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.

What does each of the elements do for my plant?

Without going too deeply into biochemistry, we can say that flowering tropical houseplants have a greater need for phosphorus, whereas leafy green house plants like a lot of nitrogen. Potassium helps fight plant diseases, and helps fruit-bearing plants by making it easier to transport other necessary elements through plant tissue.

What are the symptoms of poor feeding?

A nutrient deficiency will often cause the leaves to lose color, or to be smaller than normal. To help the plant recover, use a soluble fertilizer, so the nutrients can be absorbed faster. Too much fertilizer can result in brown leaf tips, or in wilting. This condition is easily relieved by soaking the whole pot in water to allow the excess nutrients to be washed away.

How do I fertilize my plants, and how often?

My method is to use a solution of dry fertilizer in water. Carefully following the directions on the package should make it easy not to over-fertilize your plants. I thoroughly water each plant, until the solution begins to flow out of the drain holes in the bottom of the pot. I find that feeding my plants about every other month is fine – more frequently for blooming plants in season.

By being careful in fertilizing houseplants, you can make sure that they not only survive, but thrive. And be sure to take advantage of your plant-supplier. He should be happy to give you advice on how to keep your tropical houseplants healthy.

Do you need more help with tropical houseplant care? Visit the author’s blog on tropical houseplants, where you can find help for your plants. There are also a lot of beautiful pictures, and some neat videos too.