Your plants basically need two types of nourishment to survive: water and minerals. They get minerals by absorbing them from the damp soil through their roots. Outdoor plants can often gather all the minerals they need from their surroundings by themselves. However, houseplant care requires more careful attention from the gardener, since the nutrients available from the soil in a pot are limited.

What do those numbers on the fertilizer bag mean?

The key elements required for plants are Nitrogen (N), Phosphorus (P), and Potassium (K). A fertilizer bag shows the percentages of these three nutrients as three numbers, always given in the same order. An example: an 18-6-12 label means that the bag contains 18% nitrogen, 6% phosphorus, and 12% potassium, usually in the form of nitrates, phosphates, and potassium salts. Other nutrients are also needed, but their required amounts are much lower.

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

The amount of each element needed will of course depend on the species of plant. Too much is just as bad for the plant as too little, so it requires a bit of research to learn exactly what is right for your particular tropical plant. Usually this help is available by either reading the label, or by asking your plant-supply person.