All plants need nutrients to grow and thrive, but in a houseplant's limited environment, fertilization is needed to survive. Regularly scheduled fertilization during the primary growing season will help your plants to grow robustly, flower freely and fight off pests and diseases. Proper fertilizing is not a substitute for poor plant care; adequate watering, pest and disease treatment and regular repotting all contribute to overall plant health.
All plants need nitrogen, but plants grown for their foliage need it most of all. Responsible for dark green leaves and above-ground growth, signs of a nitrogen deficiency include pale leaves and general poor health. Good sources of nitrogen include composted kitchen and garden waste, manure and liquid seaweed extract.
Plants utilize phosphate for root growth and development---without this nutrient, flowers and seeds could not form. Along with potassium, phosphorus increases a plant's resistance to disease. Symptoms of a deficiency include poor root development and stunted growth. Phosphates can be found in fish and blood and bone meal as well as seaweed.
Potassium contributes to photosynthesis. Flowering or fruiting plants need more potassium than foliage plants. Plants suffering from a potassium deficiency will not fruit or flower. Potassium sulfate is the most commonly used source of this nutrient, but it is also found in wood ashes, seaweed, and bone, fish and blood meal.
Magnesium is a component of chlorophyll and is needed by all plants. A deficiency causes yellow or brown patches around the edge or between the veins of leaves. Epsom salts are an excellent source of magnesium. There are Epsom salt formulations that are made for the garden--read and follow directions for best results.
Plants need calcium for strong cell walls and healthy growth. A lack of calcium can cause poor top growth on plants. Calcium deficiencies are a greater concern for garden and landscape plants, but your indoor plant fertilizer should contain small amounts as well.
Sulfur, a vital component of plant proteins, aids in plant growth and chlorophyll formation. Sulfur deficiencies are not a common problem for houseplants, but symptoms are similar to a nitrogen deficiency: light green leaves and poor growth. Soluble sources of sulfur include potassium sulfate, Epsom salts and gypsum.
All plants need various minor nutrients and trace elements. A deficiency of any of these nutrients or elements may cause general poor growth. Micronutrients include boron, chloride, manganese, molybdenum and zinc. Composted grass clippings contain minor nutrients and trace elements, as do most commercial fertilizers.