Its the same for plants. Well-fed plants are healthier, more productive and more beautiful. House plants' diet are entirely the plant parents' responsibility. Unlike in the wild, houseplants are dependent only on whatever is available to them in the pot which is in the house.
Plants need to be fertilized because most potting mixes does not provide the essential nutrients required for optimum growth. Even if you are able to start with a great soil mix, over time as your plants grow, they absorb nutrients and leave the soil less fertile. It is important to re pot or fertilize in order to replenish the lost nutrients in order for the plants to remain healthy and flourish.
There are 6 important nutrients that plants need in abundance. Carbon, Oxygen, Hydrogen, Nitrogen, Phosphorus and Potassium.
Plants can absorb Carbon, Oxygen and Hydrogen from the air and water. We would need to supplement them with Nitrogen, Phosphorus and Potassium which is widely available in the market in the form of N-P-K.
Nitrogen helps plants make the proteins they need to produce new tissues.
Phosphorus stimulates root growth, helps the plant set buds and flowers, improves vitality and increases seed size.
Potassium improves overall vigor of the plant. It helps the plants make carbohydrates and provides disease resistance. It also helps regulate metabolic activities.
Secondly, there are 3 nutrients that are important as well such as Calcium, Magnesium and Sulfur. However in smaller amounts.
Calcium is used by plants in cell membranes, at their growing points and to neutralize toxic materials. In addition, calcium improves soil structure and helps bind organic and inorganic particles together.
Magnesium is the only metallic component of chlorophyll. Without it, plants can't process sunlight.
Sulfur is a component of many proteins.
Lastly, here are seven trace elements or micro-nutrients that plants need in even smaller amounts. Healthy soil that is high in organic matter usually contains adequate amounts of each of these micro-nutrients. Boron, Copper, Zinc, Chlorine, Molybdenum, Iron and Manganese.
If your house plant don't bear fruit or flowers, fertilizers with a slightly higher Nitrogen value is preferable( Example. 5-3-3). For flowering types, choose those with higher Phosphorus value ( Example. 1-3-1). The fertilizers should contain some of the trace elements so that the plants can benefit greatly.
Remember to never use more than the directed amount and frequency stated on the package. You wouldn't want to burn the roots by over fertilizing.