Potting Soil vs Seed Starting Mix

If you’re new to gardening, or new to planting your own seeds instead of using purchased seedlings, you may be wondering what the difference is between potting soil and seed starting mix. Why does seed starting mix usually cost more? Is seed starting mix really necessary or worth it? And why can’t you just grab a handful of dirt out of your yard to plant your seeds in?

Potting soil and seed starting mix are two different types of soils that serve two different purposes. Discover the differences between potting soil and seed mix and how to use them correctly.

Whether you are growing a big garden or a small indoor plant, gardening becomes easier when you know the differences between potting soil and seed starting mix.

Here are the basic differences between potting soil and seed starting mix:

	Potting Soil	Seed Starter Mix
Main Purpose	to transplant plants or grow container plants	to help seeds germinate into seedlings
Typical Ingredients	field soil, compost, composted manure, sand, fertilizer, moisture retention granules, perlite, peat moss, vermiculite	coconut coir fiber, sphagnum moss, perlite, peat moss, vermiculite, diatomaceous earth
Texture	coarse and dense	fine and loose

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The Difference Between Potting Soil and Seed Starting Mix

The differences between potting soil and seed starting mix begin with their ingredients.

Seed starter mix has a fine and light texture and has the purpose of allowing seeds to germinate easily and healthily.

Potting soil is heavier, denser, and coarser, making it ideal for plants that are undergoing transplanting or container growing. (It’s important to note that potting soil is still far less dense than most yard dirt, though.) Both are important at different times of the plants’ growth stages. Do you have some leftover potting soil from last year? Learn how to tell if potting soil is still safe to use.

Many seed mixes are inert and do not contain nutrients. This is great for germinating seeds that don’t need added nutrients, and may even be harmed by them. Although exact recommendations vary, most experts recommend holding off on fertilizing seedlings until they’re 3″ tall or have a couple sets of true leaves whichever comes first (source). After that, dilute your fertilizer carefully according to the package instructions to avoid burning your delicate seedlings.

I personally use Foxfarms nutrients diluted according instructions for seedlings when I need to fertilize my young plants. The eggplant seedlings below are definitely large enough to need fertilizing:

The lack of nutrients is perfect for tiny seedlings, but harmful to growing plants that need added nutrients. That’s why you need to transplant into potting mix once the seedling is ready to make the move.

Both potting soil and seed starting mix tend to be technically soilless. Seed starting mix isn’t even always a mixture of ingredients – plain coconut coir can be used as an eco-friendly, completely renewable seed starting medium.

Is Seed Starter Mix Necessary?

Do you really need a seed starter mix for your seeds? The straight answer is no. However, seed starter mixes are highly recommended!

Some may think if the seed starter mix is only used for the first short stage of a plant’s life, it’s not necessary. This can be a costly mistake.

Seed starter mixes give the seeds optimal conditions to begin their journey in life and increase the chances of successful germination. 

The primary benefit of seed starting mix (in my opinion) is that it reduces the likelihood of damping off. Because damping off is caused by fungi and fungi-like organisms, it’s more common in humid climates.

If damping off strikes your seedlings, it can ruin your entire planting season. Even if your seedlings survive, they won’t be as strong or vigorous as truly healthy seedlings. If a longer season crop, like squash or tomatoes, are affected, you may not have time to start a second batch of seedlings and have the plants fruit successfully before the weather becomes too cold in the fall.

What to look for (and avoid) in seed starting mix

I’m personally a believer in organic gardening, so I try to find organic potting soils and seed starting mixes whenever possible. If this is your personal preference, too, select an organic seed starting mix.

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As discussed in my post on how to recharge potting soil, coconut coir is a more sustainable, environmentally friendly choice than peat moss. That’s why I typically avoid mixtures that contain peat moss. Peat moss is a popular ingredient that many people use – it’s a personal choice you’ll have to make after reading up on peat moss versus coconut coir.

There are some potting soil ingredients to be wary of. Some increase the chances of weed seeds being present, such as: 

• Field soil
• Composted manure
• Compost 

Some people do use their own finished compost for seed starting. Charles Dowding, a well-known no-dig gardener, plants directly into compost in his garden. That’s not what we’re talking about here. If you make your own compost, use it for seed starting, and it works for you – great! If you’re purchasing soil for seed starting, avoid soil with added compost.

Your seeds’ growth may be hindered or terminated due to competition with the weed seeds if you use mix containing top soil or manure.

Many potting soils come with fertilizer. For germinating seeds, this fertilizer is not only superfluous, but it may also be harmful. Worm castings are not the same thing as compost or fertilizer – they are naturally mild, slow release, and not a reason to avoid a mix for seed starting.

Sand and topsoil in the potting soil can also hinder your seeds’ development.

Can You Use Potting Soil to Start Seeds? 

Yes – you can start your seeds in potting soil. However, it’s generally not recommended for several reasons:

• Longer germination time: it makes the seeds toil a bit more to pop their seedlings through the coarser potting soil. 

