Seed quality is not uniform, and a bag often contains more than just the desired planting material. The weight purchased includes debris, weed seeds, and non-viable seeds. Buying seed by total weight, or bulk weight, can lead to planting failures and economic waste. To provide a standardized measure of planting value, the agricultural industry uses a metric that accounts for both the cleanliness and the vitality of the seed lot. This measurement ensures buyers know exactly how much usable seed they are receiving.
Defining Pure Live Seed
Pure Live Seed (PLS) is the standard industry metric that quantifies the true planting value of a seed lot. It represents the percentage of the seed’s total weight that is composed of the desired seed and is capable of germinating. PLS is the percentage of seed that is both pure and alive.
The calculation for PLS combines two quality measurements found on a seed tag: purity and germination. The formula is: PLS % = (Purity % x Germination %) / 100. This result allows farmers and landscapers to compare different seed lots accurately. A higher PLS percentage translates to a greater concentration of viable seed in every pound.
The Purity Component
The purity component focuses on the physical composition of the seed lot by weight. Purity is the percentage of the seed sample that consists solely of the desired seed species. This analysis is performed in a laboratory where a working sample is separated into four distinct parts. The remaining percentage of the bulk weight consists of material that is not the target seed.
The non-pure fraction is categorized into inert matter, weed seeds, and other crop seeds. Inert matter includes non-seed material such as chaff, stems, soil particles, stones, and broken seed pieces. Weed seeds are the reproductive units of species recognized as weeds. Other crop seeds are identifiable seeds of any plant other than the species being tested. A high purity percentage minimizes the introduction of unwanted plants and debris into the field.
The Germination Component
The germination percentage focuses on the biological viability of the pure seed fraction. It measures the percentage of pure seed capable of producing a normal, healthy seedling under optimal laboratory conditions. This test is conducted in highly controlled environments, typically over seven to twenty-eight days, depending on the species. Laboratories monitor factors like moisture, temperature, and light to determine the seed’s maximum potential to sprout.
The reported germination rate often includes seeds that germinate immediately, hard seeds, and dormant seeds. Hard seeds have a coat impermeable to water. Dormant seeds fail to germinate due to internal physiological factors, even under ideal conditions. Both hard and dormant categories are considered viable, as they possess the potential to grow later. The total percentage of live seed is the sum of prompt germination, hard seed, and dormant seed.
Practical Application of Pure Live Seed
The significance of the Pure Live Seed calculation lies in its ability to standardize seeding rates and simplify cost analysis. When planting, the goal is to achieve a specific density of established plants, requiring a targeted number of live seeds per square foot. Recommended planting rates are typically given in pounds of PLS per acre, not bulk pounds. This ensures consistency in planting density regardless of the seed lot’s quality.
PLS is used to adjust the bulk weight needed to meet the recommended rate. For example, if a planting recommendation calls for 100 pounds of PLS per acre, and the seed lot has an 80% PLS rating, the buyer must purchase 125 pounds of bulk seed to deliver the required live material. This is calculated by dividing the required PLS weight by the seed lot’s PLS percentage (100 lbs / 0.80 = 125 lbs). Utilizing the PLS calculation prevents over- or under-seeding.
The PLS percentage also allows for an economic comparison between different seed products. A seed lot with a lower price per bulk pound may be more expensive if it has a significantly lower PLS percentage. By dividing the bulk price per pound by the PLS percentage, a buyer determines the true cost per pound of live, viable seed. This calculation allows the consumer to make an informed decision based on the actual value of the seed.