Size of Medium (Leca, Diatomiote, Lava, whatever)

[Pseudo-Scientist]

What do you guys usually use? In terms of size.
I've noticed a lot of growers use larger sizes of medium, and not to point fingers or anything, but I find smaller works best.

Do you find the capillary action of larger medium is weaker?

And yes, its all relative to growing environment and medium, but without smaller particles, and the recent addition of diatomite (wow, I'm going to try some in it entirely), I wasn't getting enough capillary action to the top. Even for larger rooted plants.

I grow in a 2'x2'x4' greenhouse in my room, with 90% RH, constant air, and I'm ashamed to admit it, white LEDs (yes, I'm changing back. They suuuck).

[Ray]

You have actually asked a couple of different questions there.

They key is the size of the openings between the particles, not necessarily the size of the particles themselves, and the shape and surface texture - in addition to your ambient temperature, humidity, and air movement - of the particles play a huge role as well.

Totally uniform spheres of a single diameter provide the maximum volume of free air space - about 40%. A room full of ping-pong balls and a room full of basketballs all have the same volume of open space, but obviously the holes between the ping-pong balls are much smaller.

Moving that down to a size-of-the-orchid-roots scale, if the openings are too small, the roots can block the gas exchange pathways and suffocate themselves.

Mixing particle sizes, or moving to non-spherical shapes causes some to intrude into the void space, also lowering the free volume open to gas exchange.

As far as wicking is concerned, there are three things inherent to the particles themselves that play a role - contact area between particles, absorption by individual particles, and their willingness to release what has been absorbed.

Two smooth particles will have greater contact with each other than two with a rough surface, as the "peaks" hold them apart. Materials that are more "sponge-like" obviously have more liquid available to pass onto a neighbor, and I think that third point needs no discussion.

I don't know of this is the case, but it is also possible that your observation of your pots has led you to the wrong conclusion. It may very well be that the capillarity of both sizes are identical, but the larger free volumes between the large particles allowed evaporation to occur faster, making it appear that the liquid wasn't "climbing" as well.

You can test that, you know: soak a potful of each size for a day or two to ensure complete saturation. Place each in an identical volume reservoir and into your "greenhouse". If the reservoir in the fine grade material disappears first, your original assessment was correct.

[Pseudo-Scientist]

Interesting....very interesting. It is entirely possible that my observation was incorrect.

Thanks for helping me see the issue from a new perspective. This whole time, I've been focused on the need of particle contact for capillary action.
Honestly, I barely thought about the actual gas exchange occurring at the root zone.

Well, I'll have to re-pot one of my babies, and test that out.

Also, I do have some phals in 80% hydroton 20% large lava rock. in medium deli-containers .
How do I slow down the evaporation (if that is indeed the case).
The reservoir can be full at times, but fairly dry on top (not completely though).

Is it simply a mater of watering more often?

Lastly: Thanks Ray!
If anyone else has some thoughts, I'm all ears...

[Ray]

As evaporation is controlled by surface area, temperature, ambient humidity and air movement, and your temps and humidity are pretty well fixed (and the RH is already high), I doubt there's much you can do, but also I think you're overly concerned about the need for the medium to be totally saturated.

[razka3]

Originally Posted by Pseudo-Scientist

How do I slow down the evaporation (if that is indeed the case).
The reservoir can be full at times, but fairly dry on top (not completely though).

You can add some moist sphag on top of the LECA... seems to help on a couple of my plants. Just watch out for mold! And I wouldn't pack it on, as it would cut down on the gas exchange, just maybe lay a couple strands around the top of the pot.

[Pseudo-Scientist]

Alright, my culture isn't too bad (re: I haven't killed anything in a long time ), but I've never really thought about gas exchange in a meaningful way, at least not with SH.
But is seems to be a recurring theme on this thread....

Yeah, the roots need C02, oxygen, and....whatever (plz fill in the blanks), but i thought that since adding more holes to your SH pot wouldn't really increase airflow, that particle size wouldn't be such a big deal either. Logic being that there was really only so much airflow needed in SH (less than standard)

And we're talking gas exchange in terms of anaerobic bacteria causing root rot, right?
Or is there something else I'm missing (cation exchange for nutes, root temp fluctuations...I dunno?)

Keep me thinking gents...

[Ray]

No, we're not talking anaerobic bacteria, we're talking about the plants themselves.

Most terrestrial plants are loaded with leaf stomata, through which the gas exchange occurs. Such gas exchange involves both photosynthesis and respiration processes that occur within the plant (and probably others that I'm not even aware of).

As part of their evolution to develop water-retention strategies, in orchids the number of stomata on the leaves are very limited, and in the case of CAM (crassulacean acid metabolism) plants like phals, the stomata are completely closed in the daytime as a further moisture loss reduction strategy.

To compensate, most of the gas exchange has evolved to occur through the root system, instead.

Adding more holes to a S/H pot does increase the airflow, but at the expense of evaporating the moisture faster than the wicking can keep up with, so it defeats the purpose. Increasing the size of the pathways between particles allows more airflow, but as the evaporation is still mostly limited to the top part of the column of medium, it's acceptable.

Bacteria - aerobic or anaerobic - do not cause rot. Rot is the end result, not the beginning. Healthy tissue is protected from their attacks. If the roots are stressed (suffocation by restricting the gas exchange being the most common), they can emit phenols in order to "fight" the stressing agent. Unfortunately, unless that stress goes away soon and the phenol production stops, it becomes toxic to the roots as well (it's those phenols that show up as the dark brown or black parts of the damaged roots). Once the tissue is weakened and dead, then the bacteria can start to decompose the tissue, which we see as "rot".

[Pseudo-Scientist]

Ray, thats some good info.
Thanks for all your input.
Time for me to get acquainted with Phenols.

I re-potted 2 of my ladies (thats how I like to think of them) with larger medium...and will monitor the progress (I am a "pseudo-scientist", of course....)

Again: Thanks!

[Ray]

If I'm not mistaken, those phenols are a common way of plants "staking out a territory" and fighting the advances of other plants. It even happens in-flask, so that's one reason folks add charcoal to the agar as an absorbent.