“Total Dissolved Solids” - TDS

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Ray

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I’ll start this with an editorial comment: “Who gives a damn about TDS!”

People buy “TDS meters”, then end up using them to control their fertilizer and additive use, without really understanding what they’re doing. I’ll actually limit that a bit more - orchid growers are guilty of that more than anyone.

There are several facets to the misunderstanding. First is an understanding of what “total dissolved solids” really means.

Don’t get me wrong, I think folks grasp that the units, ppm, or parts-per-million, or milligrams per kilogram just fine and understand that it’s a mass of solute per mass of solution, but the question is, “do they grasp the meaning of the magnitude of it?”. For example, folks talk about keeping the TDS below 200 ppm to make phrags happy. Where did that number come from and what does it really mean?

I feed my plants weekly at 100 ppm N (a concentration of nitrogen, not TDS), and many folks do something relatively close to that. They tend to “freak out”, however, when confronted with the fact that the true TDS of a 100 ppm N solution is many multiples of that, varying by the fertilizer formula. 100 ppm N solutions made using MSU WW, MSU RO, & K-Lite, for example, have true TDS levels of 530, 740, & 770 ppm, respectively. My stance is “That’s interesting, but so what?”.

Now let’s throw in the meters themselves, as they are almost worthless.

A TDS meter does not measure the concentration of a solution. It is actually a cheap electrical conductivity (EC) meter to which a built-in “conversion factor” is applied. Some meters even offer a couple of different scales, further lulling people into thinking their measurements are meaningful. The trouble is that the EC/TDS conversion requires a different “factor” for each and every solution to be tested, and the built-in one is often done with a sodium chloride solution, which is nothing like a fertilizer solution.

The EC of a solution is a measure of the flow rate of electrical charges. That flow rate is determined by the charge on each ion (higher charge equals greater EC), the physical size of the ions (bigger ions move more slowly, so contribute less to the EC), and their concentrations, so how can a single “conversion factor” be correct for all solutions? It can’t, and it can be way off - I tested a known, 100 ppm N solution of MSU RO in distilled water using two TDS meters; one gave me a reading of 380, and the other 475, while the true reading was 770 ppm.

Pure water has zero TDS and EC. Let it stand around and absorb carbon dioxide from the air, and the EC increases due to the formation of carbonic acid, so a TDS meter will show an increase in the dissolved solids, too. Let that water evaporate, and the carbon goes with it, leaving you with nothing. Technically, there was a level of dissolved solids (C), but as true TDS is determined by evaporating a know mass of solvent and weighing the residue, we can safely say it was zero.

A KelpMax discussion let to this thread, so let’s look at other additives. All that I can truly say is that anything you add to your solution may affect the conductivity, giving you a “high” TDS reading, but what does it matter? Yes, KelpMax (Kelpak) does increase the EC of a solution, but within the range of recommended dosage, it makes no difference. Epsom Salt is often used to add magnesium to solutions, and it affects the EC, as well, but again, “so what?”, unless you overdo it.

As far as I’m concerned, TDS meters are only useful in monitoring changes, not in measurement of absolutes. If my known 100 ppm N solution gives me TDS readings of 100, 500, or 2000 ppm, as long as my testing tells me it’s reasonably close to that, I’m happy.

I have yet to see a decent, controlled study tell me what TDS is correct or even acceptable. Actual concentrations, sure, but not nebulously TDS readings.
 

eds

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A good write up Ray, thank you. It always amazes me how much faith people will assign to a number without much/any understanding of it!
I only really use a TDS meter to check how well my RO unit is doing and when I need to change filters / membranes.
 

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I’ll start this with an editorial comment: “Who gives a damn about TDS!”

People buy “TDS meters”, then end up using them to control their fertilizer and additive use, without really understanding what they’re doing. I’ll actually limit that a bit more - orchid growers are guilty of that more than anyone.

There are several facets to the misunderstanding. First is an understanding of what “total dissolved solids” really means.

Don’t get me wrong, I think folks grasp that the units, ppm, or parts-per-million, or milligrams per kilogram just fine and understand that it’s a mass of solute per mass of solution, but the question is, “do they grasp the meaning of the magnitude of it?”. For example, folks talk about keeping the TDS below 200 ppm to make phrags happy. Where did that number come from and what does it really mean?

I feed my plants weekly at 100 ppm N (a concentration of nitrogen, not TDS), and many folks do something relatively close to that. They tend to “freak out”, however, when confronted with the fact that the true TDS of a 100 ppm N solution is many multiples of that, varying by the fertilizer formula. 100 ppm N solutions made using MSU WW, MSU RO, & K-Lite, for example, have true TDS levels of 530, 740, & 770 ppm, respectively. My stance is “That’s interesting, but so what?”.

