Special Purpose: Seed Starter Potting Soil

FellowsPurposes are to use commonly grown plants as a bioassay to assure that these potting soils: (1) do not contain materials that would prevent the growth and development of plants, (2) will support the germination and establishment of young seedlings. For a potting soil to pass this test, seedling survival must be statistically equal to or greater than for the control soil, and plants grown must produce true leaves.


Plant species: Radish, ‘Early Scarlet’ (Raphanus sativus); Tomato, ‘Better Boy (Lycopersicum esculentum); Marigold, ‘Janie’ (Tagetes patula); Basil, ‘Sweet Green’ (Ocymum basilicum); Impatiens ‘Dazzler Red’ (Impatiens walleriana); and Zinnia “Dreamland Red’ (Zinnia elegans).
Pots: Eight (8), 4-inch square (or round) plastic pots per species.
Control potting soil: To assure testing uniformity, the MSC produces and distributes a uniform control sample. Companies wishing to conduct in-house testing should order control media directly from the association (comes in 1-CF bags).  The control soil is considered useable for a period of 4 months after the manufacture date. The expiration date is provided on every bag produced.  Along with the control soil sample, the purchaser will be issued a set of cards, which include the MSC logo and lot code of bag(s) of control soil sent.  This card is to be included in photos taken of plants at the end of all growth tests run for product registration during the 4 month period after control soil production.
Environment: standard greenhouse environment or equivalent. 75-85°F Day / 60-70°F Night.

1. All tests will include four pots of test soil and four pots of control soil for each of six test species for a total of 48 test pots. Seeds should be sown and covered exactly as in step 4.

2. Pots should be filled to overflowing and the excess brushed away so that the soil is even with the top of the pot. Pots may be grouped in trays (such as 1020 trays) for ease in handling.

3. All pots should be thoroughly watered prior to sowing to insure adequate moisture.

4. For each of the six test species, place five (5) seeds in each of the four test soil pots and five seeds in each of the 4 control soil pots for a total of 40 seeds per species. Each pot contains only one species of seed. All seeds should be covered with a very light layer of the same fresh soil before testing.

For each species of seeds
Treatment No. of pots No of seeds per pot
Control Soil 4 5
Test Soil 4 5

5. Pots should be misted using a standard 3-point fogging nozzle. Pots should be misted as needed to insure adequate moisture. Pots may need misting four times daily in summer; less in winter. Misting should occur until seedlings have established. This may be 3 to 5 days for radish, 5 to 7 days for the others.

6. After germination, plants should be watered as needed.

7. Plants should be counted to determine seedling survival.  Time will vary depending on season and species grown, and could be as early as 5 to 7 days for radish.  All plants should be counted 14 days after sowing, with data entered in the form on the tab for Seed Starter Potting Soils, in the file “MSC Data Sheets for Testing Protocols”, available on the MSC website (Link:

8. Each pot will be thinned to one plant. Plants will be grown for a total of four weeks from sowing. Plants will be watered as needed but should receive no fertilizer.

9.  Photos:  At the completion of the growth test, for all species tested, photos are to be taken of a representative plant growing in the test soil and control soil.  Each separate photo should include a card showing the MSC logo and control soil lot code, sent with the control soil sample when purchased.

Data Analysis
1. Seedling survival is a test of 2 proportions, using the 95% confidence level, and is automatically calculated, along with the pass/fail determination, from the data entered on the MSC data file.  If the proportion surviving in the potting soil is statistically equal to or greater than the control soil, proceed to step 8. If the proportion of seeds surviving in the test soil is below the control soil, the potting soil sample will fail the test.
If plants of all 6 species in the test soil produce cotyledons and the first set of true leaves, and the seedling survival is within 95% confidence level of controls, by proportion analysis, the test soil will have passed the MSC Grow Test for Seed Starter Soils.

The completed data file and photos are to be submitted with the application form for product registration.

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