How to test runoff

So I want to determine how much my plants are absorbing but I tested the run off today and it was through the roof. So I did a test with straight RO water and the medium I’m using and it was 400 PPM over just the run off from the soil? What am I doing wrong or what should I be doing. First timer here

If you put 1000 ppm through a completely inert and clean media it should come out at 1000 ppm. Anything more than that would be getting stripped from the media.

K well brand new media is running off at 400 ppm so do I subtract that and what’s left is what the run off is or??

If youre using coco or another inert media, it wont have a high ppm runoff.

What are you using?

Fox farm starter soil with all the added things. And I’m using emerald harvest grow part A&B and emerald godess. Gonna be moving to bigger pots within a week