Seed Starting Mix Vs Potting Soil: Which To Use

Picking out a bag of potting soil at your local garden center can be a bit overwhelming for the first-time gardener. If you’re trying to start seeds for the very first time, rows of regular potting soil, organic seed starting mix, and other potting soils may seem overwhelming. It’s obvious they have different uses, but what are those uses, and which one do you need for your project? Understanding the difference between seed starting mix vs. potting soil is actually pretty easy, and knowing the difference can make starting seeds indoors or transplanting tender seedlings all the easier. 

Growing seedlings is a fun way to try new varieties of veggies, flowers, and herbs and if you’re a plant lover, you’re bound to want a specific plant you just can’t find at any local garden store. When you grow plants from seed, you’ll really want to understand the difference between seed starting mix that you use when first germinating seeds, and potting soil that you transplant your baby seedlings into in order to give them more room and nutrients to grow. Let’s go over the differences in more detail. 

When first starting your outdoor garden, you’ll discover that plant growth is highly dependent on the type of soil you’re using. When first germinating your seeds, your tender seeds sprout delicate small roots that have a hard time fighting their way through dense soil. They thrive in the fluffy and light soil of a seed starter mix because that is where they have the best luck pushing their fine roots through. It’s at this stage in growing plants that your optimal growing medium is choosing from a variety of seed starting mixes. 

All About Seed Starting Mixes

Seed starter mixes aren’t always made from soil. They’re made from a variety of ingredients like sphagnum peat moss, vermiculite, coconut coir, perlite, and rice hulls. All of these ingredients are chosen for their use. An organic material like sphagnum peat moss is chosen for its organic matter and water retention that helps to provide moisture for the seeds. Starting mix often uses perlite, which is an expanded rock and due to its porosity is useful for aeration and drainage. The addition of coconut coir adds moisture to your seed-starter without causing it to become too soggy. This wetting agent works as a wonderful moisture retention tool at a crucial time, as young seedlings can easily dry out and dye before reaching maturity. Sometimes moisture retention granules such as vermiculite are also added to help with moisture at a critical period in your seeds development. 

The root growth experienced at this time happens relatively quickly. Another reason why soilless seed starting mixes are widely used is that rich soil is not necessary during the germination stage. Seeds actually don’t need additional nutrients during this time. The structure of the seed contains both the plant embryo and enough nutrients to sustain the plant until it sets its first two true seeds.

To make matters even more complicated, there is very little regulation as to what seed starters are supposed to be called. They can be called anything from soilless seed starting mixes, to soilless mixes to potting mixes. 

If you have limited space, are starting seeds early in the year, or are choosing to start seeds indoors, try going with a light and fluffy seed starting mix, and be prepared to transfer out your healthy seedlings into larger pots once they’ve reached a healthy seedling stage. 

One of the alluring parts of using a seed starting mix is the fact that when the seed starting mix first enters the bag after being produced, it is technically sterilized. It may not stay that way in between the factory and the time you buy it, but as a result, it may have fewer pathogens in it than a DIY seed starting mix or a wet and used seed starting mix laying around in the backyard. Additionally, one of the most important reasons for using sterilized seed starting mix is the lack of weed seeds in these mixes, but the bag needs to have not been contaminated while in your possession. By starting seeds in a weed-free mix, it is easier to know which is your own seed germinating, and what isn’t. For first-time growers who need the help of seed packets to know whether their seed germinated or a weed, this is a huge help. 

If you’re looking for a quick and easy way to tell the difference between a commercial potting mix and a commercial seed starting mix, there is an easy way to tell the difference. If it has no field soil, compost or sand listed on its ingredient label, it’s seed starting mix, and if it has field soil, sand, or compost, it’s potting soil. 

For a great option to make your own homemade seed starter, watch our video below!

Potting Soil

Potting soil and seed starting mix tend to differ first and foremost in the size of their mediums. With potting soil, your growing medium tends to be on the thicker side. These soil mixes tend to include larger pieces of organic material like chunks of bark or sticks. While this medium isn’t the best for delicate seeds, it is the perfect medium for container plants once they’ve reached a large enough size (once again, after they’ve put out their first set of two true leaves). 

