There are basically 2 ways to do it. First the estimative index method from Thomas Barr which is based off max plant uptake and large weekly water changes to prevent buildup. The other is through testing with a good kit..(Lamotte is good..sera or tetra does not come close.) I found those cheapo ones to be a bad indicator..(The kit gave me high NO3 reading which I thought so and stop KNO3 dosing. Plants did not grow much at all..seem kinda stunt week after week. I decide to do the estimative method and start dosing KNO3 base on possible max uptake (3-4ppm per day at max growth rate)..plant growth exploded...lots of growth. What do you think? Oh, that is provided the CO2 and rest of the nutrients are met though.
Plant Physiology by Taiz and Zeiger
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