Growstone

Kelly, one thing I've heard about this stuff is that it tends to have very high pH. I don't know if that's just an initial value that can be brought down, or if it's persistent and requires permanent buffering with more acidic substrates.

 

That, and the fact that soda-lime glass is alkaline to start with.

 

I believe you'll see an upswing in pH over time.

You can think of glass as a micro network of O-Si-O bonds, broken up with the addition of Na, Ca, Mg, etc. Over time, those alkaline earth metals react with water, forming hydroxides, and the hydroxides dissolve the glass further, opening up the structure.

As the structure opens up, the water penetration, and hence release of the metal ions, accelerates.

I had high hopes for the stuff - I AM a glass and ceramic scientist and engineer, after all - and am a big proponent of materials recycling, but the potential pH pitfalls discourage me. I'm quite sure it's fine as a soil amendment, as the complexity of the chemistry, coupled with the cation exchange capacity of the soil, can balance out the alkalinity.

 

Ray, the CO3 anion would contribute towards the basicity of the solution? Also in regards to CEC, how do you apply that to orchids? With such porous medias does the value not dramatically decrease as you get into things like 2cm bark? Or even the LECA should have a pretty low CEC right?

 

I don't know that I can answer your question reliably about the CO3 - I know that CO2 from the air reacts with water solutions to form carbonic acid, making a weakly acidic solution with pure water, but would expect calcium carbonate in a glass formulation would tend to make it more basic.

As a soil modifier, I think the degree of extraction of the ions would be small compared to the overall level of ion content, but I honestly don't see how cation exchange capacity really relates to the discussion with orchids, as we're talking extraction, not exchange.

In soil, ion exchange sites tend to be in fine organic matter and the edges of clay particles. Accordingly, the CEC in most orchid media - coarse organic matter and no clay - are orders of magnitude lower. The clay in LECA tends to be significantly vitrified, so I agree with your assessment that it would contribute little in that respect.

How rapidly the alkaline metal ions are extracted from the glass is going to depend to a great degree on just how modified the glass is. The Si-O-Si bonds I mentioned earlier are VERY strong - so strong that pure silica won't even flow when it reaches its melting point, and most strong acids won't attack it (only HF does). The addition of modifiers breaks those bonds, creating a weaker network with a much lower viscosity, allowing it to be a useful engineering material, but it also reduces its chemical durability, as well. The greater the additions, the weaker the structure, allowing solvents like water to extract them.

As an example, and taking it to the extreme opposite end of the spectrum, if you melt 2.5 parts Na2O with one part SiO2, the resulting glass is water soluble (and known by stamp collectors as "water glass").

I speculate that in order to be able to generate an extensive foam, they must have the glass pretty highly modified to reduce the melt viscocity, as that's a lot cheaper than heating it to higher temperatures to achieve the same.

 

Kelly I have been using growstones for over a year now. Depending on when you buy them they can differ considerably in terms of shape and sharpness of the edges. I prefer the rounded stuff and I usually inspect it first before purchase. I use it primarily with Paphs and Phrags and they have responded very well. I have not used it very much with miniature species.

 

Don't be too concerned about the "sharpness" of the surface texture - the root cells grow around everything, enveloping them, not "sliding past" which could lead to damage. If the plant isn't stable in the pot, that's a different story.

 

I think sharpness of the edges is more aesthetic for me. This is what I mean:

The top pieces are what I prefer (more rounded) and the bottom more rigid and less preferred. You can also see a different composition, perhaps from the type/color of glass used in recycling. I find that roots of epiphytic plants grow through and around the substrate, which I use for drainage. They do not cling to it. I have much better results when this product is kept wet.
Although not the types of orchids we are discussing, here is a Phrag. longifolium album that was repotted into this mix in the past 3 weeks.

You can see the older roots and a new one that is growing past/around the growstone without injury or problems. This plant is kept quite wet.

Kelly- I also noticed that your piece of Growstone is enormous compared to mine. Have you tried any of the smaller pieces that I showed in the pic.?

 

Chad, I had a long conversation with them, and the white stuff is old production. They pigment all of it now to look "more natural".