A fertilizer is a plant nutrient added to a soil to increase its yield. Plants need nutrients to grow and produce fruits and vegetables. Two categories of nutrients have been identified in fertilization: macronutrients and micronutrients. There are only six macronutrients and they are required in large amounts by plants: nitrogen, phosphorus, potassium, sulfur, magnesium, and calcium. However, a larger number of micronutrients are required but in trace amounts: iron, manganese, boron, zinc, copper, molybdenum, chlorine, cobalt, nickel, sodium, and silicon. Eliminate any of these elements, and plants will display abnormal growth and deficiency, or they may not reproduce.

The most popular fertilizers contain the three major nutrients: nitrogen, phosphorus, and potassium, and they are therefore referred to as NPK fertilizers. To illustrate their importance in any economy, in 2000, the world consumption of the total fertilizer nutrient (N + P2O5 + K2O) was 140 million tons, representing 52 million tons for developed countries and 88 million tons for developing countries.

Nitrogen forms part of proteins, hormones, chlorophyll, vitamins, and enzymes, and promotes stem and leaf growth. Too much nitrogen can delay fruiting, while a deficiency of it can reduce yields and induce yellowing of leaves and stunted growth. Nitrogen fertilizers are applied in organic and/or inorganic forms. Organic nitrogen fertilizers are farmyard manure, guano (excreta and remains of seabirds), dried blood, hoof, and horn. However, organic nitrogen sources must undergo microbial processes that produce nitrate nitrogen.

Inorganic nitrogen sources are directly available to plants and include the following: sodium nitrate, calcium nitrate, ammonium sulfate, ammonium nitrate, urea, calcium cyanamide, and ammonia. In addition, atmospheric nitrogen may be used as a source of plant nitrogen by the process called "nitrogen fixation." Legumes and a few other plants, in association with cyanobacteria (microscopic aquatic bacteria, for example, Anabaena azollae), convert nitrogen to biologically useful ammonia. This process occurs in small growths on the roots called "nodules." Ammonia is subsequently available for many biological molecules, such as amino acids, proteins, vitamins, and nucleic acids.

Phosphorus plays an important role in seed germination, photosynthesis, protein formation, overall growth and metabolism, and flower and fruit formation. Phosphorus deficiency induces purple stems and leaves, poor flowering and fruiting. Low soil pH (<4) ties up phosphates by favoring the formation of insoluble aluminum and iron phosphates. Phosphorus fertilizers come from different sources: bones, rock phosphate, superphosphate (a mixture of calcium dihydrogen phosphate and calcium sulfate), nitrophosphate, ammonium phosphate, basic slag (by-product in steel manufacture), etc.

Potassium contributes to the formation of sugars, carbohydrates, proteins and to cell division; adjusts water balance; enhances the flavor, color, and oil content of fruits; and is very important for leafy crops. Potassium deficiency produces a spotted, curled, or burned appearance to leaves and lowers crop yields. Potassium fertilizers are applied in the following forms: potassium chloride, potassium sulfate, potassium nitrate, and wood ash.

Other macronutrients are supplied as part of NPK fertilizers. First, sulfur is available from the sulfate of fertilizers. It contributes to the formation of amino acids, proteins, and enzymes, and is essential to chlorophyll. It also affects flavor in many vegetables. Second, magnesium is naturally present in the soil and is generally associated with potassium sulfate and calcium phosphate, used as NPK. It is a critical part of chlorophyll, and contributes to the functioning of enzymes for carbohydrates, fruit and nut formation, and the germination of seeds. Magnesium deficiency induces yellowing between the veins of older leaves, and leaves droop (hang down) as a result. Finally, calcium is also present in the soil and is available from calcium phosphate and nitrate, and lime. It activates enzymes, contributes to the structural part of cell walls, and influences water movement, cell growth, and division.

Micronutrients are not specifically applied to soil since they are naturally found in soils. However, there are some extreme cases where they must be supplied. For example, animal disorders have been linked to a lack of trace amounts of elements, not necessary for plant growth but essential for some species of animals. In some parts of Great Britain, for example, sheep and cattle suffered from "pining disease" that resulted in severe weight loss and general debilitation. The disease was found to result from a shortage of cobalt in the herbage. It has also been established that selenium deficiencies in some soils cause muscular dystrophy, while selenium excesses induce selenium toxicity in livestock.