS unit, while also considering species, source, test condition and suitability. Do not compare unrelated species or provenances solely on that number. A cheaper calculated PLS unit can still be the wrong seed for the project. Preserve the lot values and accepted definition alongside the calculation so the comparison remains reproducible when a quotation or lot changes.

Make the calculation auditable

Retain the specification, lot analysis, formula, units, rounding and actual quantity loaded. Have a second person check the first calculation or a changed lot where the project warrants it. Update the batch sheet when the lot changes; the old bulk quantity may no longer deliver the same PLS target.

Read Canadian lot review, germination and dormancy and rate basis. PLS is most useful when its definition and inputs remain visible rather than hidden inside a single calculator output.