All plants absorb minerals from the soil, whether the minerals exist in the soil naturally or as the result of additives applied by gardeners. The minerals in the soil enter the plant separately from water. Water is absorbed through the epidermis, or single-celled membrane covering the root; minerals in the soil work their way into the root through the hairs on the root. The water and minerals combine once in the root.
Fertilizer provides soil with added minerals and nutrients, which in turn are absorbed by the plant. The primary nutrients in fertilizers are nitrogen, phosphorus and potassium. Nitrogen promotes vegetative growth, the process of photosynthesis and the absorption of minerals. Phosphorus aids in the transfer and storage of glucose and the formation of nucleic acids within plants. It promotes the growth and development of flowers, seeds and roots. Potassium helps a plant tolerate cold weather and improve its fruit production. Potassium is also vital to the production of glucose, or sugar energy.
Fertilizers also contain sulfur, calcium and magnesium. These work together to balance the pH level of the soil. Most plants need a soil that is not too acidic or alkaline, while some plants, such as azaleas, thrive in acidic conditions. This is why it is important to choose a fertilizer tailored for the type of plant it is being used around. In addition to balancing the pH level of soil, these secondary nutrients also aid plants in other ways. Calcium strengthens the cell walls of plant cells. Sulfur and magnesium increase the metabolism of plants and help aid in the control of some types of plant diseases, such as mildews and leaf blights.
Finally, fertilizers contain micro-nutrients and fillers. The fillers are materials designed to make the application of the fertilizer easier. These help "bulk up" the soil, but do not directly benefit the plant. Micro-nutrients included in most fertilizers in small amounts are boron, copper, cobalt, iron, zinc, manganese and molybdenum. Boron strengthens the pollen tube and aids in the germination process. The rest of the micro-nutrients act as components of essential enzymes that aid in the plant's process of metabolism.