Removing excess salts from media

[fredfarmer]

From the number of posts and their content, the concept of “Total Dissolved Solids” or TDS is not universally understood or agreed upon. I don’t want to claim what I will write below will correct or clarify these understandings to any significant extent. I do have to explain my understanding of some of the things regarding TDS in order to justify my conclusions. I guess what I am saying is that I want to be internally consistant. Just for further information, I am a retired scientist, with degrees in Chemistry, Physics and (medical) Biochemistry. I am not a horticulturist.
I was interested in understanding more about the milieu in which my orchids had to live, especially the mixture of ionized chemicals that the roots were exposed to. I had reason to believe that the fertilizers I was using were not exactly right. They were OK, but not ideal. “Why” isn’t important at this point. So I set about trying to learn more about what that milieu was. So I started by purchasing a TDS meter.
First, I should state that a Total Dissolved Solids meter does not measure Total Dissolved Solids – it measures conductivity which is directly related to the number of ions of ionized chemicals in the solution. That’s what I was interested in anyway, so the fact that I wasn’t really measuring TDS with a TDS meter wasn’t important. Too give you an example of the difference between the two – I did the following:
I live in a part of NY that has a lot of rain, so rain water is easy to obtain in large quantities. The TDS meter, when placed in half a cup of rain water, gave a reading of 5 ppm (not 0 ppm, it’s not distilled water). I then placed ½ teaspoon of sugar in the cup and stirred it until it dissolved. The TDS meter placed in this solution of sugar in rainwater also read 5 ppm. That’s a lot of solid sugar, probably ½ gram and not much water so it’s somewhere in the range of 10,000 mg/liter or 10,000 ppm. The meter didn’t measure it because sugar doesn’t ionize. But it was a dissolved solid and it was there, because I put it there.
Now for the first experiment with orchids. For better or worse, when I fertilize my orchids, I am a soaker. My argument for this is that I want the roots to have ample opportunity to have access to the fertilizer and to absorb what they want. I use different fertilizers at different times. They only contain ionizeable salts, no urea, etc., dissolved in rain water. I have noted numerous warnings on this Board to rinse the support media (typical bark, charcoal, perlite, moss mix) with non-fertilizer solute (in my case rain water) to remove possible built up of salts on the orchids roots. I do this but often wondered if the rinsing process was to any effect. So in this experiment, I rinsed a healthy phalaenopsis, (fertilized 7 days before, and with about 150 ml of rain water poured over the media on days 2,4, and 6) with about a 1 and ½ liters of rain water (TDS reading 8) and let the orchid pot sit in the rinse water for 45 minutes. The TDS reading at the end of that “soak” was 77 ppm. So there are “leachable” ions still in the support media 7 days later. Next I poured the same leach water over the same support media and let it sit for and additional 45 minutes. The TDS reading after this second soak was 94 ppm. So the release process is slow – probably too slow to fully occur with a few minutes rinse. The conclusion is the leachable ions are definitely there and removable, but the process is slow …really slow. Enough for today. I plan to do more experiments.