Interesting Soak Water Measurements

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Aug 7, 2022
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Hey! I thought I’d kick this out to the “tribe” for opinions and input.

I was doing my usual watering today and checked soak water with my BlueLab pH/Conductivity combo meter. For what it is, I’d call that a high quality meter! My unfiltered, untreated tap water measures alkaline and relatively high ppm (I haven’t had a chance to find the village water quality report yet to ID the water components…). Usually I’m at 7.5 - 7.6 pH and approx. 230 ppm of “stuff,“ straight from my tap (exact composition TBD).

I don’t have space for an RO system but use a simple home charcoal filtration system that brings the ppm to 90-110 ppm and pH down significantly to 6.2 - 6.5.

So, today was not a fertilization day so I simply watered. I checked the soak water of all my plants. They all actually seemed to reflect my filtered tap water values - averaging 80-100 ppm and pH values from 6.1 - 6.5.

However, my Rothschildianum and my Lady Isabelle (a Roth cross as I’m sure everyone knows!) measured a ppm of 0 and a pH value of 6.8/6.7. They were both purchased from the same vendor but at different times. I also have a dianthum and a Dollgoldii front the same vendor. All 4 plants are happy and healthy overall. The dianthum and Dollgoldii both had very different pH and ppm values after watering.

I am curious if this might be a Roth specific thing? I will call the vendor tomorrow as their customer service is great and they engage with me regularly on my plants. But, I thought I’d just ask this of this group anyway! I thought based on my googling that Roths preferred acidic conditions (which 6.7-6.8 are, but barely) and the ppm value going from 90-110 from my filtered water source to 0 was very strange. I wonder if the Roths are really sucking up nutrition? Are they heavy feeders…? I didn’t think they were…

Do these numbers indicate decomposed media? They seem like happy and healthy plants. Both actually have new growth and healthy leaves (although neither have bloomed for me yet). Should I repot…? I hate to disturb roots of seemingly healthy happy plants for no reason..

I know my meter is calibrated and accurate as I have the standard solutions and check my meter prior to each use (I used to work as a chemist and would shame my own self if I didn’t verify instrument accuracy/calibration prior to use!).

Apologies in advance for something of a ramble with some questions mixed in. I’d love to hear input!
Thanks in advance!
There are a couple of interesting things there, but we need more info to make any solid sense from it.

First of all, no matter how good of an instrument it may be, your EC meter is NOT giving you a true TDS. That requires a “conversion factor”, and each and every solution requires its own factor. This article explains that. As an example, MSUWW, MSURO, and K-Lite - three fertilizers made in the same factory, using identical ingredients but in different proportions, at the same EC, the true TDS value for the MSURO solution is about 20% greater than that of the MSUWW, while the K-Lite solution is about 40% higher.

Next, activated carbon does very little to remove dissolved mineral solids. The fact that your EC meter shows a reduction after filtration suggests that the difference is due to reduction in organic compounds (a rarity in municipal water supplies, but not unheard of) and/or chlorine.

I’m not sure what your “soak water” results suggest, but if you really want to know what’s happening, pH-wise, in the individual pots, a more reliable test is the “pour through” method, which is described HERE. One thing about pH that I found surprising, was how little the solution pH affects the rhizosphere pH, which is controlled more by the plant, the medium, and the microbes contained in them.