Now let’s throw in the meters themselves, as they are almost worthless.

A TDS meter does not measure the concentration of a solution. It is actually a cheap electrical conductivity (EC) meter to which a built-in “conversion factor” is applied. Some meters even offer a couple of different scales, further lulling people into thinking their measurements are meaningful. The trouble is that the EC/TDS conversion requires a different “factor” for each and every solution to be tested, and the built-in one is often done with a sodium chloride solution, which is nothing like a fertilizer solution.

The EC of a solution is a measure of the flow rate of electrical charges. That flow rate is determined by the charge on each ion (higher charge equals greater EC), the physical size of the ions (bigger ions move more slowly, so contribute less to the EC), and their concentrations, so how can a single “conversion factor” be correct for all solutions? It can’t, and it can be way off - I tested a known, 100 ppm N solution of MSU RO in distilled water using two TDS meters; one gave me a reading of 380, and the other 475, while the true reading was 770 ppm.

Pure water has zero TDS and EC. Let it stand around and absorb carbon dioxide from the air, and the EC increases due to the formation of carbonic acid, so a TDS meter will show an increase in the dissolved solids, too. Let that water evaporate, and the carbon goes with it, leaving you with nothing. Technically, there was a level of dissolved solids (C), but as true TDS is determined by evaporating a know mass of solvent and weighing the residue, we can safely say it was zero.

A KelpMax discussion let to this thread, so let’s look at other additives. All that I can truly say is that anything you add to your solution may affect the conductivity, giving you a “high” TDS reading, but what does it matter? Yes, KelpMax (Kelpak) does increase the EC of a solution, but within the range of recommended dosage, it makes no difference. Epsom Salt is often used to add magnesium to solutions, and it affects the EC, as well, but again, “so what?”, unless you overdo it.

As far as I’m concerned, TDS meters are only useful in monitoring changes, not in measurement of absolutes. If my known 100 ppm N solution gives me TDS readings of 100, 500, or 2000 ppm, as long as my testing tells me it’s reasonably close to that, I’m happy.

I have yet to see a decent, controlled study tell me what TDS is correct or even acceptable. Actual concentrations, sure, but not nebulously TDS readings.
If you have a TDS meter and you want to make the most of it, the process is fairly straightforward. Prepare a solution at about 800 ppm using your fertilizer and distilled or RO water. Perform serial dilutions to get 400, 200 and 100 ppm. Put the TDS meter into the 800 ppm solution and adjust the trim on the meter until it reads 800 ppm. Measure each of the other solutions and the water, and record the results. Plot the results, and if they are linear, you have a roughly validated analytical procedure.

If you use tap water to fertilize your plants, you just need to measure your water and subtract that value from any measured value for fertilizer + tap water. It should be sufficient to tell the difference between 550 ppm and 600 ppm TDS for any of your fertilizer solutions.

Seems like it would be easy to construct an experiment to determine effect of high TDS on phrags. I believe I do it every time I let the medium become too dry.

Mike
 

Ray

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Mike, unfortunately, most TDS meters don't have adjustment capabilities.

Just to clarify - when you say " Prepare a solution at about 800 ppm using your fertilizer and distilled or RO water", how do you know it's 800 ppm? If you mean 0.8g/kg of solution, that's fine, but if it's a measure 800 ppm, you're no better off.

I describe a "calibration technique HERE
 

orchid527

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Mike, unfortunately, most TDS meters don't have adjustment capabilities.

Just to clarify - when you say " Prepare a solution at about 800 ppm using your fertilizer and distilled or RO water", how do you know it's 800 ppm? If you mean 0.8g/kg of solution, that's fine, but if it's a measure 800 ppm, you're no better off.

I describe a "calibration technique HERE
Ray

You need to weigh it and it needs to be the same fertilizer that you will be using on the plants. The purpose is to set equal TDS and actual concentration, removing ion size and charge density from consideration. My meter is a cheap Hanna that I bought on eBay. It looked like the dozens of others for sale, all made in China. The adjustment, shown in the instructions, was a little pot screw accessible by removing the battery holder. Mike
 

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And yet, what is the average orchid grower who does not have that technical expertise to do? I'm giving a talk to Pinelands OS later today and all I have is 1/4/tsp per gal and total 200 ppm (with pure water) as there is obviously more than just nitrogen in any fertilizer. So far it's worked for me, but is it the best? Who knows?
 

orchid527

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And yet, what is the average orchid grower who does not have that technical expertise to do? I'm giving a talk to Pinelands OS later today and all I have is 1/4/tsp per gal and total 200 ppm (with pure water) as there is obviously more than just nitrogen in any fertilizer. So far it's worked for me, but is it the best? Who knows?
What you're doing is just fine. It is nice to have a little science behind your decisions, but there is so much variability from plant to plant and from season to season, that you will never get one clear answer as to what works best. If your plants are growing well, you are doing the right thing. You don't have to be a scientist to be successful growing orchids. It's just that some of us can't leave the nerd behind when we step into the greenhouse.

