Each and every plant will have its own nutrient and water requirements.
What may be fine for one plant could be way under or way over what another plant needs.
Needs are mostly genetically related, indicas generally being heavier feeders than sativa strains.
It is a finding a balance for what each plant needs and adjusting the nutrients with a bit of fine tuning.
Even four plants of the same strain can have slightly different needs, again it comes back to which plant needs what.