Seed Starting Mix vs. Potting Soil: 5 Differences (Which is Better?)
Are you undecided if you really need seed starting mix as your garage is already full of potting soil? Are those two even different? Wondering if potting soil and seed starting mix are the same? Is one better than the other? I’ve been growing plants for a decade and I can tell by experience that they are not the same.
Seed starting mix differs from potting soil in terms of 1) nutrient content, 2) sterility, 3) texture, 4) components, and 5) accessibility. A seed starting mix is ideal for promoting germination allowing the seedlings to emerge with ease. Potting soil, on the other hand, can support the growth of more mature trees and can induce flowering and fruiting.
How does their difference translate to you and your use of seed starting mix and potting soil in your garden? Well, one of these can be used in place of the other but not vice-versa!
1. Nutrient Content
Potting soil typically has a much higher nutrient content than seed starter mixes. A potting mix is normally nutrient-rich due to compost while a seed starter mix will have little to no nutrients.
Seed-starting mixes, in general, have virtually no nutritional value. This is because seeds are just a pack of emergency nutrients that the seedling will use for its first inch of development until it grows the first roots and leaves. Just think about it!
Where will seeds take energy from to develop their stem and the first set of leaves if they don’t even have any leaves to harvest the energy from the light? In short, seeds don’t really need a lot of nutrients for germination.
A common brand (here on Amazon) clearly declares an N-P-K of 0.06-0.03-0.03. This is a common feature of the majority of seed-starting mixes.
Potting mixes, on the other hand, are nutrient-rich. This is because of the presence of compost or fertilizer—either organic or synthetic—in the mix. The majority of potting mix I played with had fertilizer in the form of slow-release pellets. They appear as small hard balls in the soil.
One of the best potting soil I tried (the Foxfarm here on Amazon) has an N-P-K of 0.3-0.45-0.05. This is up to 15 times more nutrient-dense than the average seed-starting mix—0.03 for a seed-starting mix against 0.45 for a potting mix.
If you go for a more fertilizer-aggressive potting mix, such as the Miracle-Gro, the N-P-K can be even higher.
A seed starter mix is generally sterile to ensure that no fungi development can negatively impact the growth of seedlings. In comparison, potting soil is not sterile so mold and fungi can develop in it.
What seeds need most is a constantly moist and warm environment. Warm and moisture are the perfect environment for fungi to thrive. This is a problem. Indeed, fungi can easily spoil your seeds.
Seed starter is very often sterile, reducing dramatically the possibility of any fungi developing.
A potting mix is not sterile, so there are chances for fungi to develop during germination and seedling development!
Learn more about this in-dpeth in our seed germination guide!
For many, the issue of a fungi-damaged young plant is quite important, especially if growing just a few seeds or in case they are quite pricey rare specimens.
However, this secure sterile condition is just a short-term requirement as all plants eventually need to be transferred to nutrient-dense soil or on a potting mix.
In terms of texture, a seed starting mix is normally more airy and lightweight than potting soil to allow the young plant to emerge from its seed and the medium without issues. Meanwhile, potting soil is typically heavier and more coarse to support the growth of older plants.
Seed starting mix is quite often way more airy, fluffy, and lightweight than regular potting mix. This is important to increase the germination rate.
I mean, think about it—roots developed by the energy left in a seed of a few milligrams cannot be massive. The seeds don’t have enough energy to grow thick and sturdy early on in their development.
So if the soil is very compact, hard, and coarse due to various rough organic materials in it, the seed’s roots will have less chance to develop successfully. This, in turn, reduces the chances of germination dramatically—which is also why you shouldn’t place seeds too deep into the medium.
The smaller the seeds, the more superficial they should be sown as they don’t have the energy sufficient to develop long stems.
As such, they have a better chance of growing and thriving in seed starter mixes.
On the other hand—despite still being fluffy—potting soil is generally way more coarse than a seed-starting mix. Potting soil often contains compost which, depending on the quality, might not be totally broken down.
Hence, it’s not uncommon to find medium-to-large pieces of wood materials and other organic materials. Poor-quality ones may even contain small stones. This is not a problem for a grown plant that passed the first delicate stage of development.
