What causes RADICALLY different TDS measurements (ppm) ?

While measuring TDS of the nutrient mixture and runoff of my 4 plants, I got drastically different results. How can this occur in plants that are on the same feed cycle?

Todays watering: in a two gallon watering can,
- 1.5 tsp/gal Fox Farm Big Bloom
- 1 tsp/gal Fox Farm Tiger Bloom
My vivosun meters measured 6.28 pH and 300ppm for the nutrient mixture. I made this mixture twice, so each plant got about one gallon. I aimed for a light feed with minimal runoff, just enough to test the pH and ppm. All of the plants had balanced pH runoff. But the runoff ppm varied drastically!

The plants' individual runoff values were 630ppm, 800ppm, 1775ppm, and 3600ppm! Holy mackerel! Why??? They are all fed pretty much the same. The plant with the highest measurement is the one that has been the slowest growing recently...

PS. I'm still getting the hang of ppm, what it means, and how to adjust etc. The meter was calibrated upon first use, but not before each use. My plants are in their second week of flower. Just measured my tap water and it is 160ppm

STRAIN King Louis x Cotton Candycane (supposedly)
SEED/CLONE seed
PLANT AGE germinated late January
PHASE Flowering, beginning week 2
LIGHT 1200w LED that consumes 235w
RH FACTOR 40-50%
DAY/NIGHT TEMP Day 75-80 F, Night 68-75 F
RECENT STRESSORS Transplanted April 15, supercropped April 26th, SCROG set up April 27th
PESTS fungus gnats in the PAST were vicious
TECHNIQUE LST + supercropping, now SCROG
POTS 5 gallon fabric pots
ORIGINAL SUBSTRATE Germinated in Miracle-Gro. Transplanted plastic 1.5gal pots 50/50 mix of Miracle-Gro and GreenAll Planting Mix.
CURRENT SUBSTRATE Amended soil, transplanted April 15th followed by a flush
- 45% Recipe 420 Potting Soil
- 33% Perlite
- 20% Earthworm Castings
- 2% Bone Meal 4-12-0

crittertime said:

STRAIN King Louis x Cotton Candycane (supposedly)
SEED/CLONE seed
PLANT AGE germinated late January
PHASE Flowering, beginning week 2
LIGHT 1200w LED that consumes 235w
RH FACTOR 40-50%
DAY/NIGHT TEMP Day 75-80 F, Night 68-75 F
RECENT STRESSORS Transplanted April 15, supercropped April 26th, SCROG set up April 27th
PESTS fungus gnats in the PAST were vicious
TECHNIQUE LST + supercropping, now SCROG
POTS 5 gallon fabric pots
ORIGINAL SUBSTRATE Germinated in Miracle-Gro. Transplanted plastic 1.5gal pots 50/50 mix of Miracle-Gro and GreenAll Planting Mix.
CURRENT SUBSTRATE Amended soil, transplanted April 15th followed by a flush
- 45% Recipe 420 Potting Soil
- 33% Perlite
- 20% Earthworm Castings
- 2% Bone Meal 4-12-0

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You have a a lot of amendments.


When you’re in soil your runoff ppm will generally always Be higher than your input because you have slow release Ferts In your MG so every time you water you are releasing some of those ferts.

So your readings end up higher.


Look at your plants. If they are lush lime green, heathy and happy then this is the biggest sign you’re doing well.

Realistically you can only test runoff ppm in a hydroponic system to judge what’s happening.

jondamon said:

Realistically you can only test runoff ppm in a hydroponic system to judge what’s happening.

Click to expand...

Ah good to know! Would that mean it's fruitless to compare nute solution ppm and soil runoff ppm?
My plants leaves have alway seemed rather dark to me, but this is my first grow

crittertime said:

Ah good to know! Would that mean it's fruitless to compare nute solution ppm and soil runoff ppm?
My plants leaves have alway seemed rather dark to me, but this is my first grow

Click to expand...

Stop wasting time worrying about runoff measurements especially if you're growing in soil. It's basically pointless. If you take care of what goes in then you won't have any issues. Too many people screw up healthy plants because they start checking runoff and when the numbers don't match what they incorrectly think they should be they start changing things and trying to adjust something that doesn't need to be adjusted. If the plants are healthy leave them alone.