Fertilizers are chemicals given to plants with the intention of promoting growth; they are usually applied either via the soil or by foliar spraying.

Fertilizers typically provide, in varying proportions, the three major plant nutrients (nitrogen, phosphorus, and potassium), the secondary plant nutrients (calcium, sulfur, magnesium), and sometimes trace elements (or micronutrients) with a role in plant nutrition: boron, manganese, iron, zinc, copper and molybdenum.

The three primary ingredients of fertilizers are listed on the fertilizer bags as nitrogen, phosphate and potash as three numbers, indicating the ratios in that order. Thus a 5-10-5 fertilizer would have 10 per cent phosphate in its ingredients.

Manure was once the dominant fertilizer, and is still used, but its role is greatly diminished. Fertilizer can be created either from natural organic material such as manure or compost (see also organic gardening), or artificially as through the Haber-Bosch process which produces ammonia. This ammonia is used to produce nitric acid. A reaction product of ammonia and nitric acid already gives Ammonium nitrate which is a fertilizer product. The nitric acid and ammonia also can be used in the Odda Process to produce compound fertilizers such as 15-15-15.

The Haber-Bosch process uses about one percent of the Earth's total energy supply in order to provide half of the nitrogen needed in agriculture. Organic material has the advantage of adding carbon compounds to the soil. A major source of soil fertility is the decomposing crop residue from prior years, though this is not considered "fertilizer."

Justus von Liebig wrote in 1840 the law of the minimum required by the plant.

Over-use of fertilizer can lead to algal blooms in lakes and streams that receive run-off from crop lands, and lead to long-term degradation of the soil; see in this regard eutrophication and nutrients. For these reasons, it is recommended that knowledge of the nutrient requirements of the soil vis-a-vis the crop precede applications of commercial fertilizer. In short, excess nutrient elements can cause local soil and off-site damage, as well as waste money.