Pure Live Seed (PLS) determines the amount of viable, desired seed in a given quantity of bulk seed. This percentage represents the portion of the total bulk weight that is the specific seed intended for planting and is expected to germinate. Knowing the PLS percentage is a fundamental step for efficient planting because standard seeding recommendations are often provided on a PLS basis. Failing to account for this variation can lead to poor stand establishment or wasted resources due to planting too little or too much viable seed.
Components Needed for Calculation
The calculation of Pure Live Seed relies on two specific percentages that federal and state laws require to be printed on the seed tag, often called the Analysis Tag.
Purity Percentage
The Purity Percentage indicates the percentage of the seed sample that consists of the desired crop seed, by weight. This figure excludes any inert matter, such as dirt or chaff, as well as weed seeds and other crop seeds that might be present in the bag.
Germination Percentage
The Germination Percentage represents the viability of the pure seed portion. This is the percentage of the pure seed expected to sprout and develop into a normal plant under favorable testing conditions. If a separate percentage for “hard seed” or “dormant seed” is listed, it is added to the germination percentage to determine the total expected live seed percentage.
Performing the Pure Live Seed Calculation
Calculating the Pure Live Seed percentage is a mathematical process that combines the two percentages found on the seed tag. The formula used is: Pure Live Seed Percentage = (Purity Percentage) \(\times\) (Total Germination Percentage). This multiplication quantifies the portion of the total bulk seed that is both the correct species and capable of growth.
Convert both percentages to their decimal equivalents before performing the calculation to ensure an accurate result. For example, a purity of 95% is converted to 0.95, and a germination of 90% is converted to 0.90. Multiplying these decimals yields the PLS percentage in decimal form.
Using the example of a seed lot with 95% Purity and 90% Germination, the calculation is \(0.95 \times 0.90\), which equals \(0.855\). This result means the Pure Live Seed percentage is \(85.5\%\).
Adjusting Seeding Rates Using PLS
The final step is using the calculated PLS percentage to determine the actual bulk weight of seed needed to achieve a target seeding rate. This adjustment ensures that the correct amount of viable seed is delivered to the field, regardless of the quality variations between different seed lots. Recommended seeding rates, often provided by agricultural extension services, are typically given in pounds of PLS per acre.
To find the amount of bulk seed to purchase and plant, the following formula is applied: \(\text{Actual Bulk Weight Needed per Acre} = \text{Desired PLS Rate per Acre} \div \text{Calculated PLS Percentage}\). The calculated PLS percentage is used in its decimal form for this division.
For instance, if the desired seeding rate is 10 pounds of PLS per acre, and the seed lot has a calculated PLS of \(80\%\) (or \(0.80\)), the calculation becomes \(10 \text{ lbs} \div 0.80\). The result is \(12.5\) pounds. This means that to meet the target of 10 pounds of viable seed, a grower must plant \(12.5\) pounds of the bulk seed product per acre. This adjustment prevents under-seeding or over-seeding, which wastes money on non-viable material.