Fewer components, such as peat moss and perlite, make up a seed starter mix. Potting soil, on the other hand, commonly contains fertile organic materials such as compost to promote plant growth.
Your average seed starting mix mainly contains peat moss, perlite, and some wetting agent. This is the case, for instance, of the common seedling mix here on Amazon.
Another seed starting mix, more on the eco-friendly side as it is made of coco coir instead of moss which is a finite resource, is the one below from Amazon.
Both have great water-retention capability, though coco coir is not as good as peat moss in such instances. Hence, at the end of the day, there is just one ingredient.
Discover more about what makes it a great material in our article on coco coir!
On the other side, most potting mixes are a combination of perlite, compost, and peat moss or coco coir). Such ingredients are usually present to guarantee good water retention capability and provide your plants with some structure.
Some “secondary” ingredients commonly added to improve potting soil include different types of compost, vermicast, or fertilizer which look like small colored beads, and a pH corrector like limestone.
Although both seed starter mixes and potting soil packages are available in plant shops, potting mixes are arguably much easier to find and buy compared to seed starter mixes.
Potting mix is pretty much ubiquitous in all gardening shops. Even a mall with a small gardening area will very likely sell potting mix.
Conversely, a ready-to-use seed starting mix is way more challenging to find in physical shops, and it is often not part of the gardening portions of a more generic shop.
Even online, you can find just a handful—more or less just 5—of options that are among the most familiar and reputable brands.
You may see smaller shops selling them as well but they may not disclose all the necessary information about their products.
What to Use? Is Seed Starting Mix Better Than Potting Soil?
A seed starter mix is the better option over potting soil only for seeding and germinating purposes.
Seed starting mix will increase the percentage of successfully sprouting seeds because
- It can hold an even and constant moisture level
- Offer less resistance to the seedling while emerging
- Limits the change of seedlings being affected by fungi
Seed starting mix cannot be used as a replacement for potting soil as it lacks the mineral and nutrients for long-term plant development.
With seed starter mix, you need to transplant your seedlings to a larger pot filled with sufficient potting soil around 1–2 weeks after germination—or when the seedling sprouts from the medium.
Can You Replace a Seed Starting Mix With Potting Soil?
Potting soil can be used as a replacement for the seed starting mix when growing seedlings. Seeds can grow in a slightly nutrient-rich medium. However, watering should be handled more carefully and fungi growth is more likely.
Now some of you may even be wondering if you need a seed starting mix in the first place. To be honest, the answer is no.
You don’t need to buy a big package of seed starters just to get your seeds to germinate—which makes sense since virtually all plants can grow just fine in the wild without the need for a seed-starting mix!
If you still have some potting soil left, it will serve as a good replacement for a seed starting mix. After spending decades growing plants, this is often what I do as I have more seeds than space for plants to grow.
However, as I have already discussed before, seedlings are less likely to survive in potting soil since it is not a sterile growing medium. So using potting soil may not be the most efficient way to germinate your seeds—especially rare and expensive ones.
Can You Use Potting Soil for Seedlings?
It is possible to grow seedlings in a regular potting mix. This is easy and quick to accomplish with many herbs such as basil and mint.
I grow seeds all the time both for pleasure and learning. So over the years, I’ve learned numerous tricks to ensure successful germination after sowing my precious plant seeds!
For instance, you shouldn’t place countless seeds in one tiny container—a mistake I did in an old experiment for mint seeds. I would only really do this now if I want to grow them as a microgreen. But I would otherwise strongly advise against this. Planting 3 seeds per container are generally enough.
As I’ve explained, potting soil is fine for seedlings, although it is more likely to cause issues with fungal growth than a sterile starter. Think of potting soil like starting a seed in an outdoor garden— it certainly works, but it works well much less often.
The greatest advantage of growing seeds in potting soil is that there is no need to transfer the plant later on. Transplanting, especially in different soil, can stress and kill a new plant if not done carefully.
However, a seed starting mix’s texture and water-retention capability make germination way easier. This is very important if you have expensive seeds or only a handful of them. Moreover, compared to potting mix, it is not that much more expensive.
Nevertheless, such seedlings need to be transplanted to a regular potting mix to sustain plant growth. If you don’t do this, your seedlings are bound to die sooner or later—which is a common error among newbie home gardeners.
