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Phosphorus in Fertilizer

By Richard Hoyt ; Updated September 21, 2017
Lettuce needs phosphorus.
Image by Flickr.com, courtesy of thebittenword.com

The three numbers used to describe soil amendments or nutrients in fertilizer are N, P, K, indicating nitrogen, phosphorus and potassium respectively. Nitrogen encourages the growth of stems and leaves. Potassium promotes flowering and fruiting. Phosphorus is believed to be essential to plant growth, but its use is controversial among some environmentalists.

Benefits to Plants

Phosphorus is found in every living plant cell. It helps in photosynthesis, the transformation of starches and sugars into useful nutrients and moving nutrients within a plant. It is also thought to aid in the transfer of genetic properties from one generation of plants to the next. Plants need phosphorus during periods of rapid growth, especially annuals that grow and die in one year. Plants that grow in cold weather and lettuces and other plants with shallow roots need extra phosphorus. Legumes have high phosphorus requirements. Trees, shrubs and vines, especially those grown in warm climates, need less phosphorus.

Inorganic Sources

Most phosphorus used in fertilizer comes from rock deposits formed from the fossil remains of ancient marine life. China, Brazil, India, Russia and the United States have the most phosphorus resources. Electric furnaces process the pure, expensive rock phosphate used by the chemical and food industries. Acids are used to extract gypsum and the phosphorus used in dry fertilizers. Neither process affects the availability of phosphorus to plants. Inorganic phosphorus comes in the form of phosphates, the form needed by growing plants.

Organic Sources

Compost, animal manures and sewage are all sources of phosphorus. This phosphorus is as useful to plants as that obtained from mineral sources. About 45 percent to 70 percent of phosphorus in manure is inorganic. Organic phosphorus decomposes easily in the soil. The temperature of the soil, its pH or acidity and its moisture all affect the rate of decomposition. About 60 percent to 80 percent of the total phosphorus in animal manure is available to crops in the first year. Organic sources such as bone meal provide lesser amounts of phosphorus available to plants.


Water-soluble phosphorus is the percentage of phosphate, or phosphoric acid, in a fertilizer that will dissolve in water. The phosphate that will not dissolve in water is placed in a solution of ammonium citrate. The percentage that then dissolves is said to be citrate-soluble phosphorus. This is a percentage of the total phosphate in the fertilizer. The sum of phosphate that will dissolve in both water and ammonium citrate is the percentage available to plants. This is the amount of phosphorus listed on a fertilizer label. The amount that will dissolve in water is usually greater than that which will dissolve in ammonium citrate.

Dry vs Wet

Phosphorus comes in both dry and liquid forms. Soil already contains water so the phosphorus in liquid fertilizer is no more available to plants than phosphorus in dry fertilizer.

Orthophosphate vs Polyphosphate

Phosphorus in fertilizers ordinarily comes in the orthophosphate form that the International Union of Pure and Applied Chemistry calls phosphoric acid. If water is removed from phosphoric acid, it becomes a polyphosphate, the form of phosphorus contained in liquid fertilizers sold as ammonium polyphosphate. This form of fertilizer makes possible different blends and is easier for dealers to handle. Once in the soil, water and enzymes convert it to phosphoric acid that plants can use.

Environmental Concerns

Phosphorus carried by rain water and other runoff constitutes a possible environmental hazard. The state of Maine prohibits the sale of fertilizers containing phosphorus on the grounds that phosphorus in runoff feeds green algae in lakes. Fertilizer with phosphorus may be used in Maine only if a soil test attests that it is necessary, or when it is used to establish new lawn or turf or for reseeding or over-seeding an existing lawn or turf. The state of Maine has posted a list of 26 peer-reviewed scholarly articles backing its decision to control the use of phosphorus as fertilizers.