Outdoor plants obtain many of the necessary nutrients for growth from the soil. Household plants require supplemental fertilization in order to grow efficiently. Plants exhibiting slow growth, pale leaves, weak stems and reduced flowering are often suffering from under-fertilization. Fertilization needs vary with the type of plant, age of the plant and growing season.
The majority of household plant fertilizers contain three major elements: nitrogen, phosphoric acid and potassium. Nitrogen promotes growth and produces lush, green foliage. Phosphorus encourages flowering and supports strong root growth. Potassium helps the plant fight off diseases and strengthens the stems.
According to the North Carolina State University Cooperative Extension, the majority of houseplant fertilizers are labeled 20-20-20. The three numbers indicate the percentage of nitrogen, phosphorus and potassium found within the fertilizers. Endless combinations of the three elements are available, often tailored to specific plant needs.
Most plants include a tag that indicates there should be regular fertilization. Frequency can range from every two weeks to once every three months. The fertilization needs of a household plant are dependent upon the type and age of the plant. In general, plants should be fertilized only when in a period of active growth. According to the University of Connecticut College of Agriculture and Natural Resources, houseplants usually feature active growth from March until October.
The Ohio State University Master Gardener's program recommends that gardeners use the recommended application amount of fertilizer, or even a diluted amount. Found in liquids, tablets, spikes, granulars and crystalline forms, the majority of horticulturists recommend the usage of fertilizer solutions. Apply the solution to the soil surface, continuing to add fertilizer until the solution runs out the drainage holes on the bottom of the pot.
Water-soluble fertilizers are available in liquid, powders, tablets and crystals. They are mixed with water and then poured into the planting medium; most labels for water-soluble fertilizers recommend feeding on a monthly basis.
The majority of plants purchased from a nursery feature a form of slow-release granules on the surface of the planting medium. The granules dissolve slowly over time with the application of water. If the plant is watered from the base of the planter, the fertilizer will not be leeched into the planting medium at the same rate as when watered from the top.
Fertilizer spikes are inserted into the soil and work in the same manner as slow-release granules. However, they often provide high levels of fertilizer in the immediate surrounding area, increasing the risk of root burn.
Over-fertilization symptoms include decreased growth, burned or dried edges on leaves, browned roots, and wilted plants. Over-fertilizing results in a buildup of soluble salts in the planting medium on household plants. A white crust may build up on the soil surface, around the drainage holes, or on the outside of clay pots. The salts build up over time, making it harder for the plant to absorb water.
To avoid the buildup of salts, the North Carolina State University Cooperative Extension recommends leaching the household plants once every four to six months. Pour water equal to twice the volume of the pot into the planting medium, letting it drain out the bottom of the pot. For example, a 6-inch pot will hold 10 cups of water. When leaching, 20 cups of water should be poured through the plant's soil.