Seed Starting Mix vs. Potting Soil: What’s the Difference?


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Seed Starting Mix vs Potting Soil

So many types of soil are available at the garden store that it makes my head spin! Which one do I use to start my seeds? Will my plants suffer if I use the wrong one? It’s a gardening nightmare! My garden is my life, so I dove into this subject fervently, determined to do the best for my plants.

What is the difference between a seed starting mix and potting soil? Seed starting mix is finer in texture than potting soil. It contains ingredients like peat moss, vermiculite, coconut coir, and perlite. It does not contain actual soil. Seed starting mix is lightweight, specifically designed not to weigh down seeds as they germinate. Potting soil is denser, with a coarser texture. Potting soil generally contains compost or field soil along with peat moss and vermiculite. It also usually includes some type of fertilizer, though not always.

So, now that we know the difference, the question becomes, which one to use? And, with so many options at the garden store, it truly is a time-consuming task. I don’t know about others, but I prefer to spend more time in the actual garden than at the garden store, trying to decide what on earth to buy! Hopefully, this guide will make it easier, and other gardeners can spend their free time doing what they love: growing plants.

When to use seed starting mix versus potting soil?

For the best results, the seed starting mix should be used for planting seeds and potting soil should be used for transplanting and container growing. They are both important to establish the plant and provide an ideal growing environment.

The seed starting mix is used to get the seed germinated. Then, once it reaches the seedling stage, it is transplanted to potting soil.

Potting soil may be used for seed starting, in some cases. It usually won’t harm the growth of the seeds. It just makes them work a bit harder to push up through the soil. 

Also, the potting soil may contain unnecessary fertilizer, which can burn the delicate seeds. A small amount of fertilizer isn’t harmful, and a lot depends on whether it is synthetic or not as synthetic ones tend to be stronger and cause more injury. To be safe, it is best to use the seed starting mix for germination.

Seed starting mix should not be used for transplanting, as it isn’t substantial enough to support complex root systems. Also, the seed starting mix does not contain fertilizer, which is something transplanted plants need.

How to tell the difference between potting soil and seed starting mix?

This seems like an easy question, right? After all, the bags will be labeled. Unfortunately, it just can’t be that easy. That’s life, right?

There is absolutely no uniformity in naming these items. It’s insane, really. Some companies use the term potting mix as a label for soilless seed starting mixes. Potting mix sounds a lot like potting soil, doesn’t it? Very confusing.

Other companies use the term soilless mix to indicate it’s a seed starting mix. The only way to truly understand these items is to read the ingredients. This is actually always a good idea, as a rule of thumb.

If it contains no field soil or compost or sand, it is a seed starting mix. If it includes soil, compost, or sand, it is potting soil.

Is seed starting mix even necessary?

Technically, no, seed starting mix is not necessary. However, plants will be eternally grateful if they get it. Seed starting mixes are designed to provide the perfect growing environment for seeds. Just using potting soil will be okay, though, in most cases. However, the potting soil won’t be okay to start seeds if it contains high levels of fertilizer, sand, or topsoil.

Why do I need to use potting soil or seed starting mix, anyways?

All these different soils are confusing and can get expensive. It’s tempting to forget about it all and just use topsoil from the garden. This would be devastating for plants, though.

Never use garden soil to start seeds or with container plants. It is too heavy and may contain pests, weed seeds and diseases.

The following article on our website provides an excellent detailed description of the problems with using garden or topsoil.

Can You Use Topsoil to Start Seeds?

Organic or Natural, does it make a difference?

Yes, and no. Organic options are always best for growing because they don’t contain ingredients that are potentially harmful to the environment. However, there isn’t much oversight as to what the “organic” label means in potting soil and seed starting mixes. Many companies put the organic label on their products, but these growing mediums are not approved for organic gardening.

Only products with the “OMRI” (Organic Materials Review Institute) label are certified for organic gardening.

The “natural” label means even less than organic labeling. Most potting soils and seed starting mixes are inherently natural because they are made up of ingredients from the earth, like soil, sphagnum moss and perlite. In fact, all potting soils and soilless mixes should be natural, by default. Adding the label onto the package means nothing.

