Is potting mix soil? Not at all. The potting mix composition is 100% soilless despite the confusing name on every bag. What you pour out of that bag is a blend of organic and mineral parts built to do what garden dirt can't do inside a pot.
When I first squeezed potting mix between my fingers next to garden soil, the gap shocked me. The potting mix felt spongy and light with bark chunks and tiny white perlite mixed in. The garden soil felt dense and gritty. It clumped up tight. If you try this test yourself, you'll see right away that these two products look and feel nothing alike.
Penn State Extension says your premium mixes should be soilless for two big reasons. Leaving out real soil removes the risk of diseases like pythium and fusarium that attack your roots in pots. A soilless growing medium also keeps stable air space between particles. This lets your roots breathe even when the mix is damp. Garden soil packs down in your pots and chokes off the oxygen that roots need.
Each potting mix ingredient does a specific job for your plants. Peat moss or coconut coir soaks up water and stores it for your roots to drink between waterings. Perlite is volcanic glass that creates permanent air pockets in the blend. Composted bark adds rigid structure and stops the mix from packing down. Some bags also add vermiculite to hold water and nutrients right near your root zone.
In my experience the best way to learn about potting mix is to compare it to your garden soil side by side. I tested three different brands last year against my backyard dirt using the same herbs in matching pots. Every potting mix brand grew taller, greener basil than the dirt pots produced. Your results will be similar no matter what brand you grab.
You can tell if a bag has real soil in it before you open it. Pick it up first. A bag of pure potting mix feels light for its size since none of the potting mix ingredients are dense minerals. Squeeze the bag. It should give like a pillow, not sag like a sandbag. Then flip it over and read the label. Peat, coir, perlite, or bark should sit at the top of the list. If you see "topsoil" or "garden soil" listed, put that bag back on the shelf.
After you open the bag, grab a handful and press it in your fist. Good potting mix should hold its shape for a second then crumble apart when you open your hand. If it sticks in a tight ball, the mix is too dense for most pots. If it falls apart right away, it may drain too fast and need more coir mixed in. This squeeze test takes five seconds and tells you how the mix will handle water in your containers.
Read the full article: Potting Soil Guide for Beginners
