Fertilizer is a natural or manufactured material applied to soil or plants primarily to supply nutrients needed for growth. Fertilizers include mineral salts, industrially synthesized compounds, and nutrient-bearing organic materials such as manure and compost. They differ from soil amendments used chiefly to change soil properties, although some materials perform both functions. Fertilizer effectiveness depends on nutrient availability, crop requirements, soil conditions, and application methods; excessive or poorly timed applications can contaminate surface water and groundwater. (epa.gov)
Nutrients and plant growth
The primary nutrients supplied by fertilizers are nitrogen (N), phosphorus (P), and potassium (K). Nitrogen is a constituent of proteins and chlorophyll. Phosphorus participates in energy transfer and forms part of nucleic acids and adenosine triphosphate (ATP). Potassium supports enzyme activity, the movement of sugars, and regulation of plant water balance. These nutrients have overlapping physiological roles rather than acting exclusively on leaves, roots, or flowers. (extension.missouri.edu)
Plants also require calcium, magnesium, and sulfur in relatively substantial quantities, and micronutrients—including iron, manganese, zinc, copper, boron, molybdenum, chlorine, and nickel—in smaller quantities. “Micronutrient” describes the amount required, not lesser biological importance. Fertilizers may supply these elements individually or in mixtures. Nutrient additions are useful when the existing supply is insufficient; a product containing all three primary nutrients is not necessarily appropriate for every soil or crop. (extension.missouri.edu)
Types and labeling
Mineral fertilizers contain nutrients in inorganic compounds or manufactured products such as urea. They may provide a single primary nutrient or combinations of nutrients. Common examples include ammonium salts, phosphate fertilizers, and potassium chloride. Products are sold as granules, powders, or liquids, with differences in concentration, solubility, handling, and release characteristics. (extension.umn.edu)
Organic fertilizers derive from biological materials such as livestock manure, compost, and plant or animal residues. Their nutrient concentrations and release rates vary with the source and processing. Much of their organically bound nutrient content becomes available through microbial decomposition and mineralization. They can also contribute organic matter that influences soil structure and water retention. Biological origin alone does not establish eligibility for certified organic farming, which has separate input requirements. (extension.missouri.edu)
Under the conventional labeling system used in the United States and many other countries, fertilizer grades give three percentages by weight: nitrogen as N, available phosphate expressed as P₂O₅, and soluble potash expressed as K₂O. Thus, a 10–10–10 fertilizer contains 10 percent nitrogen and the stated oxide-equivalent amounts of phosphorus and potassium—not 10 percent elemental phosphorus and potassium. These oxide expressions are reporting conventions rather than a description of the compounds necessarily present. (extension.umd.edu)
Manufacture and historical development
Industrial nitrogen fertilizer production centers on ammonia. The Haber–Bosch process combines atmospheric nitrogen with hydrogen under elevated temperature and pressure using a catalyst. Hydrogen production has conventionally relied heavily on natural gas and other fossil feedstocks. Ammonia can be applied directly or converted into products such as urea and ammonium nitrate. Its manufacture therefore connects fertilizer supply with industrial energy use and carbon dioxide emissions. (iea.blob.core.windows.net)
Most phosphate fertilizers are manufactured by treating phosphate rock with acid to increase phosphorus solubility. Potassium fertilizers originate chiefly from mineral salt deposits; potassium chloride and potassium sulfate are important products. These distinct production routes mean that nitrogen, phosphorus, and potassium fertilizers rely on different raw materials and processing systems. (extension.umn.edu)
An important nineteenth-century development was John Bennet Lawes’s production of superphosphate by treating bones with sulfuric acid, patented in 1842. In 1843, Lawes and Joseph Henry Gilbert began the long-term field experiments associated with Rothamsted Research. These experiments helped establish principles of crop nutrition and provided continuing comparisons of mineral fertilizers, manure, and unfertilized treatments. (rothamsted.ac.uk)
Application and behavior in soil
Fertilizers may be broadcast across a field, placed in bands near crops, incorporated into soil, or applied in solution. Some nutrients can also be delivered through leaf sprays or irrigation systems. Nutrient management uses soil testing, crop requirements, and application timing to match the nutrient supply with plant uptake while limiting losses. Placement matters because nutrients differ in mobility and concentrated fertilizer can damage seeds or roots. (extension.umn.edu)
Applied nutrients undergo biological and chemical transformations. Urea converts to ammonium, while nitrification converts ammonium to nitrate within the nitrogen cycle. Surface-applied urea can lose nitrogen through ammonia volatilization. Phosphate can react with iron, aluminum, or calcium compounds, reducing its immediate availability. Such retention does not necessarily make nutrients permanently unavailable. Soil pH, moisture, mineral composition, and temperature influence these processes. (extension.umn.edu)
Environmental effects and nutrient efficiency
Nutrients not taken up by crops may remain in soil or move beyond the treated area. Surface runoff and erosion transport nutrients to waterways, while infiltration can carry them toward groundwater. Excess nitrogen and phosphorus contribute to eutrophication, including excessive algal growth and oxygen depletion as algae decay. Both mineral fertilizers and manure can contribute to these losses. (epa.gov)
Fertilizer use also affects atmospheric emissions. Agricultural soils receiving synthetic or organic nitrogen can emit nitrous oxide, a greenhouse gas. Environmental management consequently addresses both nutrient losses after application and emissions associated with manufacture. Approaches include matching rates and timing to crop demand, controlling runoff, and developing lower-emission ammonia production using hydrogen from electrolysis powered by low-carbon electricity. (epa.gov)