In elementary school, you probably learned that plants make their own food. So why do you need to feed them?
While it is true that plants make their own food, they still need the raw ingredients. Sometimes, gardeners need to provide several of the nutrients required by many plants that may not be available in adequate quantities for healthy plant growth.
If your plants are growing in fertile garden soil, they probably have most of the nutrients in adequate amounts. If any elements are in short supply, they most likely are nitrogen (N), phosphorus (P), or potassium (K). These are the three elements in ordinary garden fertilizers and are required in relatively large quantities by plants.
Nitrogen promotes good growth of stems and leaves or vegetative growth, while phosphorus promotes flowering. Potassium is important in a lot of reactions within the plant. It is important that these elements be in balance with each other for healthy plant development. Too much of any element can be harmful.
Under certain conditions, you may need to add other nutrients for normal plant development. Certain plants have higher requirements for some micronutrients such as iron or zinc. Certain soils, such as those that are very sandy, may be deficient in some of the micronutrients.
If you suspect a problem, consider having a soil test done. A soil test provides information on the amounts of P, K, pH (the acidity of the soil), and organic matter in your soil. Some testing services also indicate the amounts of other nutrients such as magnesium (Mg); however, since most micronutrient problems are not widespread, the standard tests usually do not include these.
The local office of the Cooperative Extension Service has information on how to take a soil sample and where to send it. Home test kits, available from garden centers, will give you a general idea of the amount of nutrients in your soil. Follow the directions carefully.
Nitrogen is more difficult to test for. If plants appear yellowish or stunted, they may need additional nitrogen. The soil test results should indicate any possible problems. Based on the results, you will know if additional P or K is needed. The pH affects the availability of many of the nutrients. Adjusting the pH, by either adding lime to make the soil less acidic or adding aluminum sulfate to make it more acidic, may be all it takes to correct some nutrient problems. For example, iron deficiency in plants such as azaleas, rhododendrons, and blueberries often can be corrected by lowering the pH.
Good soil preparation goes a long way in preventing nutrient problems. A well-drained soil, high in organic matter, allows for the development of a good root system able to take up adequate amounts of water and nutrients. Compost is an excellent source of plant nutrients and contributes to the development of a well-aerated, well-drained soil. Soils with high organic matter sustain large populations of worms which improve the nutrient status of your soil.