What is a Purity Test?
A purity test is conducted on approximately 2,500 seeds which are broken down into four components: Pure seed, Other crop seed, Weed seed, and Inert matter. The components are then weighed from which percentages are calculated. The percentage is based on the weight of each component, not the number of seeds.
What is a "Pure Seed"?
A pure seed is based on the Pure Seed Unit (PSU) definition given for each species in the Association of Official Seed Analysts (AOSA) Rules for Testing Seeds. There are over 50 different PSU's in the AOSA Rules. The PSU is used by all seed labs, so they test seeds uniformly.
What is "Other Crop" and "Weed Seed"?
Other crop and weed seed are contaminating species in a seed lot. There are definitions in the AOSA Rules as to whether a species would be considered other crop or a weed seed based on the kind of seed being tested.
What is "Inert Matter"?
Inert matter is comprised of broken seeds of the kind being tested, chaff, dirt, or any other material that is not a seed.
An example of the kind of rules used in a purity test, the picture below shows a purity sample broken down into pure seed, other crop, weed seed and inert material. in the test, a weed seed (downy brome) was found to have less than one-half of its root-shoot axis, which makes this seed inert matter.