Once established and growing, is the time they need and consume it the most. The idea is to maintain the balance of nutrients by avoiding buildup.
For example: Aside trace elements plants in an aquarium that is rich in Co2, phosphorus and potassium, will turn nitrate into a nutrients that is consumed instead of building up.
Without the other essential elements this process can slow down to a halt. Resulting in problems with plants and nitrate.
There are many situation where 2 thing can happen, 1: You need to add more, Because the environment is poor. 2: You need to add less, because it's rich.
Only testing for key element can provide an answer to that or a diligent water change schedule that insure nothing can possibly be accumulating.