PHYSICAL PURITY TEST

1. Exercise No. 06
PHYSICAL PURITY TEST
Objective:
To determine whether the submitted seed sample conforms to
be prescribed quality standards in regard to purity components,
objectionable weed seeds, other inseparable crop seeds and
seeds of other distinguishable varieties.

2. • Principle:
In accomplishing the objectives of purity analysis,
each and every seed as well as particle is examined
and separated into the following component parts
(i) pure seeds
(ii) other crop seeds
(iii) weed seeds and
(iv) inert matter.
The composition of the sample is determined and the
results are expressed as percentage on weight basis.

3. Pure seeds (PS):
The pure seed shall refer to the species stated by sender or
found to predominate in the test and shall include all
botanical varieties and cultivars of the species. Generally,
they are intact seeds or broken seeds that are more than
one half of their original size whether immature,
undersized, shrivelled, diseased or germinated provided
they can be definitely identified as that of the species.
Other crop seeds (OCS):
Shall include seeds and seed like structures of any plant
species other than that of pure seed. The classification for
immature, damaged, diseased and empty seeds of other
crop is the same as that provided for pure seed.
Weed seeds (WS):
Shall include the weed seeds, bulbets or tubers of plants
which are recognized as weeds by laws, official regulation
or by general usage

4. Inert matter (IM): Shall include seeds, seed like structures and
other matters as follows:
• Pieces of broken or damaged seeds, achenes, mericarps
and caryopsis which are one half the original size or less.
• Broken or empty glumes, lemmas, paleas, unattached
sterile florets and florets with caryopsis less than the
minimum size prescribed.
• Seeds of the Leguminosae, Cruciferaceae and
Coniferaceae with the seed coat entirely removed are
regarded as inert matter.
• Split cotyledons of peanut, cowpea, and soybean are also
classed as inert matter.
• Other inert matter shall include soil, sand, stems, leaves,
pieces of bark, cone scales, wings, flowers, nematode
galls, fungal bodies (such as ergot, other sclerotia and
smut balls), caryopsis of Gramineae replaced by insect
larvae.
• Any other matter that is not seed.

5. Equipments and materials required:
• ‘ L’ shaped purity working table.
• Physical purity work board with illuminating light.
• Wide field hand-lens or Magnifying glass of 3 to 7 X magnification.
• Binocular microscope.
• Balances viz., i) One with a capacity of 100—200 gm with accuracy of 0.1
mg (analytical balance) and ii) One with a capacity of 1000 gm with
accuracy of 10 mg (precision balance).
• Sieves and sample pans.
• Seed blowers and dividers.
• Spoons, spatula, forceps, scalpel, needles, shallow trays, watch glass, etc.
• Small seed containers to hold separated seeds and other matters (vials
with corks).

6. Procedure:
• The size of the submitted sample sent to the Seed Testing Laboratory is
larger than the working sample to be used for purity analysis. Generally, a
purity working sample is a weight estimated to contain 2,500 to 3,000
seeds subject to a minimum of 0.5 grams and a maximum of 1,000 grams
or otherwise not less than the specified weight as per ISTA rules, 2004.
• The analysis may be made on one working sample of this weight or on two
sub samples of at least half this weight, each drawn independently from
the submitted sample. Prior to purity analysis, the submitted sample is
thoroughly mixed three times with the help of mechanical seed dividers.
Then, the correct size of the working sample is obtained from the
submitted sample by repeated halving or by abstracting and subsequently
combining small portions. In any case, the working sample must not be
altered by adding or removing seed by hand. In case, if the submitted
sample contains very large quantity of extraneous materials, then the
preliminary separation of small inert matters like chaff, dirt and other light
matters should be removed by using seed blower and screens.

7. • The physical purity analysis should be done on the clean
surface of a purity working board Examine each and every
seed and inert matter of the working sample. Separate the
sample by the forceps into four components such as pure
seed, other crop seed, weed seed and inert matter
• the purity separations are also mane by taking the
advantage of seed shape. The round seeds can be separated
from flat seeds and chaffs by allowing the seeds to roll down
on a slightly inclined surface. Then weigh each component
on an appropriate balance to the desired decimal place.
Check original weight against final weight as a check against
loss of material or other error. The difference between
original and final weights should not be more than 6 per
cent.

8. Calculation of results:
• The percentage by weight of each of the components shall be calculated
to one decimal place. Percentages must be based on the sum of the
weights of the components or on the original weight of the working
sample used for physical purity analysis.
Component Weight (g) Percentage
Pure seed (PS) A= AE x 100 =
Other crop seed (OCS) B= BE x 100 =
Weed seed (WS) C= CE x 100 =
Inert matter (IM) D= DE x 100 =
Final weight E= 100 per cent

9. Percentage of purity analysis up to “one decimal place”
Percentage of all components
should be total to
“100”
If a component is nil “0.0”
If a component is ≤ 0.05% “Traces”
If a component is 0.05 to 0.1% “0.1%”
Percentage of components on “sum of weight all components”
Not on the weight of “original working sample”
The name of each crop species “with Latin name”
If, difficult “only by genus”
Reporting results:
Note: If gain or loss in weight during purity analysis exceeds 5% of the initial weight, a re-
test is required.

10. Study questions:
• Write the use of seed blower in physical purity
test
• How to test physical purity of pelleted seeds
• Write the relation between physical purity and
planting value of seeds
• What do you mean by planting value of
seeds?