organic or inorganic material containing one or more of the nutrients—mainly nitrogen, phosphorus, and potassium, and other essential elements required for plant growth. Added to the soil or other medium, fertilizers provide plant nutrients that are naturally lacking or that have been removed by harvesting or grazing, or by physical processes such as leaching or erosion. Organic fertilizers include animal and green manure, fish and bone meal, and compost (see also humus). Microorganisms in the soil decompose organic material, making its elements available for use by plants. Inorganic or artificial fertilizers (also called chemical or mineral fertilizers) are formulated in appropriate concentrations and combinations for various crops and growing conditions. The most popular inorganic fertilizers include: anhydrous ammonia, a gas that is 82% nitrogen; urea, a solid compound containing 46% nitrogen; superphosphate; and diammonium phosphate, containing 18% nitrogen and 46% phosphate. Fertilizers may be spread over the soil surface or plowed under, drilled into deep or shallow layers of the soil, applied in bands under the rows where the seeds are to be sown, drilled into the bands at the time of planting, or side-dressed between planted rows. Nitrogen fertilizer washing from farms into surface waters promotes overgrowth of aquatic vegetation, which degrades water quality and can cause eutrophication. Use of inorganic nitrogen suppresses nitrogen-fixing soil bacteria, making agriculture increasingly dependant on artificial fertilizer. See nitrogen cycle.

See publications of the U.S. Dept. of Agriculture.

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