How can ‘over-potting’ be a thing when plants grow straight from the Earth’s surface with infinite amounts of soil available?

‘Over-potting’ isn’t about ‘too much soil’. It specifically refers to the relationship between a plants ability to uptake water, and the soil it’s planting in, ability to dry out.

Most plants require a wet/dry cycle to properly grow. If you put a teeny-tiny plant in a big giant pot and water the pot until it’s entirely saturated, the amount of soil in the pot will take longer to dry out than the plant’s ability to uptake the water. Thus, the plant can literally ‘drown’ because in addition to water, their roots require oxygen, and they die.

Plants in the ground have this problem also….it’s just that there’s usually plenty of other plants competing for that water and they deal with it.

“Overpotting” is also a bit of a oversimplification. Just because a pot seems too big or too small doesn’t mean it will do inherently poorly or well. A large pot that’s well-draining with lots of perlite, sand, bark, and other media that’s poor at retaining water can dry out faster than a much smaller pot with rich soil with like vermiculite and packed mosses. How quickly a pot dries out is all about the pot shape, the media, the pot material, how you watered the plant, how wet tolerant your plant is, how much water the plant uses, temperature, humidity, etc. One of the best things to learn about houseplant care is learning how to adjust your potting and watering to fit your lifestyle and your plant’s needs.

>LI5: How can ‘over-potting’ be a thing when plants grow straight from the Earth’s surface with infinite amounts of soil available?

Because there isn’t “infinite soil” available. Many plants are specialized to grow in very thin, nutrient-poor soils and will rot or suffer other ill-effects in soil they’re not adapted for.

A lot of people kill Blueberries for example (a plant that grows on thin, sandy soil or even nearly bare rock) by planting them in a rich, nutrient-rich soil. They grow on barrens with pines and oaks similarly adapted for that habitat.

Moisture has a profound effect too, a big pot full of spongy soil will hold onto a lot more water than a plant can use and will cause root rot. I grow cacti in the Northeast were it is very humid and I have to keep even large cacti in comparatively tiny pots to ensure they dry out as quick as possible.

Besides what other people have said, I think you might be very surprised at how shallow and not infinite soil is in many places.

In the ground, water can drain away in every direction. In a pot, it can only drain out through a small hole in the bottom. Moisture is retained much more easily in a pot, especially in the dense, rich potting soil sold by most nurseries, leading to rot.