I live in an apartment building, and all my plants grow in pots or small flower beds. Since I moved here, 7 years ago, I have only watered the pots and revolved the "soil", eventually adding more vegetable compost to fill the pots.
I believe that a splash of NPK fertilizer would be good, but I am concerned about poisoning the plants. Is there a possibility to overdo the amount of such nutrients? What kind of damage could happen?
Yes excess NPK can damage plants, although how much N, P, or K is too much depends on the plant.
For example, you shouldn't add too much N to the soil for radishes and carrots because that will causes them to grow lots of leaves and small vegetables. Conversely, grass-type plants, like corn, and vegetables you grow for their leaves, like kale and lettuce, will use a ton of Nitrogen.
For general knowledge, I recommend you read all the questions tagged fertilizer.
