Pure Live Seed

Two numbers print on every seed tag and neither of them alone tells you what you bought. Purity is the share by weight that is the species named on the bag, the rest being chaff, inert matter, other crop seed and weed seed. Germination is the share of that species which sprouted in a laboratory test. Pure live seed multiplies them, and it is the only figure on which two lots can honestly be compared.

A forage tag reading 98.5% purity and 90% germination gives $0.985 \times 0.90 = 0.8865$, so 88.65% of the bag is seed that is both the right species and alive. Put another way, 100 kg of that bag delivers 88.65 kg of useful seed and 11.35 kg of freight. Now compare it against a cheaper lot at 92% purity and 78% germination: $0.92 \times 0.78 = 0.718$. If the first lot costs $4.20/kg and the second $3.60/kg, the real prices are $4.20/0.8865 = \$4.74$ and $3.60/0.718 = \$5.01$ per kilogram of pure live seed. The cheap bag is the expensive one.

The classic mistake is buying forage and turf seed by the kilogram of bag. It matters far more in these crops than in cereals because their PLS figures are genuinely poor — small-seeded grasses and legumes are hard to clean, and lots in the 70s are ordinary rather than scandalous. Certified cereal seed usually runs above 95% PLS, which is why nobody in a wheat-growing district thinks about the number and everybody in a forage district does.

Two things PLS does not cover are worth knowing. Hard seed, which is common in legumes, is alive but impermeable, and a tag may report it separately from germination — it will establish eventually, over months or years, which is useful in a permanent pasture and useless in an annual. And the weed seed content within that impure fraction is worth reading in detail rather than as a percentage: a tag showing 0.1% weed seed is showing you several hundred weed seeds per kilogram, and if one of them is a noxious perennial the tidy purity figure has hidden the most important thing on the label.