Besides studying abnormal appearances, you can learn whether or not to give specific nutrients to a plant depending on what stage of growth it is in, and even what variety or species of plant it is.

Young Growth

Nitrogen is especially helpful to give, though adequate levels of all nutrients are critical at this phase.

Newly Transplanted

Will benefit especially from higher amounts of root-supporting nutrients, like phosphorus and potassium.

Blossoming

Benefits from phosphorus, potassium, and calcium in particular.

Fruiting

It’s important to include decent amounts of phosphorus and potassium, while not overdoing it with nitrogen.