The importance of making the right choice

So many options are out there all making the same claim that they are the best. Seed starting and potting soil mixes range widely in prices and claims, and sorting through them all is time-consuming. Being an informed buyer, though, is the best way to get the best mix to grow plants. The more expensive ones aren’t always better. 

Making the right choice is important because the wrong product may impede the seed or plant’s growth, forcing a start over. Not fun! It is best to start with the correct product and save the time and heartache of failed or poorly growing plants.

How to choose a seed starting mix

An effectual seed starting mix will contain some combination of these ingredients: sphagnum moss, peat moss, coconut coir, perlite, vermiculite, and a wetting agent.

  • Avoid any products that contain “forest products.” Forest products are a dense filler ingredient found in inferior seed starting mixes.
  • Do not buy any mix with fertilizer or lime in it. The combination of moss, perlite, coconut coir, and organic material already provides enough nutrients to get the seeds started. The seeds do not need any additional fertilizer. And, in fact, fertilizer at this stage can burn or harm the seeds. More fertilizer is not always a good thing!
  • Do not buy any bags that are wet, damp, or heavy. The mix may become compacted, the opposite of what is desired. It also may contain sand as a filler. Only purchase bags that are dry and lightweight.
  • Do not purchase any product that does not list the ingredients. Who knows what is in there!

How to choose a potting soil

  • Look for the type of fertilizer and amount, if listed. Most potting soils have fertilizer added, which is great. However, a potting mix heavy on fertilizer isn’t great for all plants. Fertilizer encourages a plant to grow, but this may be at the expense of fruits. Organic fertilizers are much better than synthetic ones.
  • Do not buy any bags that are wet or heavy. Water activates fertilizer, and this is not something that should happen before it is even used. Also, heavy bags may contain excess sand filler.

Are specialty potting soils worth it?

In many cases, yes. It depends on the particular advertised use. For example, succulents and cacti absolutely need specialty soil. Sand is an essential element for these plants, while for other plants, sand is detrimental. Orchids and African violets also require special potting soil.

Sow for yourselves righteousness; reap steadfast love; break up your fallow ground, for it is the time to seek the Lord, that he may come and rain righteousness upon you.

Hosea 10:12

The problem with peat moss

Peat moss has long been used to create soil mixes because it is an almost perfect medium. It is lightweight, natural, and retains water and air exceptionally well. Peat moss is excavated from bogs that took hundreds of thousands of years to form.

Recently, the sustainability of this practice is being brought into question. The bogs being excavated are destroyed during the process, with little chance for recovery. This is not a reliably renewable resource. Peat bogs are an important ecological environment, and their destruction hurts the natural world.

  • Peat moss is different from sphagnum moss. Sphagnum moss is the young, green form of peat moss and is renewable. To make things confusing, some labels say sphagnum peat moss, which is the same as peat moss. If it has the word peat in it, it is the non-environmentally friendly type.

Many companies are using coconut coir and rice hulls in their mixes as an environmentally friendly substitute. Coconut coir, in particular, is an exceptional replacement for peat moss.

Can I make my own potting soil or seed starting mix?

Healthy Soil

Yes! In fact, it is the best thing to do. Gardeners who do this know exactly what is in it. Of course, it is more time-consuming, which can be a big downside. However, the benefits are great, and it will save money in the long-term. Learn how to make homemade potting soil with this guide.


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Resources:

University of Connecticut Home & Garden Center

Michigan State University

University of Maryland, Home and Garden Extension

University of Massachusetts, Amherst, The Center for Agriculture, Food, and the Environment

University of Vermont Extension




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Corey Leichty

Hi, I’m Corey and I love using gardening as a way to provide food for my family, learn life lessons alongside my wife, Andrea, and teach life lessons to my two sons. Do you have gardening questions? Not finding what you are looking for? Please feel free to Ask a Question (Click Here!) and I will get back with you as soon as I can!

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