What is the Best Seed Starting Mix?
The best seed starting mix is one that completely lists all of its components, including beneficial fungi for plants called mycorrhizae. This allows a seedling’s roots to effectively absorb water and nutrients.
I have to admit that there are a few brands that gained a reputation over the years due to their reliability and quality ingredients. However, if I would need to pick up only one seed starting mix, I would go for the product below from Amazon.
This seed starting mix goes the extra mile compared to the competitor due to a special ingredient, which is still relatively new and unknown in the home gardening world.
I am talking about mycorrhizae. This is a special type of fungi which literally means “fungi’s roots.” As its name suggests, this beneficial organism creates a root-like system that “fuses” with the plant root system.
Simply put, mycorrhizae act as an extension of plant roots. This means that even a young plant can reach way water and nutrients fast and efficiently without waiting to develop a larger root system!
Plants grow larger, with more flowers and fruits, and have better resistance pests, pathogens, and stresses such as drought, when grown in mediums containing mycorrhizae.University of Manchester Study
More specifically, mycorrhizae increase the germination rate and help promote a plant’s further development even when moved it is to a different soil.
Here, some words of caution are needed when talking about mycorrhizae in the seed starting mix.
- Not Universally Beneficial. Even though such fungi do help, their effects vary significantly from plant to plant. For instance, they are almost ineffective on cabbage or some types of flowers.
- Can’t Withstand Pesticides. If you apply pesticides, these are also likely to negatively affect the rhizome and roots of the plant.
- Varying Effects. Plants and herbs respond differently. One might take longer than the other before showing positive effects.
- Slow Rate of Results. The response of plants is usually way slower than compared to fertilizer. As with every natural phenomenon, this is expected. However, this results in the long-term benefit of a more sustainable way of gardening.
Seed Starting Mix and Potting Soil: Are They Soil?
Neither seed start mixes nor potting soil is equivalent to regular soil. Rather, they are a mixture of various decomposed organic materials like moss and coco coir that are fine or no longer whole. Soil, in contrast, is composed mainly of sand, silt, and clay.
You have to know that potting soil, despite its name, actually isn’t like regular soil at all. What I mean by this is that you will not find any sand, silt, or clay in it.
It is simply a formulated growing medium—with each company having its own unique recipe—made by mixing peat moss, compost, perlite, fertilizer, and a few other materials such as lime for pH correction.
Some companies use coco coir as a replacement for peat moss. Others avoid adding slow-release fertilizer balls and instead use organic alternatives like guano or fish meal.
Can You Make Your Own Seed Starting Mix?
To make a seed starter mix from scratch, one can combine 5 units of dry peat moss or coconut coir with one unit of perlite.
Such a simple recipe for seed-starting will make this growing medium airy enough for the seeds to sprout while being able to retain sufficient moisture for a longer period.
Before sowing your seeds, just remember to add water. Add as much water as the perlite you add—1 unit of water per unit of perlite.
Remember, to do this before you sow your seeds. The growing medium should be moist before you placed any kind of seed in it for successful germination!
What is the best seed starting mix for vegetables?
The best seed starting mix for vegetables is by far the Foxfarm lightweight here. This seed starting mix can be used for all types of seedlings even though its additional benefits might vary depending on the crop.
How do grow seeds for a higher germination rate?
For a higher germination rate, high temperatures and moisture levels need to be guaranteed. For more, check the complete guide to seed germination and the required germination time and temperature of various plants including herbs.
Summary of Seed Starting Mix vs Potting Soil
Seed starter mixes are designed to improve the germination process but not the growth of the plant further growth. Improving the germination process simply means having a growing medium with physical and chemical characteristics to increase the germination rate and reduce germination time. Indeed, the growing medium plays a massive role in this.
On the other hand, potting soil is designed to sustain the long-term growth of herbs and plants but it is not specifically designed to ensure or improve germination. Moreover, as it is not sterile, seeds started in this medium are at risk of dying due to fungi development. Nevertheless, a potting mix can be used to grow seedlings, though with less success.
- “Potting soils and seed-starting mixes for your garden” by Gary Heilig in the Michigan State University Extension
- “Mycorrhizae” by Alan C Gange and Robin Sen in ResearchGate
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