Potting soil is generally made up of compost, composted manure, or field soil as well as sphagnum peat moss (or sometimes coconut coir) and vermiculite. It may have fertilizer mixed in, something that you should be aware of if sowing seeds as seed starting soil can’t have too much fertilizer in it without accidentally ‘burning’ the seeds. Potting soil ingredients usually differ among brands, which can be frustrating to the gardener just getting started. However, as you begin to understand which ingredients can help with which aspect of growing, you may find that this can often help to your benefit. But there are other things to take into consideration as well; large chunky mixes may or may not actually make contact with the seed, which means moisture penetration into the seed may be slowed. Soil-based blends can contain large quantities of clay, and that can hold way too much moisture. 

In general, potting soil may actually contain soil, which means it may also contain soil microbiologicals such as fungi or bacteria, and depending on the soil, it might be stuff that lingers such as the fungal causatives of problems like damping off.

Potting mix usually is soilless (and may be sterilized), but often contains larger particulate matter like forest products (aka wood bits), and may or may not include a fertilizer that’s totally unnecessary for seed starting. Some potting mixes are optimized to particular uses, such as a raised bed mix with lots of added forest products added for long-term moisture retention and excellent drainage.

Peat moss has a slightly acidic pH and is great on its own for plants that prefer a more acidic variety such as blueberries. For other crops, like beans and brassicas, you might go with a mix that includes not just the peat but other ingredients like compost, as that usually provides a more neutral environment.

Take, for example, starting tomato seeds. These seeds are of a smaller size and would have a difficult time getting started in most potting soils. The coarser texture of the potting soil or the garden soil may inhibit growth by not providing the correct amount of continuous moisture. However, after the tomato plants grow into healthy young plants, say in a seed starting soilless mix, you can then start transplanting your seeds into your choice of potting soils. Tomatoes have an aversion to ‘wet legs’, i.e. continuously damp roots and so you might want potting soil that has better drainage. As you gain familiarity with your seeds and what’s available you’ll get a better idea of what to look for in a seed starting soil. 

While some gardeners do use potting soil for starting seeds, you might want to take into account what type of seed they’re using. When you sow seeds large enough for a potting mix, you’re sowing seeds that should be about the size of your pinkie fingernail. Imagine one of the larger sunflower seeds, pea seeds, or even beans. Starting seeds is more of an art form than a science and getting to know your own seed options well enough can take time. Organic gardening is different for every gardener, and getting it right involves a lot of trial and error to find out what works best for you. 

Home gardeners can easily make their own potting soil at home for transplanting seedlings, for their potted plants, or for another all-purpose potting mix. Farmers and gardeners alike can make their potting mix at home using a simple formula. Try mixing together equal parts compost, peat moss, and perlite. Mix and strain through a fine-mesh hardware cloth and add (optional) a natural fertilizer like kelp meal or worm castings. Powdered azomite is another great addition to a homemade potting mix as it contains trace minerals harder to find. 

When getting to know potting soil and seed starting mix keep in mind that only OMRI listed potting soils and mixes are actually guaranteed to be organic. Potting soil and seed starting mixes can call themselves organic without that on the label, but the OMRI designation means that the blend has been confirmed to be organic by the Organic Materials Review Institute, or OMRI. All components used in an OMRI-rated organic soil blend must be confirmed to be from organic sources, so you don’t end up with any odd chemical fertilizers or other questionable components. Companies that are not OMRI-certified may use partially organic ingredients, but there may be some inorganic or chemically-derived additives too.

Hopefully, this gives you a good idea of seed starting mix vs potting soil and why they have such different uses. Remember, there are many different variations out there, so it’s important to make sure you have what’s right for your specific plants.

Don’t know which starter pots to choose from? Try the Epic 6-Cell or Epic 4-Cell starter trays! These make transplanting easy, and will last you a lifetime.

Frequently Asked Questions

Q: Can I use seed starting mix instead of potting soil?

A: Seed starting mix is optimized to help seeds germinate and generally doesn’t contain much, if any, nutrient value. It’s meant to be ideal for germination, but not great for growing stuff later. Since most people up-pot or transplant their seedlings into other soil, seed starters can be used to get things growing and you can then plant it into a potting mix.

Q: What is the best soil for starting seeds?

A: While the best soil is “the one that will get you to plant seeds,” if you’re using older seeds or seeds with low germination rates, a dedicated seed starting mix will get you a better chance at success overall. The fine particulate ensures good moisture-to-seed contact, and that enables the seed to hydrate fully and spring fully to life. Larger seeds like sunflower seeds can be planted in potting blends as discussed earlier, as the larger seed size gives it more surface area to absorb moisture from the soil. Ideally, you’ll want the potting blend to hold at least enough moisture that it feels like a wrung-out sponge, but not so much that it turns muddy.