When I set out on my gardening journey more than 15 years ago, I had no idea that there was a difference in potting soil vs. seed starting mix. Not a clue! I just grabbed any old bag of dirt off the shelf and called it good. I used that dirt for seed starting, potting deck flowers, and house plants. Then I wondered why my seeds weren’t germinating. Now I know better.
So just what is the difference between potting soil and seed starting mix? There are quite a few differences from composition to ingredients, nutrients and more. Let’s explore.
Difference In Ingredients
Let’s first look at the difference in ingredients for both seed-starting mixes and potting soil. Each growing medium has a different makeup that leads to differing nutrients, texture, and purpose.
Ingredients In Seed Starting Mix
Seed starting mix isn’t made with regular garden soil and often is a soilless medium. The main ingredients include:
- Sphagnum peat moss is a partially decomposed organic matter from peat bogs and helps the mixture retain moisture in soil mixes.
- Coconut coir, also known as coco coir or coconut fiber, is made from the outer husks of coconuts. It’s added to soil mixes to help it retain moisture, aerate the soil, and prevent soil compaction.
- Vermiculite is a mineral used as a soil amendment to improve water retention, aerate soil, and aid nutrient takeup for young plants.
- Perlite is a lightweight material made from volcanic glass that helps promote good drainage and prevent root rot.
Some seed starting mixes contain other elements to provide more nutrients for encouraging plant growth. It’s common to find worm castings, bone meal, kelp meal, and more. These additives encourage healthy plant growth after seed germination has occurred. But seedlings don’t need added nutrition early on as they have all the nutrients they need to get started from the seed casing.
A note on peat moss vs. coco coir: Some companies’ seed start mixes are transitioning from using peat moss to coconut coir. The main reason is sustainability. I like looking for alternatives to peat whenever I can when shopping for a seed starter mix. I can’t find a peat-free mix locally so I’ve started making my own seed start mix. More to come once I fine tune the mixture.
Ingredients In Potting Soil
The ingredients in regular potting soil vary greatly but often have two key elements that differentiate it from the seed start mix.
One is the addition of compost and organic materials. The decomposed organic matter comes from a wide variety of sources but they all add alot of nutritional value to the mix. This is also why it’s not uncommon to find pieces of sticks or brush in a potting soil mix.
The second, is added soil amendments to help young seedlings grow. While the amendments in the seed start mix are more for short-term growth, like healthy root development, the regular potting mix has nutrients aimed for more long-term growth.
Some common ingredients of potting mixes include peat moss, coconut coir, vermiculite, perlite, worm castings, compost, bark, sand, chemical fertilizer (in non-organic potting mix), activated charcoal, kelp meal, bone meal, and more.
Difference In Nutrition
The key nutritional difference between potting soil and seed start mix is aimed towards the short-term vs. long-term needs of the plants.
Many seed start mixes have very basic or short-term nutrition. The mix is the right choice for germination but not for long-term growth. Many ingredients of seed start mix aim to help the plants germinate by retaining moisture without excess water, encourage healthy root growth, and provide for the plant’s early growth. Seedlings don’t need fertilizer at first as everything the plant needs early on comes from the seed casing.
Potting soil, on the other hand, is rich in compost providing more long-term nutrition for plant growth. Peat moss and coconut coir are added to help lighten up the mixture while other nutrients are often added for best results with longer-term plant growth.
Difference In Texture
The other big difference between potting soil and seed start mix is in the texture of the medium.
The seed starting mix has a much finer texture to help the medium hold water. Germination is greatly affected by moisture. If the mixture dries out or remains too wet, young seedlings can die off before they even break the surface of the soil.
Potting soil has a much coarser texture attributed to the compost. Sometimes, it’s necessary to strain the soil mixture to remove larger pieces, like sticks, that haven’t decomposed yet.
In the picture below, the top saucer is filled with a seed germination mix while the bottom saucer is a more traditional potting mix. Specifically, I used Black Gold Natural and Organic Seedling Mix (current favorite store-bought mix but it’s not peat free) and Happy Frog Potting Soil (my current favorite potting soil). Do you see the small chunks in the potting soil and how coarse it is?
Which To Use: Potting Soil Or Seed Start Mix?
Experienced gardeners use both. Seed start mix has the perfect ingredients for germination and the early stages of plant growth while potting soil is too heavy for a seed to push its way up through.
Once a seed has germinated the seedling breaks through the soil surface, and it needs more nutrients that the seed starting mix doesn’t have. After the seedling has its first pair of true leaves is a great time to up pot those seedlings into larger pots with potting soil or transplant mix.
Can I Use Potting Soil To Start Seeds
It’s not a great idea, but you could make it work. To use potting soil to start seeds, you’ll want to sift out any larger material. Small seeds don’t germinate well in heavier soils. You may also consider adding some finer materials, like peat moss or coconut coir.
There are some seeds that I started using potting soil. Onion seeds, for example, need to be started early and stay in pots for a considerable amount of time before transplanting them into the kitchen garden. Because I want them to have longer-term nutrition that the seed starting mix doesn’t have, I layer soil in the pots. Sifted potting soil makes up the bottom third of the pot, while seed start mix is layered on top. The combo helps my seeds germinate while providing more nutrients for the seedlings until they can be moved to garden beds.
Can I Use Seed Starting Mix As Potting Soil
Again, it’s not a great idea or the purpose of seed starting mix, but you could make it work with some adjustments. Mainly, you would need to add fertilizers, compost teas, or other organic material to the seedlings throughout their long-term stay in the seed start mix.
I’ve done this before but it is a lot of work and I often don’t keep up with it to encourage healthy plant development. For example, with onion seedlings, I used to start them in seed start mix and keep them in the medium for months. Whenever I did this without adding more nutrients to the seeding mix, my onions were stunted and never produced much during the growing season. Adding compost teas regularly worked, but it’s time consuming.
Choosing Seed Starting Soil To Start Seeds Indoors
So when it comes to choosing the best seed-starting soil to start your seeds indoors, keep in mind these basic concepts. Seed starting mix is great for germination due to its fine texture and short-term plant nutrition while potting mix is better for long-term seedling growth but poor for germination. Both have different purposes that help with different stages in the plant’s lifecycle. The subtle differences between the two will make a big difference in your gardening.
When you’re shopping and choosing the soil type and brand, first look for the type of soil you need. Is it for seed starting or potting plants? Then look at the ingredients and find one that suits your needs.
For example, if you’re starting seeds and prefer more sustainable products, first look for seed start mix and then scan the ingredients to see which one uses coco coir bricks instead of peat moss. Later, you can plant your new seedlings in larger pots if they need more growth time before transplanting outside.
Another example, is if you’re up potting pepper plants after they’ve germinated and prefer organic gardening. Look first for potting mixes and then scan ingredients to ensure the compost used is organic.
I hope this helps you understand the differences in potting mix vs. seed starting mix and how both play a role in the garden. And if you’re still unsure, most local garden centers have a ton of products along with knowledgeable staff who can help you choose the right soil for your gardening needs.
Happy gardening all!
