Fabric pots? Try taking a sample with a syringe from the bottom inch and test that.I'm trying this type of grow this run and am happy with the results. I was growing under 2 SE 1000w HPS and 1 Diablo but my 420 sale Diablo came in yesterday so I switched one of the HPS lights out. Everything is looking good but I can't get the runoff under control. I feed @3.0 or 1500PPM & 5.5 PH. I was watering 7 x a day and the runoff kept creeping up. Finally hit 2700 two weeks ago so I started a flush, 1st cut the feed in half, then increased amount of water per feed. Took about a week and I got the runoff down to 1600. I'm only feeding 3x a day now but more per cycle. Anyway as soon as I went to a normal 20% runoff schedule the runoff shot up quick. Plants don't look to have an issue but I want to be proactive if this could lead to a problem, any suggestions?
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Later in the grow some salts can accumulate on the fabric from previous drying and skew the numbers. If no decline in plant health or issues this could be the case.
What type of nutrients are you using? That ppm seems awfully high