By: Lowell Busman, John Lamb, Gyles Randall, George Rehm, and Michael Schmitt Presented by: Julie Baur April 5, 2001

Introduction n Phosphorus (P) is classified as a macronutrient because of the large amounts of P required by plants n One of three nutrients added to soils by fertilizers n Main role is transferring energy n Adequate amounts stimulate early plant growth and faster maturing time

Introduction (2) n Too much P in soils is a threat to water quality n When lakes and rivers have too much P, algae growth is in excess n Too much algae leads to a decrease in available dissolved oxygen because of algae decay, this can lead to a decline in the fish population

The Phosphorus Cycle

The Phosphorus Cycle (2) n P can be found in soils, minerals, living organisms, and water n P is not found in elemental form--is extremely reactive and will combine with oxygen when exposed to air n In soils, mainly exists as PO 4 -3 n In water, exists as H 2 PO 4 - (acidic) or HPO 4 -2 (basic)

The Phosphorus Cycle (3) n Phosphate compounds are not very soluble in water n Soil water and surface water usually contain about 10 ppb or more of dissolved P

Forms of Phosphorus in Soils P in soils exists in three “pools” n Solution P n Active P n Fixed P

Forms of Phosphorus in Soils (2) Solution P Pool: n Very small, usually contains only a fraction of a pound of P per acre n Only pool from which plants will take up P n Only pool with measured mobility n Needs to be continuously replenished

Forms of Phosphorus in Soils (3) Active P Pool: n An acre of land may contain several pounds to a few hundred pounds of P in the active P pool n Consists of P in the solid phase n Is what replenishes the solution P pool

Forms of Phosphorus in Soils (4) n The amount of P absorbed by plants increases as the amount of P in soils increases

Forms of Phosphorus in Soils (5) Fixed P Pool: n Contains inorganic P compounds that are very insoluble and organic P compounds that are resistant to mineralization n Remains in soils for years without becoming available to plants

Forms of Phosphorus in Soils (6) Fixed P Pool: n Little impact on fertility of soils n Some slow conversion between fixed P pool and active P pool

Fate of Phosphorus Added to Soils n P in fertilizers and manure is initially soluble and available n After initial contact, reactions begin occurring that make P less soluble and less available n Rates of reactions depend on soil conditions such as pH, moisture, temperature, and present minerals

Fate of Phosphorus Added to Soils (2) Moisture n Begins to dissolve the P particle n Then moves away from the fertilizer particle n Movement is slow, but is increased by rainfall or irrigation n Gradually react with minerals and becomes fixed

Fate of Phosphorus Added to Soils (3) pH n Maintaining a soil pH between 6 and 7 results in the most efficient use of P

Fate of Phosphorus Added to Soils (4) n Fine- textured soils can hold hundreds of pounds of P per acre n Coarse-textured soils can hold much less n Loading soils with P will generally not hurt the crops, but will usually result in an increased P concentration in nearby bodies of water

Soil Phosphorus and Water Quality n P is considered unique since it not toxic itself, but can have detrimental effects on water quality n So, there is a lot of concern about transportation of P from soils into nearby water bodies

Conclusion n P is an essential element for plants n The amounts of P added to soils via fertilization needs to be monitored n Even though excess P is not hazardous to plants, it is hazardous to marine life