A bag labelled plant food contains nutrients, but it does not replace the sugars a green plant makes through photosynthesis. Light supplies energy; roots usually obtain mineral nutrients dissolved in water. Both processes contribute to growth.

Fertiliser adds concentrated nutrients. It can help where supply is inadequate for the plant and crop you want, including many container crops. It cannot repair every cause of yellow leaves or poor growth.

Think of nutrients as materials for a working workshop. Supplying more material is useful only if it addresses a need. Check the plant, its conditions and any feed already present before adding another dose.