One of the great myths of gardening is that fertilizer is plant food. It’s not. Plants make their own food in their leaves and stems with the energy of the sun. A better way to think of fertilizer is like a vitamin and mineral supplement. Plants need the nutrients in fertilizer to grow and stay healthy, but will only benefit from nutrients in the proper amounts. Fight the misconception that “more is better” - a little fertilizer is much better than a lot. Plants that are suffering from disease, insect attack, or neglect will not benefit from extra fertilization. Try to determine the problem, cure that, and then continue the fertilizing program when the plant’s general health has been restored.

Here are a few rules of thumb with fertilizing:

Never fertilize dry soil. Water all plants thorough ly the day prior to fertilizing. Nutrients in contact with dry roots will burn sensitive root hairs.
Wait 3 weeks before fertilizing newly bought plants or transplants.
Never fertilize your plants on days when the air temperature is over 85 F.
Different plants have different fertilizer needs - find out what your plant prefers, and keep a regular schedule.
If your plant seems unhealthy - don’t try to use fertilizer as medicine. There could be lots of reasons besides lack of nutrients that cause a plant to be pale or stunted.