Starting seeds is one of the most exciting parts of gardening Seeing those first sprouts emerge from the soil never gets old! But to get seeds off to a strong start, it’s important to use the right growing medium. When it comes to potting soil vs. seed starting mix, which is better for starting seeds?
The short answer is that seed starting mixes are specially formulated for germinating seeds, while potting soils are intended for transplanting seedlings or established plants. But with some considerations, potting soil can work in a pinch for starting seeds. Follow these tips to improve your chances of seed starting success with potting soil
Seed Starting Mix vs. Potting Soil – What’s the Difference?
To understand why seed starting mixes tend to work better for seeds, it helps to look at the key differences between seed starting and potting mixes:
Texture
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Seed starting mixes have a light, fluffy texture that allows tender roots to penetrate and get oxygen.
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Potting soils often contain compost or soil that creates a dense, heavier texture.
Ingredients
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Seed starting mixes are soilless, containing vermiculite, peat moss, perlite.
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Potting soils include compost, composted manures, topsoil, and other organic matter.
Nutrients
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Seeds contain their own nutrients and seed starting mixes contain little to no added fertilizer.
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Potting soils frequently have added organic fertilizers that can be too strong for sprouting seeds.
Particle Size
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Seed starting ingredients are very finely screened to avoid blocking delicate roots.
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Potting soils contain a range of larger particle sizes. Bits of bark, soil clumps, and organic matter may impede seedlings.
Can You Use Potting Soil to Start Seeds?
While potting soil may not be ideal, you certainly can use it to start seeds in a pinch. Follow these tips to improve your chances of success:
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Sift potting soil through a screen to remove large pieces that could inhibit seedling growth.
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Consider mixing potting soil 50/50 with a seed starting mix to lighten the texture.
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Use only lightly fertilized or unfertilized potting mixes to avoid burning tender roots.
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Water carefully to keep soil moist but not soaked for good air circulation.
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Add a thin layer of vermiculite or seed starting mix on top for better seed-to-soil contact.
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Take care when transplanting to avoid damaging delicate seedling roots in dense soil.
When to Use Potting Soil vs. Seed Starting Mix
As a general rule of thumb, follow these guidelines:
Use Seed Starting Mix For:
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Very small seeds like petunias, tomatoes, peppers, etc.
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Older seeds with lower germination rates.
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Seeds requiring light for germination like begonias.
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During early seedling stage for best root growth.
Potting Soil Works for:
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Larger seeds like beans, cucumbers, squash.
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Transplanting seedlings into containers.
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Seeds with high germination rates if sifted.
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Quick-growing seeds planted directly in containers.
Making Your Own Seed Starting Mix
You can easily DIY an inexpensive seed starting mix at home with ingredients like:
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1 part sphagnum peat moss or coco coir for moisture retention
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1 part vermiculite, perlite, or sand for drainage
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1 part compost (screened through 1/4″ mesh)
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Mix thoroughly and add optional organic fertilizer like worm castings.
How to Prepare Potting Soil for Seeds
To ready potting soil for seed starting, screen out large pieces and amend as needed:
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Pass through 1/4″ hardware cloth screen to remove debris.
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Mix in perlite, vermiculite, or sand to reduce compaction and improve drainage.
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Blend in seed starting mix to lighten texture.
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Add thin layer of vermiculite on top of pots for better seed contact.
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Moisten soil before seeding, then water carefully to maintain even moisture.
Key Tips for Starting Seeds Successfully
Follow these best practices when starting seeds in any medium:
Provide Warmth
Warm soil temperatures are critical. Use a seedling heat mat if needed to maintain 75-80°F soil temp.
Allow Proper Light
Give seeds the light conditions recommended on the seed packet. Most need 12-16 hours under grow lights or direct sun.
Water Carefully
Water thoroughly after planting then keep soil evenly moist but not saturated. Err on the drier side.
