I read about how air pruning can help your roots, because it stops the roots from growing when they reach the edge of the pot. I also read that roots need oxygen in their soil, in order to breath.
Can someone give me an explanation as to why in one case, air stops the roots from growing, but in the other case, air is good for the roots?
My guess is that air pruning stops roots because of the "dryness" of the air, and it's not the actual "air" that's stopping the growth?
Nature is complex.
Roots usually look for ground, in order to get water, minerals and other nutrients. For this reason, many plants stop growing roots if they find that they are growing in the wrong direction. Orchids are often epiphytes (growing on trees), so they do not look for soil, just for other secure places; and there are many other exceptions (nature is complex).
Not all roots requires oxygen, e.g. on aquatic plants or in general plants in very wet soil. In general oxygen means not so wet, so no rot on roots, or just less compact soil.
We tend also to forget that roots are nearly useless if there is not fungi or bacteria to help it to absorb nutrients (try to plant a vegetable on a sterile soil!). There is often a symbiosis, so non-aquatic plants want bacteria and fungi which need oxygen.
And lastly, nutrients are often in form of rock (or sand or silt/loam or clay), but also organic material (and manure). They need to be decomposed, in order to be absorbed by the roots. Oxygen helps a lot of additional bacteria (which can live also without plants) to decompose such stuff. Oxygen is also very reactive (chemically), so O2 also breaks stuff (metals: rust), which are then more easily absorbed by roots (smaller and they flow on water). [Wet plants do not have usually such problem: water break and bring nutrients to them, but carnivore plants (which growth often on aquatic/wet environments) needs meats, because lacking of some nutrient on water (or often just because acidity inhibits absorbing of some nutrients).
Note: root absorption is done mainly on the root cap (tip of roots), so on younger part. Breaking the roots [warning, just few] helps them to form new caps near the trunk, so with more ease to transport nutrient on the top.
Nature is complex, and evolution changes the plant to survive on special environment, so we cannot generalize.
Note: I never studied biology, so for details we have biology.stackexchange.com, which probably will negate most of thing I wrote.
I have to reiterate. Soil is soil, all soil is great soil. What ever type of soil you have you just need to know the proper management of that soil.
No soil comes with NUTRIENTS. Ugh. There might be tidbits but even in the deep dark undisturbed forests with dark, rich looking soil, there is NO chemistry with which the plant can take up to use for photosynthesis. Photosynthesis is the only way plants make their own food. There is no other food for plants. I wish that people would stop using the word nutrient or the word food in terms of fertilizer.
Fertilizer is JUST basic chemicals necessary for our artificial gardens our needs for food. Nature does not waste chemistry and nature doesn't like overpopulation so nature doesn't allow 'nutrients' or this chemistry I am talking about to happen! Population control...
We have to not only KNOW what the chemistry in our soil is at the beginning we need to know enough to ADD chemistry to grow what we want to grow. No soil should ever be considered NUTRITIOUS, having the right balance of N P or K plus minuscule amounts of a dozen plus micro chemicals (not nutrients).
If one doesn't understand chemistry and organic chemistry and botany and soils they need to learn this stuff to be able to grow plants and food. And sigh, talking about fertilizer; Less is Best, More is Death and None is Dumb. This No Till or No Fertilizer trend is ridiculous.
Roots in soil need air pockets. Air filled pore space...not large pore spaces but readily draining pores in the soil. That is one of the major reasons for using potting soil in pots. Very little soil, lots of uncompactable pore spaces, no disease or insects or weed seeds as it has been sterilized.
The dryness of the roots between pot and soil is the limitation. Roots will dry out. Other roots will be made if they can be kept moist and dark and maintain their access to lots of free air. Even aquatic plants need air, oxygenated water. Stagnant water is only meant for anaerobic life (stinky).
Epiphytes have multiple avenues to get water, nutrients, air and sunlight. They are a free loader of sorts borrowing from their host without harming the host.
And part of our problem as humans is equating plant needs with animal needs/processes. Plants do NOT BREATHE. No lungs, nothing like our respiratory systems as animals. Vastly different processes happening. Plants for all intents and purposes are ALIENS to us humans. Brilliant and after all plants are older than us humans and animals! They actually communicate, been proven, albeit a chemical communication than auditory. Understanding the processes happening in your plants, the needs of your plants is the first step.
Roots are specialised organs and the roots that grow in ground expect to have mineralised elements available to them in a solution surrounding the roots for them to function. Often such roots will swap complex carbohydrates that have been manufactured in the leaves in return for fungi providing these mineralised elements. In air pruning the roots hit air instead, they dry out and so can't initiate oxygen absorption, and the roots die back to where there is sufficient moisture.
I've noticed that when using my air pruning pots, the roots will grow through the air gap into the ground if it's moist enough i.e. not enough air flow under the pots. So, this allows dissolved oxygen to reach the roots when it's this moist. And roots will often grow in moving water if there's a high enough oxygen content. I have an apple tree that is growing in water which proves this, and it's the basis of hydroponics.
