Newly purchased houseplants usually have enough fertilizer in their soil to nourish them for a while. But any houseplant that has been in the same pot for more than a few months needs to be fed, especially when lengthening days begin to stimulate new growth.
There are a few different ways to go about this feeding. Simplest is just to mix fertilizer right into the potting soil before you even pot up a plant.
If you use a purchased potting soil, check if fertilizer has already been added.
If not, or if you make your own potting soil, you can use ordinary garden fertilizer, such as 5-10-5, at the rate of about a tablespoon per gallon of soil. This fertilizer does have the disadvantage, however, of leaching somewhat too readily out of the pot as the plant is watered.
Specially formulated, slow-release synthetic fertilizers circumvent this problem by slowly bleeding their nutrients into the soil in response to warmth (Nitroform, for example), moisture (Magamp) or both warmth and moisture (Osmocote).
Natural fertilizers also release nutrients slowly into the soil in response to warmth and moisture. Most natural fertilizers can be relied on to supply only one of the three major nutrients--nitrogen, phosphorous and potassium--needed by plants, so use different natural fertilizers in combination.
For example, soybean meal or cottonseed meal can supply nitrogen; bone meal can supply phosphorus, and wood ash can supply potassium. Compost and dried manure are natural fertilizers that provide balanced nutrition, but to provide sufficient nourishment they need to make up about a quarter, by volume, of the potting mix.
If you are going to pasteurize the potting soil (and this is not always necessary), add fertilizer to the mix after the soil has cooled. Especially in potting mixes lacking garden soil, micronutrients also might be needed, though not if natural fertilizers are used.
No matter what nourishment a potting mix initially contains, more fertilizer eventually will be needed. You can supply additional fertilizer to a growing plant by periodically scratching a quarter-teaspoon of any concentrated fertilizer mentioned above into the surface of the soil. If using a dilute fertilizer such as compost or dried manure, blanket the whole surface of the soil with a quarter-inch layer. In either case, nutrients will wash down to the roots each time the plant is watered.
Another way to fertilize houseplants is with fertilizer "spikes" or "tabs" sold for this purpose. You push these compressed fertilizers into the soil near the plant, and nutrients slowly leach from them into the soil each time you water.
Plants also can be fed with specially formulated powders or concentrated solutions that you dilute into your watering can. These fertilizers must be soluble. Common garden fertilizer, for instance, will not wholly dissolve in water. An annoyance with soluble powders is that they usually are dyed blue, and this blue dye stains fingers and clothes.
Also, after some time, the powders tend to cake up, so you have to chip away at the powder to get the teaspoonful or tablespoonful you need. Liquid concentrates are more convenient but are more expensive because you are paying, in part, for packaged water.
Natural fertilizers that are soluble and can be fed as you water include fish emulsion (but hold your nose--even the supposedly deodorized stuff retains some odor) and seaweed, in concentrated liquid or granular form.
No matter which method of fertilizing plants you use, always follow the directions for the specific fertilizer.
And watch your plants--their growth and their leaf color--to see if you are feeding them correctly.
Always err on the side of underfeeding, which just slows growth, rather than overfeeding, which can burn roots and increase susceptibility to pests.