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.

[HantsomeOrchids]

I would imagine they slowly get pushed into the interior pores / caverns in the media. eg. bark will hold water, so surely itll also hold minerals, right? the leech rate
would be similar to the rate at which the media absorbs it which ig would have an upper maximum of... the drying time? the issue though is that since the ions dont evaporate they just get stuck there.

i suspect in general its better to soak the media in slightly high fertiliser then just let it leach out with every clean watering. but thats riskier

honestly id love to know HOW MUCH these guys actually need of what. like... orchids grow really slowly... they might be fine with a single yearly fertilise soak of the media. you can grow tomatoes in good mulch without fertiliser! but thats super buffered with low solubles.

[Ray]

You are "preaching from my bible" on TDS. I have several related articles on my website.

Mineral accumulation is inevitable in all media, but the rate of accumulation and difficulty of "cleansing" is directly related to drying. No drying, no precipitation.

Precipitated minerals do not redissolve readily, so maintaining the airy medium moist at all times slows the precipitation rate (an advantage of "semi-hydroponics).

If you do the calculations of the mass balance related to carbon fixation (i.e., "growth") you will find that in order for any plant to gain one pound of mass (454 g), it only needs to absorb and process about 5 g of N-P-K nutrition. It requires a about 200 pounds (~90 kg) water in those reactions - plus the 95%+ of absorbed water lost to transpiration.

If you back-calculate the nutrition+water needed for the mass gain, for that pound to be added over a 4-year period, that would imply a demand for one pint (~475 ml) of a 55 ppm nutrient solution weekly, but if that's what you applied, you'd end up with a desiccated, undernourished plant.

The bottom line is that we don't know the quantity nor rate of the applied solutions an orchid absorbs, but it varies all over the map and we do know that it is very limited compared to terrestrial plants.

While such calculations are interesting to "nerdy" folks like me (and apparently you), after over 50 years of growing, I have found that you can pretty much pick your favorite fertilizer formula (changing them is unnecessary if yours if complete), and drench with a 100 ppm N solution once a week to have happy, growing, well-performing orchids.

[ArronOB]

Fredfarmer, I am far from a scientist so i read this as a layperson slowly changing over to inert media. I have always wondered just how much truth there is in the need to flush out salts, so what I take from your post is these three things:
1. There is indeed an issue with dissolved salts accumulating in inert media.
2. Regular flushing with running water has limited benefit.
3. You need a prolonged soak to remove the problem residues.

Have I understood that correctly?

If so then it’s the first time I’ve read it with a bit of experimentation behind it and I thank you for that.

Cheers

[Ray]

I will add that it's fairly easy to demonstrate the issue.

Soak two new sponges in water containing a lot of food coloring.

Let them drain until dripping stops, then put one in a ziplock bag and seal it, letting the other one dry out.

Take both and put them into two containers of plain water. The dye will come out of the wet one tremendously more quickly from the wet one because there was no "precipitation" of the dissolved solids.

After an hour put them in a new, plain water bath for another soak.

It will take many soaks for the dry one to "come clean" - suggesting even prolonged soaking of potting media isn't all that great for "removing" residues. Reducing, sure, but I'd bet the food coloring redissolves much more quickly than do mineral salts.

[DirtyCoconuts]

i am not a scientist but i have used inert media for several years and here is one thing i have noticed.

it is better to update and refresh the media than try to flush it of the salts. see the sponge example above.

having said that. i have had great success using 90-95* water flushes for a few phals and catts that i cannot repot easily-

i set my shower and then let the plants get a 10 minute flush about every 6 months.

I have also started when i repot or have a plant die. i take that media and it goes into an old turkey fryer pot and once that is full i will fill it with water and simmer it for a while adding water to flush it over and over. i had trouble with perlite and floating leca but the rest is easy fill, boil, add water from hose, repeat.

then i dump that out into a large barrel with holes at the bottoms and use it as needed for future plants and as a soil additive.

[fredfarmer]

AaronOB - Yes, I agree with the concllusions you have drawn. There are some "control" experiments I have to do which will take some time. Meanwhile, it raises an issue I/we have to think about, i.e. what can be done about it? I let you know if I come up with anything.

[alecStewart1]

I'm not expert but I'm guessing due to the type of medium and how orchids grow they're quite a bit different than some of the other plants I have.

For plants that grow in a peat mixture, the peat will naturally decay as time goes on and leaves some humic acid in and on the soil. Sometimes that effect new growth on a plant, as humic acid on the surface might burn the new growth a little. The 2 ways people recommend fixing this is getting a paper towel and soaking up the excess humic acid on top of the soil, or just top watering and throwing out the resulting orange-ish water that drains out the bottom of the pot. But most orchids don't grow in peat, so maybe it's not as big of an issue compared to mineral and salt build up for orchids.

Just from cursory knowledge and not super scientific nor researched guess, the only way you might be able to "properly" drain out excess salts is with something like deionized water. Since it's been deionized, minerals and salt could reionize any molecules passing by and, thus, on paper/in theory could pull some more minerals and salts out with the draining water compared to just distilled or rain water.

That's just a complete guess on my part, though, and I could be absolutely incorrect. Even if it is somewhat accurate, recalling what Ray has said, how much benefit is there really to doing that extra work and spending more money? Maybe not much.

[Clawhammer]

Great topic, love the approach, glad to have you in the forum!

My very non scientific observations that are somewhat contrary to these conclusions. I keep most of my orchids in my basement grow room where trays are plumbed and I water liberally with a hose injected with a target of 25ppm of N (except winter). Every plant gets thoroughly flushed every watering. Not a bit of efflorescence in the whole grow room except on the floor (800+ plants).

I also keep a bunch of phals in terra cotta with moss. Flushing is challenging (impossible) so I am very conservative with the fertilizer. However, every pot is quickly covered with efflorescence in short order.

[Leafmite]

Interesting. I rely on flushing for all my plants...soil, lava rock or mounted.