Plant fertilizers have three main ingredients: Nitrogen (N), Phosphate (P) and Potassium (K). Fertilizer is labeled with three numbers pertaining to the fertilizer's percentage of each respective element. For example, a fertilizer labeled 5-10-5 contains 5% nitrogen, 10% phosphorus and 5% potassium. The rest of the bag contains secondary nutrients, such as calcium and magnesium, and filler, which is typically organic matter, sawdust, or sand. The filler is essential, as adding only nitrogen, phosphorus and potassium could chemically burn or kill the plants.
Nitrogen essentially promotes fast plant growth. However, a forced, fast-growing plant tends to be weak overall and is more susceptible to insect damage and disease. Always use a nitrogen-rich fertilizer after the plant has been established. It is best to use a slow-release fertilizer when attempting to increase the nitrogen content. Nitrogen is released periodically, giving your plants a light boost instead of a shot. Organic fertilizers are naturally slow releasing.
Phosphorus encourages strong root growth, prolific blooming and fruit production. A fertilizer high in phosphorus should be used to amend the soil before planting and used throughout the plant's infancy. Phosphorus is encouraged for vegetable gardens and flowering plants, such as fruit tree and roses. Organic fertilizers typically use bone meal to boost the phosphorus content.
Potassium, or potash, boosts your plant's hardiness to disease and insect damage. It strengthens the overall plant and is best used before and during the winter months. Winterizer fertilizers are typically high in potassium. However, always use fertilizer high in potassium with extreme caution and care. These fertilizers have a high salt index, according to Donnan.com, and over-application can burn or kill the plant.
Each fertilizer contains trace amounts of secondary nutrients or natural elements necessary to organic growth. These elements are calcium, magnesium and sulfur and small amounts of copper, iron, boron, zinc, chlorine, manganese and molybdenum. These elements are found in all organic material. Commercial fertilizers may use compost to obtain these elements. Organic fertilizers contain these ingredients naturally.
Filler can be anything from sand and sawdust to granulated limestone. Its purpose is to separate the nitrogen, phosphorus and potassium and act as a buffer for the plants. If you were to add only nitrogen, phosphorus and potassium without any filler, the plants would be chemically burned and could die as a result. While the majority of the bag may be labeled "filler," its existence is essential to the plant's health and the fertilizer's purpose.