Use Clean Containers
Sterilize seed trays and pots before use to prevent mold and disease.
Label Diligently
Label each flat or pot with plant name and variety for easy tracking.
Fertilize Minimally
Avoid fertilizer until seedlings develop true leaves. Then use half strength liquid fertilizer.
Harden Off Gradually
Move seedlings outside for short periods to gradually acclimate them before transplanting.
Potting Up Seedlings
Whether you start seeds in potting soil or seed starting mix, most will need to be transplanted into fresh potting soil as they grow. Follow these tips when it’s time to pot up your seedlings:
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Wait until seedlings develop 2-3 sets of true leaves before transplanting into larger containers.
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Water seedlings well the day before to minimize transplant shock.
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Carefully loosen soil with a fork and lift out seedlings gently by the leaves.
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Set seedling slightly deeper in new potting soil, up to the first set of leaves.
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Firm soil gently around roots and water well to help settle roots.
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Place in partial shade for a few days after repotting to allow adjustment.
While specially formulated seed starting mixes provide ideal conditions for germinating seeds, potting soil can be used with some extra care. Focus on lightening and enriching bagged potting soils to create a suitable seed starting medium. With attention to proper moisture, light, warmth, and fertility, seeds can thrive in potting soil. Pair seed starting best practices with the right medium, and your seedlings will be off to a vigorous start!
Potting soils and seed-starting mixes for your garden
Gary Heilig, – February 28, 2013
Healthy garden vegetables and flowers begin with good soils.
March is the month when smart gardeners gear-up for the growing season by starting vegetable and flowering seedlings. One important factor that will determine the degree of success is the potting media.
All seed-starting mixes and potting soils are not created equally. You can find items from Miracle Grow, Scott’s, Farfard, Schultz, Happy Frog, Hyponex, and other brands at garden centers in the area. Most experienced gardeners have a favorite potting medium, but new gardeners might not know what to choose.
According to Michigan State University Extension, there is a difference between potting soil and seed-starting mix. Soilless seed-starting mixes have a finer texture and are made from ingredients such as milled peat moss, perlite, coconut coir fiber and vermiculite. Although potting soils may be used to start seeds, they tend to have a more coarse texture and may contain field soil, compost or composted manure along with vermiculite, peat moss or perlite. Some seed-starting or potting mixes may contain fertilizer as an additive. Read the package. Some products contain enough fertilizer to provide seedlings with sufficient nutrients to last up to three months, while others may have no added nutrients.
Some manufacturers add moisture retention granules to their soil mixes that get many times bigger when they absorb water. This is done because peat moss absorbs and helps keep water in the soil. These are especially beneficial for container gardening during dry weather or when you forget to water your plants. When potting soil ingredients include field soil, compost or manure, they may also contain some weed seeds. Although this may be an inconvenience, few seeds will be viable if the soil has been pasteurized.
People who garden organically like to use organic potting mix, but just because it says “organic” on the package doesn’t mean you can use it to start organic seedlings. Read the package closely to determine if the soil and other additives are approved for organic production.
You can save some money by making your own seed-starting or potting mix if you are a smart gardener. One third of the mix should be spaghnum peat moss or coconut coir fiber. The other third should be finely screened compost, and the last third should be vermiculite. Add about 1 to 2 cups of worm compost to a 5 gallon bucket of your soil mix. I also like to stretch commercial potting media by adding up to 50 percent screened compost. I generally use this mixture when transplanting seedlings into larger pots.
When I purchase commercial potting media, I like to select those that have a guarantee. To me, a guarantee indicates that a company is willing to stand behind their product. Keep the sales receipt just in case you are not satisfied with the performance of the product. It is generally required to obtain a refund.
For more information on a wide variety of smart gardening articles, or to find out about smart gardening classes and events, visit www.migarden.msu.edu.
We tested popular seed starting soils: you’ll be surprised to learn which one did best!
FAQ
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