Why do over-watered plants get root rot but we can grow plants hydroponically?

If I over-water a house plant, the roots can get starved for oxygen and rot. Why can you grow thriving plants in nothing but water?

They Continuously add oxygen (air) to the water used in hydroponics To avoid creating anaerobic zones.

Don’t some systems pump nutrient into the growing medium and then drain it?

Root rot can be a bigger problem for hydroponic plants than plants in soil. They aren’t somehow protected from it.

Root rot usually is caused by lack of oxygen. In hydroponic systems, you can increase the aeration and adjust the nutrient balance to treat it. If you overwater a potted plant, you don’t have those treatment options, so it’s important to not overwater it from the start.

I am growing a plant from a cutting by sprouting it in water. No special aeration or anything else.

Water can hold a certain amount of dissolved gases, and most of those come from the atmosphere. In a deep water hydroponic system where the plant roots are fully submerged all the time, air is pumped into the water which both helps replenish the oxygen that is used up and helps circulate the water around.

In a cup of water there is so little water that it’s pretty easy for oxygen to continue dissolving in and so little being used by the tiny roots developing that you don’t need to aerate the water.

In a pot full of soil, there is far less ability for oxygen to dissolve down into the water; likely only the top few mm could have gas exchange due to all the soil particles. Also, there is no circulation of that water… it just sits stagnant. Once the oxygen in that mud gets used up more can’t get back in fast enough and the roots start to die.

Tiny plant roots in an open cup of water is like sitting on a big porch with screen windows. Not perfect air circulation, but plenty of fresh air coming in compared to how much you’re using. A full grown plant with roots flooded by stagnant water is like being in a closet with the door closed and the door seams taped shut.

Do plant roots consume the oxygen? If not, where does it go?

This is the basic explanation I recall. It’s not just the water, it’s a stagnation environment that is the problem.

Root rot is also a function of the type of plant and its particular growing conditions.

As for oxygen depletion in the soil - this can be problematic even outdoors under normally good growing conditions, when there’s been heavy rain followed by bright hot sunshine. Gaseous exchange at root level is impaired, so paradoxically you see (usually temporary) wilting.

Absolutely. Plant tissues consume oxygen to metabolize sugars, the same as animals do. The roots, submerged in water, endure damage, and die, just like animals do. The aerial parts, that have chlorophyll, make oxygen, and in twilight, consume that same oxygen to metabolize the sugars they plant has made. And on a bright sunny day, excess oxygen escapes. Hydroponic system that grow roots under water thoroughly oxygenate and only grow fast-growing plants, such as lettuce, and there’s still some roots in the open (although very humid) air. Just look at the images, all have some in the air: https://www.google.com/search?q=Deep+water+culture&client=firefox-b-1-d&source=lnms&tbm=isch&sa=X&ved=2ahUKEwj527W1vOjpAhX_g3IEHYpPC_IQ_AUoA3oECAwQBQ

“Hydroponics” is a very general term for a variety of growing techniques. Years ago (statute of limitations expired) I played around with hydroponically growing cannabis. The two techniques I used were flood and drain (nutrient containing fluid periodically pumped into system then allowed to drain) and aeroponic bubbling bucket (a plant is suspended above liquid nutrient solution while a pump forces air through an aquarium air stone, causing splashing).