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Does this mean the potassium you were dosing, presumably the K part of the KNO3, was all taken up by the plants and/or adsorbed by the substrate? (If the substrate material has a reasonable CEC it should adsorb some potassium.)
I suggest adding some or more K2SO4 to the PPI formula, dosing for ten days, and rechecking for potassium. Repeat that until you get a measurable value. Then dose that mix for a week or 10 days and re-measure potassium, repeating several times, to see if it accumulates and how fast.
I suggest adding some or more K2SO4 to the PPI formula, dosing for ten days, and rechecking for potassium. Repeat that until you get a measurable value. Then dose that mix for a week or 10 days and re-measure potassium, repeating several times, to see if it accumulates and how fast.