I am reminded of a time I was standing at my display at a show in Ohio. A person walked up and said that the display lacked artistic qualities and that it looked like it was built by an engineer. I'm not an engineer, but I took it as a compliment. Different strokes for different folks. Mike
 

Ray

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My position is simply that TDS be dumped as a measurement when discussing or handling fertilizer. I have, long since. It’s fine as a monitoring method, but the values are entirely too suspect to be a meaningful measurement.

Professional nursery folks use ppm N or it’s known (published by the manufacturer) EC to control feeding. I use ppm N because it’s the most important nutrient element, and ppm in this case is meaningful. There is a really easy way to estimate fertilizer use to achieve ppm N levels:

Divide 8 by the %N content of the fertilizer. The result is the teaspoons/gallon for a 100 ppm N solution. (10.4/%N gives the result in ml/L). Yes, it is a gross estimate, as we don’t know the exact bulk density of the powder or liquid, and it’s safe to round up or down for simplicity at those concentrations, but it’s still a better “guesstimate” than TDS will ever be.
 
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richgarrison

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hanging this out there... be gentle... :)

Since many (most?) of these TDS meters also have a 'conductivity' mode, is there a reason that a grower mixing fertilizer according to a manufacturers direction, where the manufacturer has published the expected conductivity for a particular solution strength, couldn't verify that what they have mixed, matches that strength?

and if it does, know what concentration of N or other elements is likely in that solution, also based on the manufacturers published label?
 

eds

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As mentioned above, the meters aren't accurate enough compared to each other.

Giving a weight or volume measurement is much less likely to go wrong for most hobbyists.
 

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My position is simply that TDS be dumped as a measurement when discussing or handling fertilizer. I have, long since. It’s fine as a monitoring method, but the values are entirely too suspect to be a meaningful measurement.

Professional nursery folks use ppm N or it’s known (published by the manufacturer) EC to control feeding. I use ppm N because it’s the most important nutrient element, and ppm in this case is meaningful. There is a really easy way to estimate fertilizer use to achieve ppm N levels:

Divide 8 by the %N content of the fertilizer. The result is the teaspoons/gallon for a 100 ppm N solution. (10.4/%N gives the result in ml/L). Yes, it is a gross estimate, as we don’t know the exact bulk density of the powder or liquid, and it’s safe to round up or down for simplicity at those concentrations, but it’s still a better “guesstimate” than TDS will ever be.
So, Ray, using your own K-Lite, that comes out to 2/3 tsp/gal, which is more than I use now, watering 2 times per week. But as I recall there's a time element as well, which I thought was one week, and yet your container says 25 ppm N for every watering, for me that's in S/H. I should test my TDS meter and see how it measures those amounts (though I believe, if I understand correctly, this measurement may be consistent with the same fertilizer, but might be quite different, if for example, I go to measure one that's organic). So my quarter tsp per watering may not be that far off (I think that converts with my meter to about 200 ppm, but I'd have to check again). At that level it doesn't seem to damage the "sensitive" ones like Disas.
 

Ray

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hanging this out there... be gentle... :)

Since many (most?) of these TDS meters also have a 'conductivity' mode, is there a reason that a grower mixing fertilizer according to a manufacturers direction, where the manufacturer has published the expected conductivity for a particular solution strength, couldn't verify that what they have mixed, matches that strength?

and if it does, know what concentration of N or other elements is likely in that solution, also based on the manufacturers published label?
None of the TDS meters I ever owned had an "EC Mode". I now own a dedicated EC meter. But yes, if the EC meter is calibrated and accurate, it should - in conjunction with published EC data for fertilizers - tell you the concentration of nitrogen, and knowing the complete fertilizer formula, you could calculate the ppm levels of everything else, as well. I agree with eds - knowing the mass of powder per mass of solution is the most accurate way to measure.
So, Ray, using your own K-Lite, that comes out to 2/3 tsp/gal, which is more than I use now, watering 2 times per week. But as I recall there's a time element as well, which I thought was one week, and yet your container says 25 ppm N for every watering, for me that's in S/H.
I think it pays to think in terms of "exposure", too. Here's a very gross and simplified example of what I mean, NOT factual info:

► Case 1: Plant grown in straight, coarse Orchiata. You hit it with a fertilizer solution, and between pour-through and evaporation, the solution may only be present for absorption for two days, so you water three times a week. in that case I'd shoot for 25-50 ppm N at each watering.
► Case 2: Same plant grown in a mix that includes more water-retentive ingredients and in an environment that doesn't favor evaporation so strongly, so the plant has more time to absorb the solution, so you water only once a week. For that, I'd target 75-125 ppm N.
► Case 3: Same plant grown in semi-hydroponics, where the roots are exposed to the nutrient solution at all times - in that case, I go with 25 ppm N if I water daily or weekly. (Daily means it's exposed to 25 ppm more steadily; weekly, it must be decreasing with time as the plant absorbs it AND increasing as the water is evaporation, so maybe there's a balance somewhere in there.)