• Germination success rate may go down: The germination stage will be less optimal with potting soil, and sometimes, though rarely, seeds may cease to grow at all.

• Higher chance of fungi, mold, and damping off: Sterilized seed starting mixes protect your delicate seedlings from damping off. Damping off is caused by one of several fungi or fungi-like organism and can kill or severely stunt seedlings.

How Potting Soils Help Plants Grow

Potting soils are made specifically for plants that will grow in pots and containers. Hence it is called potting soil. 

Potting soil is superior to “dirt” from your yard if you have a container garden. Benefits of potting soil include:

• Moisture retention: Potting soils usually contain peat moss or added moisture retention granules. These ingredients help container plants’ roots retain moisture. This is especially important for plants growing in dry and arid environments. 

• Additional nutrients: The soil within the potting soil provides nutrients. Many potting soils may have compost and fertilizer, too; these also give the plants a lot of nutrients to grow and stay verdant. 

• Aeration: Ingredients like perlite, vermiculite, or coconut coir keep the potting soil from being too dense. This allows the roots to grow more freely and for air to circulate around them. 

You do not need to lug heavy, dirty bags around the store and haul them in your car – you can purchase compressed potting soil bricks with a coconut coir base that are much easier to handle:

Can You Plant Seeds Directly into Soil?

Directly planting seeds into the soil outdoors, or direct sowing, is the best choice for some crops.

Know your plants before you decide where to germinate the seeds. Consider the following before you plant the seeds:

• Will this plant’s seedlings transplant well? (If not, direct sowing is a better option)
• What seasonal conditions does the plant prefer?
• What is the time period for the seed to grow into seedlings?
• How sensitive is the plant seed or seedling to outdoor pests?

Generally speaking, root crops and other plants with a long taproot need to be direct seeded. Several other popular summer crops, like corn and beans, are also direct seeded. Crops to direct seed include:

• Carrot
• Radish
• Turnip
• Cilantro
• Fennel
• Corn
• Sunflowers
• Beans

Please note that links in the list above take you to posts with in-depth information on growing and caring for each specific plant. Make sure to check out these growing guides for a successful garden!

The Benefits of Starting Seeds Indoors

The many advantages of starting seeds indoors if it’s appropriate for the specific plant you’re growing. As discussed above, not all plants are suited to transplanting.

• Earlier harvest – indoor seed starting means you don’t have to wait for the right season and climate to come early. Simply grow the seeds early indoors and transplant them outdoors when the weather allows.  
• Easier to control conditions – watering, light, heat, and humidity are all easier to control indoors. This way, you don’t have to worry about excess water or too much sunlight and heat for delicate seeds and seedlings. 
• Fewer pests – there should be fewer pests indoors. Just make sure you remain wary of the health of the plants, especially if you have other house plants.

If you start seeds indoors, always follow a hardening off process to acclimate your seedlings to the outdoors. Learn more in this post on how to prepare seedlings for transplanting.

Some popular warm-season plants benefit from being started indoors and transplanted outside after the danger of frost is past. This allows you to grow longer season, heat loving veggies like pumpkins and tomatoes even if you have a relatively short summer. You can also start cool weather favorites like broccoli while it’s still too hot (or too cold!) so they can be grown fall and winter in many climates.

Some popular plants that are usually started inside include:

• Tomato
• Eggplant
• Pepper
• Broccoli
• Cabbage
• Melon
• Squash

The links above take you to posts with in-depth information about each vegetable. Make sure to stop by posts for any vegetables you’re interested in to learn about specific seeding requirements.

For an excellent, in-depth look at starting your own seeds and how to ensure growing success, read The New Seed-Starers Handbook. It covers organic methods for growing and has advice on picking your growing medium, ideal growing containers, transplanting, and more.

Can You Use Seed Starter Mixes to Pot Plants?

Seed starter mixes should not be used to pot plants. Remember that seed starter mixes have little to no nutrient value. This makes seed starter mixes unsuitable for transplanting plants or for container growing. IT also tends to cost more – don’t make your gardening habit more expensive than it needs to be!

What to Know Before Buying Seed Starters and Potting Soils

The difference between seed starters and potting soils is important to know. Unfortunately, it is not always easy to tell the two apart when buying them at the store. The names and taglines of the products can be confusing.

Some seed starter mixes may be labeled as potting mix, which can easily be confused with potting soil. Other seed starter products are labeled with the word “soilless,” but this isn’t very helpful since most potting soils are technically soilless, too. To be confident in what you are buying, always check the ingredients of your product.

If there is no ingredients list, stop and do not purchase the product.

Seed starter mixes ingredients will not have field soil, sand, compost, or composted manure. If any of these four ingredients are present, then you can know you have potting soil in your hands. 

Always buy starter seed mixes and potting soil bags that are dry. Products that are wet or damp and heavier due to water and are at greater risk for carrying diseases like the fungi that cause damping off.

Conclusion

Now you know what seed starting mix is and what is potting soil. Now it’s time to put your knowledge to use – check out these growing guides on Together Time Family so you can have a successful, bountiful garden!