By runoff readings as I mentioned. Cant flush the plant out to get a good runoff reading. You want as little of fresh water as possible to mix with soil. Water a little let seep water a little let seep. Until water starts barly dripping from pot. Catch that water and only enough to stick a probe in to get a reading. Works with ph as well if you're already getting a ppm reading and dont want to do a slurry test.
