Potted plant food replaces soil nutrients used through the growth of potted plants. Because a plant may not need to be frequently re-potted with new soil, plant food, also known as fertilizer, can be added to the pot to help improve plant health and growth through proper nutrition. Different kinds of plants do better with different kinds of plant foods.
As plants grow, they take nutrients from the soil for use in creating green plant matter and flowers. Plant food for potted plants can serve a couple of functions. One function is to replace soil nutrients consumed through growth. The second function is to increase soil nutrition in ways that will encourage more rapid growth.
Two general classes of fertilizers are available, both for potted plants and plants growing in the soil. Organic fertilizers use natural substances that must be broken down further by soil microbes before providing nutrients to plants. Chemical fertilizers are synthetic chemicals that are in forms that are ready for use in plants.
Potted plant fertilizers can come as liquids, powders, or long-release spikes or crystals. Long-release fertilizers release their nutrients slowly to increase the amount of time required between fertilization.
Fertilizer contents are shown on the packaging as N-P-K levels. N-P-K stands for nitrogen, phosphorus and potassium. An N-P-K number is shown as something like 10-10-10 or 20-10-10. Each number indicates the percentage by weight of the particular nutrient. For example, 20-10-10 is called high nitrogen fertilizer because of its higher percentage of nitrogen.
Chemical fertilizers can work very quickly because they are already in forms that the plant can use. Organic fertilizers, however, can take longer to work because of the need for further breakdown by soil microorganisms. Some potted plants, like bonsai, require frequent fertilization because the water drains from the pot quickly.
Although they work quickly, chemical fertilizers have some inherent risks. Because they are in a form that can be quickly used by plants, there is a risk of over-fertilization if you don't carefully follow package instructions. Organic fertilizers have a lower risk of over-fertilization and subsequent damage to plants.