In recirculating hydroponic systems producing leafy crops, one of the main factors in the control of the grower is the frequency of tank changes (ie pumping out and turning over the recirculating solution). Since we generally manage the solution based on EC, we are assessing the total amount of solutes in the water. We don’t know the balance of each nutrient individually, so tank changes are carried out to try and maintain necessary levels of nutrients. Essentially, our goal is to change the nutrient solution often enough that we don’t have detrimental buildups of unused ions or depletions of important nutrients. Also keep in mind that different water sources have different background ion levels that can slow or speed up imbalances in the nutrient solution.
This tank change practice is really based on cost efficiency. There are nutrient solution management systems that have the ability to manage based on individual ions, but these are much higher cost than the typical systems installed in small to mid scale greenhouses. This hydroponics system cost savings comes at the price of more frequent tank changes that maintain a safe margin of nutrients in solution.