I should test my TDS meter and see how it measures those amounts (though I believe, if I understand correctly, this measurement may be consistent with the same fertilizer, but might be quite different, if for example, I go to measure one that's organic). So my quarter tsp per watering may not be that far off (I think that converts with my meter to about 200 ppm, but I'd have to check again). At that level it doesn't seem to damage the "sensitive" ones like Disas.
Bill Argo, the MSU PhD who formulated the "MSU Fertilizers" and derived K-Lite for me, published a 5-part series on orchid nutrition in the IPA magazine several years ago. I have them at my website. The 5th - "Putting it all together" - has several tables that provide data for several fertilizer formulas, whether that is mass, ppm N, volume, or EC.
 

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Good points, Ray. Another difference with S/H. As I'm sure you are aware, some growers believe fertilize first, water after, others the opposite. Of course, with S/H, it's really fertilize all the time. Ironically, the 1/6 tsp required for that (regardless of how many times a week one supplements the water reservoir, which may be more accurate than to say "water" is not that much less than the 1/4 tsp. I recommended in my talk last night. Incidentally, only one person in the group remembered that you had given then not one, but two, previous talks on the same topic (Pinelands in NJ). Presumably those were in person. And yet not one of them admitted to growing that way! LOL. However, Zoom talks have changed everything, especially for smaller societies, and I expect most future talks will be done remotely. I expect it's also reduced the requested honorarium, at least it has for me.

Oddly, I called this talk "Throw out the bark for the last time" without thinking how close it might be to "Throw out Barkalow for the last time" (joke). But I do promote your products. Your talk is the best on the subject of S/H, I think, and I also recommend yours on nutrition as well--solid information, clearly and accurately presented. Would that all society talks were like that.
 
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Ray

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I have pretty much given up doing talks on S/H culture, although a few societies have requested it in the time of Zoom. These days, it’s mostly “Understanding Orchid Nutrition” and another about the use of microbes and phytochemicals in orchid growing, which I’ll be Zooming to a club in Southern California next month, but I believe that will be my last talk.
 

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Five Cities, I hope, as they share theirs. Ray, your talks are among the best I've heard, and frankly are better than mine. Traveling speakers that require air travel are largely now a thing of the past. I lowered my price a d say bring as many as you like. Without travel 2 hours is not a lot of time. I hope you don't stop. Times have changed but not the need for good speakers.
 

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This got me looking at my calendar. I am currently scheduled for the an in-person presentation at the Tidewater OS on 2/6, and Zooming to Marin County on the 22nd.

Not sure the first will happen with the omicron variant running wild.
 

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Pure water has zero TDS and EC. Let it stand around and absorb carbon dioxide from the air, and the EC increases due to the formation of carbonic acid, so a TDS meter will show an increase in the dissolved solids, too. Let that water evaporate, and the carbon goes with it, leaving you with nothing. Technically, there was a level of dissolved solids (C), but as true TDS is determined by evaporating a know mass of solvent and weighing the residue, we can safely say it was zero.
This explains something that has been puzzling me. Water from my RO tap measures 1ppm but the same RO water in my reservoir is normally around 10ppm.
Thanks
-Keith
 

Sky7Bear

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This got me looking at my calendar. I am currently scheduled for the an in-person presentation at the Tidewater OS on 2/6, and Zooming to Marin County on the 22nd.

Not sure the first will happen with the omicron variant running wild.
I suspect in person lectures will largely be superseded in the future. Which talk are you giving at Marin? They meet the same night as Mt. Baker and we often "share" on Zoom. I don't know if our new VP will continue that practice, but it has worked well in the past. We had planned to meet in person in January, but given the statistics for this area that I saw today, I'd say unlikely.

I just got out my TDS meter and inspected it more closely. It has a Mode button that can be switched between TDS, uS/cm, which I believe means microSiemens/centimeter (which means nothing to me) and temp in C and F (not sure why).

So I took 1/4 tsp of K Lite and put it in a gallon of RO water, and it came out 253 ppm (about what I expected) and 538 uS/cm. So, Ray, what do I make of that? (I don't have Greek letters easily available, but I'm sure you understand.)
 

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