In order to survive, your houseplants need two kinds of nourishment. We discussed one of these, water, in Part II of this series. They also need minerals. When a plant is outdoors, it can usually get enough minerals from its surroundings all by itself. But houseplants need more attention – the nutrients available to them indoors must be provided by the gardener.

What are those numbers on the bag of fertilizer?

Fertilizers contain the key elements needed: Nitrogen (N), Phosphorus (P), and Potassium (K) in varying proportions. The numbers given on a bag of fertilizer show the relative amounts of these three elements. For example, an 18-6-12 fertilizer contains 18% N, 6% P2O5, and 12% K2O. Other elements are also necessary, but in much lower amounts.

How do I decide what proportions to use for my plants?

This of course depends on what type of plant you are fertilizing. It is very important not to give your plant too much or too little food. This is a decision that must be made for each individual plant. You should carefully read both the label that came with your plant if there is one, and the label on the bag of fertilizer. If you’re still not sure, ask the person at the plant store for advice.