All living organisms require a form of nutrition to survive, and plants are no exception. Plants produce chemical energy from sunlight using photosynthesis, and they extract other mineral nutrients from the earth through their roots and, over time, can deplete their surroundings of nutrients. Fertilizer applications restore nutrients to the soil that plants need to grow.
Pick up a bag of commercial fertilizer, and you see that it is identified with three numbers. "5-2-0," reads a bag of Milorganite fertilizer. Even organic fertilizers are identified this way. Fish meal, for example, is 5-3-3.
Fertilizer numbers identify the percentage of the three primary macronutrients--nitrogen, phosphorus and potassium--found in the fertilizer. The fish meal in the example contains 5 percent nitrogen, 3 percent phosphorus and 3 percent potassium. Fertilizer numbers are also called NPK ratios because they express the proportion of those essential ingredients available in the fertilizer.
The first number on the fertilizer label identifies the percentage of nitrogen it contains. Plants require nitrogen for many metabolic processes, and it is an essential component of protein and DNA. Nitrogen assists plants in using energy efficiently. Although nitrogen is abundantly available in the atmosphere, it must undergo a change called "fixation" before it is available to plants. Plants deficient in nitrogen become spindly, and the older leaves turn yellow. Likewise, overuse of nitrogen can be harmful, burning plants and polluting the ecosystem.
Phosphorus quantities in fertilizer are indicated by the second number on the label. The soil gains phosphorus through the weathering of certain rocks, and it helps plants with photosynthesis, respiration, energy storage and water usage. Phosphorus-deficient plants weaken, develop a bluish tint to their leaves and fail to produce flowers and fruits. Although overuse of phosphorus doesn't put the plant at risk, it does run off and pollute the environment.
The final number in the NPK ratio indicates the percentage of potassium available in the fertilizer. Potassium assists plants in metabolism and also controls the stomata, the pores on the undersides of leaves essential for gas exchange. Because the stomata also control water loss through evaporation, plants with inadequate potassium dry out and weaken.
Perusing commercial fertilizers at your gardening center reveals that there are various NPK ratios available. Some contain a balance of the three nutrients, while others offer only one nutrient or a mix of all three. Because all soils are different and because overuse of a nutrient can damage the plant or local ecosystem, have your soil tested before selecting a fertilizer. Local agricultural extension offices often do tests for free or, at least, can recommend a testing service. Soil testing assures that you are applying needed nutrients and not wasting your money or harming your plants by overapplying nutrients that aren't needed. Soil test reports also recommend an NPK ratio for fertilizer application.