Testing for Physical Purity - Seed Testing - page 41

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Testing for
Physical
Purity

Along with germination, purity tests are one of the oldest and most common tests performed by seed
analysts. Their purpose is to determine the physical composition of a seed lot by performing a detailed
and precise separation on a small representative working sample . The procedure consists of separating
the sample into four components (pure seed, other crop seed, weed seed, and inert matter) so the percent
composition by weight of each may be determined. These components are shown in Figure 4.1. Pure seed is
the portion of the working sample represented by the crop species for which the lot is being tested. In actual
practice, it includes the percentage of each crop species present in levels of 5% or more, or levels less than
5% if shown on the label. Other crop seed refers to crop seeds present in concentrations of less than 5%
of total sample weight. Weed seed denotes the percentage of seeds present of plants considered as weeds.
Sometimes this designation may be somewhat arbitrary since a plant may be considered a crop in one state
or country but a weed elsewhere. For any particular region (state, country), the analyst uses well-accepted
guidelines for classifying seeds as crops or weeds. The Association of Oficial Seed Analysts (AOSA)
includes a classiication of weed and crop seeds in its seed testing rules. Inert matter denotes the portion
of the sample that is not seed. It consists of materials such as chaff, stems, shells, stones and soil particles,
but may also include pieces of broken, damaged, or immature crop or weed seeds that do not qualify as
pure seed units. It is worthy of note that the International Seed Testing Association (ISTA) rules specify a
three-way separation of components into pure seed, other seeds, and inert matter.

Figure 4.1. Sample components are separated into pure seed, other crop seed, weed seed, and inert matter in
the purity analysis according to the AOSA Rules for Testing Seeds.

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