Materials added to the soil, or applied directly to crop foliage, to supply elements needed for plant nutrition. These materials may be in the form of solids, semisolids, slurry suspensions, pure liquids, aqueous solutions, or gases.

The chemical elements nitrogren, phosphorus, and potassium are the macronutrients, or primary fertilizer elements, which are required in greatest quantity. Sulfur, calcium, and magnesium, called secondary elements, are also necessary to the health and growth of vegetation, but they are required in lesser amounts compared to the macronutrients. The other elements of agronomic importance, called micronutrients and provided for plant ingestion in small (or trace) amounts, include boron, cobalt, copper, iron, manganese, molybdenum, and zinc. All these fertilizer elements, along with other chemical elements, occur naturally in agricultural soils in varying concentrations and mineral compositions which may or may not be in forms readily accessible to root systems of plants. The addition of fertilizer to soils used for the production of commercial crops is necessary to correct natural deficiencies and to replace the components absorbed by the crops in their growth.

Crop requirements of fertilizer components could be satisfied by the spreading of individual materials for each element deficient in the soil. However, economy favors the single application of a balanced mixture that satisfies all nutritional needs of a crop. Many commercial fertilizers therefore contain more than one of the primary fertilizer elements.

The compositions of fertilizer mixtures, in terms of the primary fertilizer elements, are identified by an N-P-K code: N denotes elemental nitrogen; P denotes the anhydride of phosphoric acid (P₂O₅); K denotes the oxide of potassium (K₂O). All are expressed numerically in percentage composition, or units of 20 lb each per short ton (10 kg per metric ton) of finished fertilizer as packaged. Formula 8-32-16 thus contains a mixture aggregating 8 wt % N in some form of nitrogen compounds, 32 wt % P₂O₅ in some form of phosphates, and 16 wt % K₂O in some form of potassium compounds, to give a product with a total of 56 fertilizer units. The commercial N-P-K formulas are generally in whole numbers. None of the N-P-K formulas totals 100% plant nutrients because the formulas indicate only the nutrient portions of the primary-element compounds and do not account for any other materials present.

Aqueous solutions of urea, ammonia, and ammonium nitrate (UAN solutions) are used directly by the farmers as well as in the preparation of granular N-P-K products by mixing with other materials. UAN solutions are also spread directly by field application or used to prepare complete N-P-K fertilizer solutions or suspensions. Suspension fertilizers consist of aqueous slurries of fine crystals in saturated solutions that are stabilized by small amounts of gelling materials, such as attapulgite clay. Suspensions can be maintained in uniform composition during spreading on the fields, and give better dispersion than granular material. Fertilizing

Organic fertilizers are organic materials of vegetable and animal origin which contain certain macro, secondary, or micro nutrients that can be utilized by plants after application to agricultural soils. The primary nutrient sources of vegetable origin are crop residues, green manures, oilseed cakes, seaweeds, and miscellaneous food processing and distillery wastes. Also included in this category is biologically fixed nitrogen from legumes in association with root-nodulating bacteria of the genus Rhizobium. Animal sources include animal manures and urine, sewage sludge, septage, latrine wastes, and to a lesser extent materials such as blood meal, bone meal, and fish scraps. Often organic fertilizers are of mixed animal and vegetable origin, such as most farmyard manures, rural and urban composts, and sewage effluents and sludges. Nitrogen fixation Soil microbiology