Why / How does water culture work but ...

[Cymbaline]

I guess another silly question ..

How does water culture ( growing orchids in water ) work with some orchids but if you plant it in soil or if it's medium starts hold too much water it would probably rot?

Both are very wet medium so why does one work and another wouldn't?

I know the roots in water culture has to adapt to water, right? But why then doesn't it just adapt to soil or soggy medium?

This board has a lot of smart , knowledgeable people who knows the science behind what they talk about so I'm really just curious about this.

I promise I won't keep asking silly questions like this but it always made me wonder.

Thank you

[Subrosa]

I would suspect that the organic nature of soil works against the plants in multiple ways. Firstly, decomposition of organic matter depletes the oxygen available to the roots, and makes diffusion of fresh oxygen from the atmosphere more difficult than simply diffusing through the air/water interface at the surface of the water. Additionally I suspect that such conditions are more conducive to pathogenic bacteria and fungi. This is all speculation on my part.

[Ray]

When root cells grow, they "tailor" themselves to function optimally in the environment into which they have grown. Once they have grown, they cannot change. If the environment changes - during repotting, or as the medium decomposes and gets compact, as Subrosa mentioned, the roots are no longer optimal for that environment, so start to fail.

Even a case where the medium didn't change, as when you purchase a plant from a professional grower, the environment around it has changed, so it functions differently than the existing roots are "tuned" to, so may start to fail then, as well.

[mook1178]

I agree with the availability of oxygen. To add to that and along the line of decomposition is the build up of CO2. The CO2, if not removed by air flow, will greatly acidify the surrounding media by reacting with water and creating carbonic acid. The exact same process as ocean acidification. The difference though between the salt water and fresh water in our orchid media is the amount of alkalinity. Salt water has a lot of alkalinity, buffering and reducing the amount of carbonic acid made. In fresh water, there is little to no alkalinity. Therefore the pH can drop lower and quicker than the same amount of CO2 added to salt water.

In FWC, the root tops are really only supposed to be in the water, resulting in very little water in the container. The small amount of water and proper airflow allowed gas exchange in the water to happen. Therefore, very little build up of CO2 and quick replenishment of O2.

[SouthPark]

Originally Posted by Cymbaline

I guess another silly question ..
How does water culture ( growing orchids in water ) work with some orchids but if you plant it in soil or if it's medium starts hold too much water it would probably rot?

Both are very wet medium so why does one work and another wouldn't?

Somebody taught me that roots of orchids require oxygen to survive. In 'water culture' or roots immersed in water, still-water can eventually run too low in oxygen (even with water-adapted roots). So I think that water culture growers do change the water every once in a while. Or some use aerator systems - sort of like a fishtank to keep stirring the water surface in order for oxygen in the air to get into the water.

In soil that is too wet - the roots can drown if the soil becomes saturated with water and not enough oxygen for the roots.

If the medium is sand or sand/soil mix - and if this medium is deep enough and drains nicely, then I think it's possible that at least some orchids can still grow and survive. I mentioned once in a forum post that I've encountered a situation where a Cattleya type orchid grew roots into sand/soil. And when I decided to move the pot, I found that the clump of roots in the sand/soil pit under the drainage grate was not a small clump. It was a large clump of roots under the soil surface - not too deep under the surface - but certainly in very good shape. The roots looked excellent.

I didn't keep the roots in that sand though - just for safety reasons ----- mentioned by subrosa, like the possibility of pathogens getting picked up.

[Subrosa]

Originally Posted by mook1178

I agree with the availability of oxygen. To add to that and along the line of decomposition is the build up of CO2. The CO2, if not removed by air flow, will greatly acidify the surrounding media by reacting with water and creating carbonic acid. The exact same process as ocean acidification. The difference though between the salt water and fresh water in our orchid media is the amount of alkalinity. Salt water has a lot of alkalinity, buffering and reducing the amount of carbonic acid made. In fresh water, there is little to no alkalinity. Therefore the pH can drop lower and quicker than the same amount of CO2 added to salt water.

In FWC, the root tops are really only supposed to be in the water, resulting in very little water in the container. The small amount of water and proper airflow allowed gas exchange in the water to happen. Therefore, very little build up of CO2 and quick replenishment of O2.

Roots can grow quite deeply into the water after they've adapted to it. Another consideration re O2 and CO2 is that orchid roots (at least the Vanda and Phalaenopsis I keep in water culture) are photosynthetic, which would certainly have a bearing upon the relative levels of the 2 gases dissolved in the water.