ir management is probably the single, most significant aspect of orchid culture, but a subject that is usually never directly addressed in discussions about how to grow them.
Let's start with the basics: Orchids, for the most part, are epiphytes - "air plants" - growing attached to trees or rock outcroppings, or in the leaf litter on the forest floor, and have their extensive root systems rambling around the surface, where they are exposed to lots of air. They are not parasites, but have evolved to the point of not needing a soil medium from which to take nutrients, instead gleaning their existence by absorbing their water and nutritional needs primarily from nutrient-bearing rainwater cascading on them from the canopies of the forests and jungles in which they live. One of the more observable aspects of that evolution is the water-storage mechanisms of the plants, such as pseudobulbs or thick, fleshy leaves, and the development of the sponge-like layer of cells on the root surface, the gray or silvery velamen that turns mostly transparent when saturated, showing the green inner cells of the root. Another part of that evolutionary development, but one we tend to ignore, is that of gas exchange.
We are all aware of the fact that "animals take in oxygen and give off carbon dioxide, while plants take in carbon dioxide and give off oxygen". (Actually, plants do both.) In animals, that gas exchange occurs through the lungs or gills, while in plants, it occurs pretty much over their entire surface. Orchids, because they have evolved to hold onto moisture in between potentially infrequent periods of rainfall, tend to have limited gas exchange from the leaves and pseudobulbs, so have shifted much of that important activity to the root system.
As we "domesticate" our plants, putting them in pots, we unnaturally confine the root system and "set up" our plants for potential problems, some of which have led to the proliferation of orchid-culture myths.
In a pot, the volume of air to which the roots are exposed - and with which the gas exchange must happen - is limited to the pore space in the potting medium and is refreshed only through whatever openings are available in the pot itself. The more densely the medium is packed, the less volume of air is available to the root system, and some of those "air pockets" are effectively cut off from the others, so do not get refreshed. When we water those potted plants, some of the liquid is absorbed by the particles of potting medium, but another part of it is held by surface tension in little pockets between the particles, effectively reducing the free air volume and further cutting off the connecting pathways between the remaining pores, reducing the refresh rate even more. The plants are still going through their essential gas exchange processes, but they have to deal with a root environment of "polluted"-, rather than fresh air to do so. That situation sets up two stressful scenarios - suffocation and poisoning.
(As an aside, it is the gas-flow limiting effect of the trapped water that leads to problems, not the presence of water itself, although that's what we've all been taught about "overwatering causing root rot". Consider that plants grown hydroponically have constant water exposure without rotting.)
Gas exchange occurs through osmosis, a natural process in which the concentrations of ions on either side of a membrane - root cell walls in this case - try to equalize. In free air, the carbon dioxide content is typically around 0.04%, so any level slightly higher than that easily leaves the root cells and disperses in the air. If the reduced airflow of the compact or saturated medium restricts that, the gas concentration in those isolated pockets gets much higher, stopping or even reversing the osmotic process, and suffocation and cell death can occur. Additionally, as it cannot be dispersed easily into the air, the carbon dioxide can react with the water to form carbonic acid, which can poison the roots.
In both cases, the plant responds to the stress by producing phenols in an attempt to "kill" the stressful agent. Unfortunately, those phenols are toxic to the roots themselves, so if that stress is not eliminated relatively quickly, root death will occur. It is those phenols, by the way, that give the roots the brown and black colors we see in "rotted" roots.
So what is the practical significance of this to the grower?
When selecting a potting medium for the plant, above all, think in terms of maintaining free and adequate flow of air to the root system. Consider the pore size within the medium and how it relates to root size (yes, roots can clog air flow too), and how “bridging” water droplets will play a role upon watering. Choice of medium can affect the latter, as some materials are more sponge-like than others, so “suck up” those bridging droplets and release them slowly. (Another aside here – the old adage about letting the potting medium dry out between waterings has nothing to do with letting the roots dry, it’s done to allow those bridging water droplets disappear so the plants’ roots can “breathe” again.)
Also consider the moisture needs of the plant and your ability to provide an adequate supply. Obviously a mounted plant should have no air supply issues, but in the home, it’s most difficult to water mounted plants often enough, and maintain the humidity to give the plant time to absorb the water before it evaporates. To compensate for that, we utilize potting media that absorb and hold moisture to different degrees and for various lengths of time. We can utilize different particle sizes as well (smaller particles have higher surface-to-volume ratios than do larger ones, and can absorb more water more quickly), but then we might be compromising the size and volume of those passageways for air to reach the roots.
Keep in mind that uniform particle size in the medium provides the maximum void space. A room full of basketballs and a room full of ping-pong balls have the same void space (you can do the math yourself), but when you start to mix particle sizes, the small ones fill the voids between the big ones and the free volume is reduced.
Armed with this information, if you aim for an airy potting medium that works with your watering habits to provide your plants with an adequate moisture supply, your growing should be headache free. Just remember this priority list:
Air → Water →Air → Fertilizer →Air!