Yes, you need potting soil for any plant growing in a pot with a closed bottom. Garden soil does not work in containers because it packs down, blocks drainage, and chokes your roots. University research shows that water moves through soil in a very different way inside a pot than it does in the ground.
I tested this during my second year of container growing. I filled five pots with garden soil from my backyard beds and five more with bagged potting mix. Same tomato starts, same sunny spot, same water schedule. By week four the garden soil in containers had turned into a dense slab with cracks on top. Those plants barely grew six inches tall. The potting mix group was already two feet tall and setting fruit.
The UMD Extension explains why use potting soil instead of garden dirt. Garden soil particles are too small and pack too tight for air and water to flow in a pot. In the ground, gravity pulls water down through feet of earth below your roots. In a pot the water hits the bottom in inches and pools there. Dense soil makes this worse by holding moisture against your roots until they rot and die.
How much garden soil is too much? UMD Extension says it should make up no more than 10% of the volume in large outdoor pots. For small indoor pots, the answer is zero percent garden soil. Your smallest pots need perfect drainage since there's less mix volume to soak up extra water. A six-inch pot filled with garden dirt can stay soggy for a full week after one heavy watering.
Is potting soil necessary for healthy container plants? Yes, because it fixes every problem that dirt creates. It stays fluffy so your roots can spread. It drains fast so your roots get oxygen between waterings. It starts with a balanced pH near 6.2 so nutrients stay available. And it comes free of weed seeds and disease bugs that ride along in garden soil.
The one case where you can skip potting mix is large raised beds with open bottoms sitting on the ground. Water drains into the native soil below, so you can use a garden soil blend without the packing issues you get in closed pots. But any container with a solid bottom and holes needs proper potting mix. Don't skip this step to save a few dollars. Dead plants cost more than a bag of good mix every single time.
Read the full article: Potting Soil Guide for Beginners
