Any organic or inorganic material of natural or synthetic origin (other than liming materials) that is added to a soil to supply one or more plant nutrients essential to the growth of plants. These may include:

• acid-forming - Fertilizer that, after application to and reaction with soil, increases residual acidity and decreases soil pH.
• blended - A mechanical mixture of different fertilizer materials.
• bulk-blended - A physical mixture of dry granular fertilizer materials to produce specific fertilizer ratios and grades. Individual granules in the bulk blended fertilizer do not have the same ratio and content of plant food as does the mixture as a whole.
• complete - A chemical compound or a blend of compounds that contains significant quantities of all three primary nutrients, N, P, and K. It may contain other plant nutrients.
• compound - A fertilizer formulated with two or more plant nutrients.
• controlled-release - A fertilizer term used interchangeably with delayed release, slow release, controlled availability, slow acting, and metered release to designate a controlled dissolution of fertilizer at a lower rate than conventional water-soluble fertilizers. Controlled-release properties may result from coatings on water-soluble fertilizers or from low dissolution and/or mineralization rates of fertilizer materials in soil.
• granular - Fertilizer particles sized between an upper and lower limit or between two screen sizes, usually within the range of 1 to 4 mm and often more closely sized. The desired size may be obtained by agglomerating smaller particles, crushing and screening larger particles, controlling size in crystallization processes, or prilling.
• injected - Placement of fertilizer into the soil either through use of pressure or nonpressure systems.
• inorganic - A fertilizer material in which carbon is not an essential component of its basic chemical structure.
• liquid - Fertilizer wholly or partially in solution that can be handled as a liquid, including clear liquids and liquids containing solids in suspension.
• mixed - Two or more fertilizer materials blended or granulated together into individual mixes. The term includes dry mix powders, granulated, clear liquid, suspension, and slurry mixtures.
• organic - A material containing carbon and one or more plant nutrients in addition to hydrogen and/or oxygen.
• pop-up - Fertilizer placed in small amounts in direct contact with the seed.
• salt index - The ratio of the decrease in osmotic potential of a solution containing a fertilizer compound or mixture to that produced by the same weight of NaNO3 x 100.
• sidedressed - A fertilizer application usually banded to the side of crop rows after plant emergence.
• slow-release - See fertilizer, controlled-release.
• starter - A fertilizer applied in relatively small amounts with or near the seed usually during planting for the purpose of accelerating early growth of the crop plants.
• suspension - A fluid fertilizer containing dissolved and undissolved plant nutrients. The undissolved plant nutrients are kept in suspension with a suspending agent, usually a swelling type clay. The suspension must be flowable enough to be mixed, pumped, agitated, and applied to the soil in a homogeneous mixture.
• top-dressed - A non-incorporated surface application of fertilizer to a soil after the crop has been established.