I am only here because debating this is more entertaining than anything else.
I don't believe these links that are attached to selling something. "Proven winners" "pro mix" sounds like advertising more than science.
I am a bit disappointed in the UCDavis link because it is about hydroponics.
Before we even begin to discuss oxygen and roots we have to know what type of hydroponic system is being used. The paper does not say, so it is essentially useless.
If we are to assume it is a hydroponic system where roots are constantly surrounded by water, we can use the argument that it is only validating what we are arguing. Since constant water would mean ONLY getting Oxygen from water.
Sorce
I agree you point on the Proven Winners/pro-mix link, their goal is to sell something ultimately. I've never used or sold this soil, and where I work has no contract with any plant distributer (looking especially at you, Monrovia). Honestly, I just did a quick and dirty search at work to find these links, but they all (university, peer reviewed paper, horticulture whatever) say the same thing -roots need air/oxygen. I try to always support things scientifically, and that ain't that, but still think these companies are saying this because its true, and their bread and butter depends on it. That said, I too am very weary of the hucksters.
The hydroponics paper I thought was especially poignant because it directly gets at this deal of non-aquatic plants requiring oxygen at their roots, even especially if submerged in water. This directly relates to bonsai soil and putting a tree in a small pot. The first two lines of the abstract of that paper are:
Plants need oxygen to perform cellular respiration. Plants absorb oxygen through their roots.
Dissolved oxygen (DO) is just normal oxygen -not freely floating in the air, but in a liquid.
While there are a bazillion factors that determine how much DO is in water it is usually in nature something like a fraction of 1%.
Henry's Law says: At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. I understand this as a gas such as O2 will enter water at varying degrees under different circumstances. A big factor is surface area of the pond or ocean in relation to pressure around (elevation, etc.) / (wide anderson flat or drainage holes everywhere pond basket or colander vs. wood box). Another factor would be like when it rains and the fish start biting.
Photosynthetic plants are the only organisms on the planet we know of that can split a water molecule naturally. So every creature on the planet requiring oxygen gets it through the air or as DO. An octopus breathes DO in the water, but does not pull it out of the water itself, to do so would take a great amount of energy, rather it filters it out.
But I think, unlike an octopus, roots (species dependent) can both uptake oxygen from the air in a soil, or sometimes from the DO (technically air/gas) in water. A window of opportunity. We don't think of our lungs being wet, we'll drown, but they are juicy right? I thinking now of roots as if we had no skin or ribcage, and the only way we could drink or breathe would be to put our chest into dirt, but I believe there is this window of drying-but-not-dry where oxygen uptake will take place, but unlike us they don't have to breathe all of the time, oxygen or CO2.
