Some TDS measurements


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It shouldn't be so hard to find Sarracenia media! TDS (total dissolved solids) is a conductivity measurement so a measure of how much ionic content in a solution i.e. the amount of salts dissolved (note that 'salts' is not just sodium chloride). It does not indicate pH, nor organic nutrients in the solution. Nor does it give a measure of cation exchange capacity, nor any idea of which minerals are in solution. However, it is quick and easy to measure with a cheap TDS meter and is probably the first best quickest indicator to see if a certain media is going to be worth trying for something like Sarracenia. Generally, I think below about 50ppm is good. Here are some tests for some recent media I obtained after only an hour in solution of the roof collected rainwater, in ascending order:

Tile roof collected rainwater: 30ppm

Klasmann-Dielmann Lithuanian fine sphagnum peat: 30ppm (and the 'best looking' peat I have ever seen)

Melcourt horticultural potting grit: 45ppm

Perlite: 70ppm (note that it is possible this was slightly contaminated by a cute young cat that seemed to need a litter tray in my greenhouse)

Melcourt Growbark Pine: 95ppm

Melcourt Composted Fine Bark: 115ppm (contains quite big pieces of bark in comparison with Growbark)

Westland sphagnum peat: 200ppm

coirproducts.co.uk coir block 5kg: >1000ppm

It would seem that TDS reading is inversely related to how easy it is to get hold of the media.

(I wonder what became of Davion?)

Edited by Zarniwoop
slight change
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Interesting but I don't think the PPM of most planting mediums is indicative of being good or bad.

Water high in TDS is due to calcium carbonate, sodium etc. So it is specific compounds that can cause problems rather than every dissolved solid. 

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You may have a relatively high concentration of some organic acids (tanic, etc.), and surely other substances that dissociate in water (so they add to the conductivity), and the plants be perfectly happy. I would worry with high ppm values though, let's say, e.g., well above 200.

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I did not mean to say it was a comprehensive test. I meant that it is quick and easy and useful. If you get a very low TDS reading then you know it has not been washed with seawater, for example. Another example is that sphagnum moss peat should not be as high as the Westland brand above. Many of us have had bad experiences with Westland (but some people have found it ok - possibly batch variation, possibly something more complex). A meter costs £5 and the test takes as long as it takes you to mix the media with rainwater.

I would be interested to hear of anyone growing Sarracenia at their best in water with a TDS measurement much over 100ppm.

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The water you put on your pots is not the same as the water it comes out after passing through the substrate, which is the one the roots are in contact with. Even if you continuously add only 50 ppm water with something in principle harmless, it may reach harmful concentrations after time. But a little bit of the most usual salts in water (carbonates, sulphates) does not have a detrimental effect, as they are retained, complexed, metabolized, etc. with time, or they are not enough to harm before repotting.  If that amount is exceeded, the problems start sooner or later depending of the specific substances and the concentration they reach, as we all know.

You, however, do not continuously add a solution of 200 ppm tanic acid, just to put an example (it is obviously an unrealistic extreme to be illustrative), so the ppm in the substrate after it first contact with distilled water and the ppm of the water you use to regularly water the plants are not comparable.

Edited by jcz
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