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United States Department of Agriculture

Agricultural Research Service

Title: Phosphorus Availability

Author
item Sharpley, Andrew

Submitted to: Handbook of Soil Science
Publication Type: Book / Chapter
Publication Acceptance Date: October 2, 1997
Publication Date: N/A

Interpretive Summary: Phosphorus is an essential nutrient for plant growth. Several complex and interrelated processes determine how much phosphorus is present in soil and whether it is in a form which can be taken up by plants. Most soils have inadequate natural levels to provide enough of this nutrient for crop production. This is because the natural processes of phosphorus supply in soils by the weathering of soil minerals and breakdown of organic matter and plant residues containing phosphorus, occur too slowly to supply crop needs. To meet these needs, phosphorus applications to soil as fertilizer or animal manure, can be estimated from chemical tests and field crop trials. After phosphorus is applied to soil, however, it reacts with various components in the soil and slowly becomes less available to crops. Knowledge of these processes allows us to manage soils in a way that we can maximize the amounts of phosphorus available for crop use.

Technical Abstract: Phosphorus (P) is an essential nutrient for plant growth. The low concentration (100 - 3000 mg P/kg) and solubility (<0.01 mg P/L) of P in soils, however, make it a critical nutrient limiting plant growth. The components, forms, availability, and cycling of P in soil are determined by complex and interrelated soil, chemical, and biological processes. Soil P exists in inorganic and organic forms. Inorganic P forms are dominated by hydrous sesquioxides, amorphous, and crystalline Al and Fe compounds in acidic, noncalcareous soils and by Ca compounds in alkaline, calcareous soils. Organic P forms include relatively labile phospholipids and fulvic acids; more resistant forms are comprised of inositols and humic acids. In natural ecosystems, P availability in soil is controlled by the sorption, desorption, and precipitation of P released during weathering and dissolution of rocks and minerals of low solubility. Soil P availability is generally inadequate for crop needs in production agriculture, thus, P is added as fertilizers or animal manures to build-up or maintain soil P availability at predetermined optimum levels. This paper describes these processes occurring in the soil, how they are affected by agricultural management, and how we attempt to optimize soil P availability for crop production.

Last Modified: 8/19/2014
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