For example, a purity of 50% (the amount of whole seed) multiplied by 80% germination (how much seed grows when tested) equals 40% PLS. To put it simply, in one pound of this example seed, about 40% of it is pure live seed. So to get 1 lb. pls, you would have to use 2.5 lbs. of bulk seed from that example seed. From another sample with a 82% pls, you would need 1.2 lbs. of bulk seed.
How the planting rates are set: USDA sets the PLS-per-acre planting recommendations for each species of seed. We use this to guide how we help you design your project. For example, if you have one acre you would like to spread a grass seed on, and the grass seed you chose has a recommendation of 3 pls per acre, we can then look at the lots of seed we have available and gather a bulk quantity that will reach that pls level — and you are charged on how much viable seed you get, not just bulk pounds, which includes non seed items like stems and hulls.
Why is PLS important? Think of this similar to the octane rating on gasoline. Where one gallon of 87 gasoline costs less than one gallon of 92 octane, because for the same amount of volume, one has a higher fuel quantity. So comparing one pound of seed to another there might be a large difference as to how much whole viable seed is present. PLS corrects for that. Therefore you know when you buy seed with a PLS rating that it is of a certain quality and has a certain likelihood of growing. Like gasoline, you know how much "energy" is in that bulk pound.
Why not have the same amount of seed per pound across the board? Plants have variability and one season a field of grass might produce a lot of seed, then another season the same field might produce little seed or produce more stems and hulls than seed. And although seed processing is pretty refined, it cannot pull all the extra stuff out if those items are of similar size to the seed.
If you compare the two different harvests based on weight alone, you will get a different amount of live seed each time. Pricing on bulk weight alone is not fair to you, the consumer, who wants the same amount of plants from one year to the next. So we take the extra step to test all of our lots of seed to ensure you are getting the right amount of seed for your project.