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SEED ANALYSIS FACT SHEET: PURITY
The purpose of a purity analysis is to:
1. To determine the physical composition and quality of a seed
lot. The first step is to verify the identification of the species
in question. One also needs to identify the other crop seed, weed seed,
and noxious weed seeds. Then further examination of the inert material
identifies soil, insect parts, plant material, and ergot.
2. To provide labeling information for state and federal seed
act compliance, thus facilitating interstate and international seed trade.
3. To identify and quantify any noxious weeds present.
This helps to prevent the spread of noxious weeds.
4. To help limit the uncertainties of crop production by providing
information useful for planting rates by the calculation of PLS (pure live
seed) and TVS (total viable seed).
5. To help in making quality control decisions during harvesting
and conditioning, and for deciding the best uses for a seed lot.
6. To check for and prevent adulteration of seed lots through
tolerance testing. There are natural variations that occur in non-uniform
products such as seeds. For this reason tolerances were developed to
determine if there is a significant or “real” difference between
two tests or an official test and the seed tag information.
7. To facilitate the standardization under which seed is sold.
If AOSA rules are used by all seed analysts, then tests can be duplicated
readily. (If standard rules are not available for a particular species
then the method used should be stated with the test results.)
8. To provide “pure seed for planting” for the germination
test. Specific AOSA rules are to be applied when determining the pure
seed portion. This helps with standardization when the analyst applies
the rules to each species tested. The method with which the pure seed
is determined can radically effect the germination results.
THE PURITY TEST
Submitted sample: first and foremost a representative sample must be sent
to the seed testing laboratory. When this is done, a purity test can
be worked that will represent the whole lot in question, the most basic function
of the purity test.
Working sample: The submitted sample is mixed and randomly divided
into the weight specified in the AOSA rules that will provide 2500 seeds
for purity examination , and 25,000 seeds for the noxious weed examination.
Component parts: The working sample is divided by hand on a purity
board under good light, using magnifier light for work with small seeds,
tweezers, a seed pushing wedge, and a lot of hand eye coordination.
The component parts are:
Pure Seed: Includes all seeds of each kind and/or cultivator under
consideration which are present in excess of 5% of the whole.
Other Crop Seed: Seed of plants grown as crops (other than the kind
or cultivator included in pure) shall be considered other crop seeds, unless
recognized as weed seeds by laws, regulations, or by general usage.
Weed Seed: Seeds, florets, bulblets, tubers, or sporocarps of plants
recognized as weeds by laws, regulation or by general usage shall be considered
weed seeds. Further classification of species is determined with the
use of the reference, “Uniform Classification of Weed and Crop Seed”
which is published by the Association of Official Seed Analysts.
Inert Matter: Soil particles, stones, chaff, stems, leaves, flowers, cone
scales, pieces of bark, pieces or resin, etc.. Pieces of broken and
damaged seed units of crops which are half the original size or less.
Damaged weed seed with over half the embryo missing.
Each of the four component parts is weighed and a percentage is calculated
from the sum of the four component parts. This purity information is
used to tag the seed for sale and is reported on the analyst’s Report
THE NOXIOUS WEED EXAM
The noxious weed exam is worked on a sample ten times the purity weight.
This is equivalent to looking at 25,000 seeds. The analyst becomes
proficient at spotting only noxious weeds and ignoring all the other components.
Noxious weed exams can be requested for “all states” or with
a particular state in mind. (Hawaii’s prohibited noxious weed
list includes over 100 species and is not routinely done unless requested.)
A Federal Noxious Weed List is included in the Federal Seed Act, otherwise
the classification of noxious weeds is left up to individual states and is
determined by the agricultural needs of each state. There are two classifications
of noxious weeds. Prohibited noxious weeds render the lot not fit for
sale. Restricted noxious weeds require the lot to be labeled as to
the rate of their occurrence.
Advice to Seed Producers
It is always a prudent choice to have your seeds tested by an experienced
analyst. It is even better if the analyst is experienced with the species
to be tested. Two analysts who test flower seeds on a regular basis
are more apt to have results within tolerance, when testing a flower seed
sample, than an analyst who tests only corn. Of course the reverse
would be true also. When a species is not listed in the AOSA Rules,
and there are no standard rules to follow, then dealing with an experienced
analyst becomes even more important. Do not look at price only for
deciding who will test your valuable seed crop. Remember to deal with
an experienced seed analyst and to send in a representative sample for testing.
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