The purity test is a visual examination of a seed sample for contaminants which occur in seed lots, such as other crop seeds, weed seeds, and various inert matter. This also includes a noxious weed examination, in which seeds of any species that are classified as noxious weeds are reported.
This test is also referred to more specifically as a physical or mechanical purity test. This is to distinguish it from genetic or varietal purity tests which determine traits that are not visually detectable in the seed.
The purity test/noxious weed examination is a two-part procedure. The purity test is an examination of a relatively small sample in which all impurities are separated and quantified. This gives the percentages of all components in the sample. All contaminating species of weeds and other crops are identified and quantified as to the rate of occurrence in the seed lot, usually as the number of seeds per pound.
In the noxious weed examination, a larger portion of seed (usually ten times the purity test amount) is examined specifically for seeds of species which are classified as noxious. Typically, this applies to any species considered to be noxious in the US, and is commonly referred to as an all states noxious weed examination. Federal noxious weeds are also reported. Seed lots containing federal noxious weeds are not eligible for interstate transport.
The ISTA Rules (International) and Canadian M&P (specific to Canadian seed trade) have different procedures for conducting purity tests and for reporting the results, but the principles are the same. Impurities are identified and quantified, and the rate of occurrence is reported.