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

Choosing the right potting soil can feel overwhelming for first-time gardeners. The rows of regular potting soil, organic seed starting mix, and other soil types may seem confusing. While it’s clear they have different purposes, understanding those differences and knowing which one you need makes starting seeds or transplanting seedlings much easier.

When starting your garden, you’ll quickly discover that plant growth heavily depends on the type of soil you use. During germination, delicate seed roots struggle to grow in dense soil. Instead, they thrive in the light, fluffy texture of a seed starting mix, allowing their fine roots to push through with ease. At this stage, selecting the right seed starting mix is crucial for giving your seeds the best chance to sprout.

It’s important to understand the difference between seed starting mix, used for germinating seeds, and potting soil, used for transplanting seedlings to give them more space and nutrients to grow. Let’s break down these differences in more detail.

Seed Starting Mixes

Seed starter mixes aren’t always made from traditional soil. Instead, they consist of various ingredients. Sphagnum peat moss, vermiculite, coconut coir, perlite, and rice hulls, are all carefully chosen for their specific benefits:

• Organic materials like sphagnum peat moss provide moisture retention, helping seeds stay hydrated.
• Perlite, an expanded rock, is included for its porosity, aiding in aeration and drainage.
• Coconut coir adds moisture retention without making the mix overly soggy, which is crucial since young seedlings are highly vulnerable to drying out.
• Vermiculite, another moisture-retaining component, is often added to ensure consistent hydration during this critical growth stage.

Root growth at this stage is relatively rapid, but nutrient-rich soil isn’t necessary. Seeds contain all the nutrients needed to sustain the plant until the first two true leaves develop. The primary function of a seed starter mix is to provide an optimal environment for germination. This focuses on structure, aeration, and moisture retention rather than nutrient content.

To complicate things further, the terminology for seed starter mixes isn’t well-regulated. These mixes may be labeled as soilless seed starting mixes, soilless mixes, or even potting mixes, which can add to the confusion for new gardeners.

When to Use Seed Starting Mix

If you have limited space, are starting seeds early in the year, or plan to start seeds indoors, opt for a light and fluffy seed starting mix. Be prepared to transplant your healthy seedlings into larger pots once they reach the appropriate stage.

One of the advantages of seed starting mix is that it is technically sterilized when it is first packaged. While it may not remain completely sterile between production and purchase, it often contains fewer pathogens than a DIY mix.

Another key benefit of sterilized seed starting mix is the absence of weed seeds. This ensures that any sprouts you see are from your planted seeds, making it easier to identify germination success. For first-time growers, a weed-free mix eliminates confusion and makes the process much simpler.

Finding Seed Starting Mix

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

Potting Soil

Seed starting mix and potting soil primarily differ in the size of their components. Potting soil typically has a coarser texture, often containing larger pieces of organic material like bark or sticks. While this medium isn’t ideal for delicate seeds, it’s perfect for container plants once they’ve grown larger, usually after producing their first two true leaves.

Potting soil is generally composed of materials like compost, composted manure, or field soil, along with sphagnum peat moss (or sometimes coconut coir) and vermiculite. It may also contain fertilizers. The specific ingredients vary by brand, which can be frustrating for beginners. However, as you learn which components serve different purposes, you may find this variability works to your advantage.

Potting soil often contains actual soil, which means it may also harbor microorganisms like fungi or bacteria. While some of these can be beneficial, others, such as those that cause damping off, can be problematic.

Potting mix, on the other hand, is usually soilless and sometimes sterilized. It often includes larger particles like wood bits and may also contain unnecessary fertilizers for seed starting. Some potting mixes are designed for specific uses, such as raised bed mixes, which include forest products to enhance moisture retention and drainage long term.

When to Use Potting Soil

Peat moss has a slightly acidic pH, making it ideal for plants that prefer acidic conditions, such as blueberries. For crops like beans and brassicas, you might opt for a mix that includes peat moss along with other ingredients like compost, which typically creates a more neutral environment.

Take tomato seeds, for example. These small seeds can struggle to germinate in coarse potting soils or garden soils, as the texture may prevent adequate moisture retention. Instead, starting tomato seeds in a fine, soilless seed starting mix provides the consistent moisture they need. Once the seedlings grow into healthy young plants, you can transplant them into potting soil.

Since tomatoes dislike ‘wet legs’ (constantly damp roots), a potting soil with good drainage is ideal. As you gain experience with different seeds and soil options, you’ll develop a better understanding of what to look for in a seed starting mix.

While some gardeners use potting soil for seed starting, the choice often depends on the type of seeds. Larger seeds, such as sunflower seeds, peas, or beans (about the size of a pinkie fingernail) can handle coarser potting mixes.

Finding Potting Soil

Home gardeners can easily create their own potting soil for transplanting seedlings, potted plants, or as an all-purpose mix.

A simple formula involves mixing equal parts compost, peat moss, and perlite. After blending, strain the mixture through fine-mesh hardware cloth and optionally add a natural fertilizer like kelp meal or worm castings. You can also include powdered azomite, which provides trace minerals that are harder to find in other ingredients.

When choosing between potting soil and seed starting mix, it’s important to note that only OMRI-listed products are guaranteed to be organic. While many products may label themselves as organic, the OMRI designation signifies certification by the Organic Materials Review Institute.

This certification confirms that all components in the mix come from organic sources, ensuring no chemical fertilizers or synthetic additives are included. Products without OMRI certification may still contain some organic ingredients but could also include inorganic or chemically-derived components.

Frequently Asked Questions