Porous materials like perlite can store water within themselves. There water retention is high. As you know perlite also floats because there is trapped air inside. I think people may get confused that the trap air is available to the roots, but it is not. Also trapped air doesn’t create oxygen (air) movement within a medium. Air is trapped in perlite internally by its structure and in the porous outer surface when water covers the outer gap.
Irregular-shaped particles have a greater surface area and therefore have higher water retention then smooth round particles.
Smaller particles have greater surface area and pore space to size, and the smaller the particles the closer they pack together trapping water between the particles. The closer in uniformity of the size of particles also helps in water retention.
So with an all perlite mix of uniformly small particles you can hold a lot of water and stop any flow of air. Orchids have various tolerances in the amount of water retention in a medium they are willing to stand. Frequency of watering and the pot material (plastic or clay) used also effect how well or long an orchid root can remain healthy in such a potting mix. I have seen orchid roots grow into standing water and remain submerged in water for long periods of time and remain healthy and grow, but you can’t plunge an orchid root in such a condition that it did not grow into and expect the root to remain healthy for long.
In all fine orchid mixes where you want some water retention plus oxygen movement, it is better to include some large particles to create air gaps and pockets. In a fine orchid mix the more you vary the size, degree of porosity, and shape (irregular to smooth and round) of the particles the better balance you will have between water retention and oxygen movement. Your mix will last longer.
You probably could get away with an all perlite mix of small particles in a semi-hydroponic situation as describe on Ray Barkalow - First Rays Orchids - www.firstrays.com
Though on a quick read of Ray’s Semi-Hydroponics Basic he writes “The medium is another key component of the culture technique; it must be inert so it won't decompose, fairly uniform in particle size so it provides lots of free air space, and must provide good capillary (wicking) action to keep it uniformly moist. “
I would say the uniform particle size helps in providing good capillary (wicking) action and capturing evaporation not free airspace.
Mark Sullivan
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