Applying fertilizer is part of many gardeners' regular routine, and they can see just by watching their garden grow that fertilizers promote healthier and more productive plants. What exactly goes on in those handfuls of fertilizer or compost carefully placed around growing plants is more of a mystery. Fertilizers provide nutrients that bolster plants' essential life processes.
Like all living organisms, plants draw nutrients from their environment and use them in metabolic processes related to growth and health. Just as a pantry will eventually run out of food if not restocked, plants will gradually deplete the soil of nutrients if they are not restored. Natural processes, such as the decay of plants at the end of autumn, restore nutrients to the soil without human intervention. In the more intensive planting typical of gardening and agriculture, however, plants often use more than can be restored without the application of fertilizer.
Plants require an array of minerals and micronutrients, with nitrogen, phosphorus and potassium as the three most used and best known. Each nutrient serves a different function within the plant, and most contribute to multiple metabolic processes. Nutrient deficiencies tend to manifest as poor growth and productivity and higher rates of disease.
Many nutrients provided by fertilizer are required for photosynthesis and other metabolic processes related to the use, transfer and storage of energy, allowing plants to conduct these processes more efficiently. Because plants need energy to grow and produce flowers and fruit, increasing the availability of energy naturally encourages growth and productivity. For this reason, gardeners often apply fertilizer when they want plants to grow larger, faster or produce more flowers or fruit.
Other nutrients assist in processes that allow plants to maintain their health and resistance to disease. Plants grown in optimal conditions tend to be less susceptible to disease, pests and nutrient deficiencies, and soil health is a major component of that. Nutrient deficiencies themselves can cause health problems in plants or make them more susceptible to disease.
Fertilizer is not a cure-all for plant problems, however, and gardeners should be judicious in their use of fertilizer. Some nutrients found in most fertilizers, such as nitrogen, can be harmful to a plant if overapplied. Nitrogen, for example, forms salts in the soil that draw water from the plant, causing it to dry out and "burn."
Not all soils are created equal, and different soils--as well as different plant species--have varying nutrient needs. Although mild fertilizers, such as compost, can be applied safely at just about any time and in any conditions, before beginning a more intensive fertilization program, be sure to have your soil tested. Local agricultural extension offices will usually do this, often for free, and recommend a fertilization program. Once you've determined your garden's needs, always follow the instructions on the packaging to achieve the best results.