Phosphorus Sorption by Soils

Phosphorus sorption refers to the abiotic retention of inorganic phosphorus in soils. In the following text, we will review few concepts and terminology related to inorganic phosphorus retention in soils. For details on the terminology, the reader is referred to basic soil chemistry textbooks (e.g., McBride, 1994 Bohn et al., 1985). 

Desorption It refers to the release of adsorbed inorganic phosphorus from the mineral surfaces into soil pore water. Depletion of phosphorus from soil pore water results in the release of phosphorus from mineral surfaces until the new equilibrium is reached. The balance between phosphorus adsorption and desorption maintains the equilibrium between solid phases and phosphorus in soil pore water. This phenomenon is defined as phosphate buffering analogous to pH buffering. 

FIGURE 9.16 Schematic showing the partitioning of phosphorus in adsorbed and solution phases. 

FIGURE 9.17 Chemical reactions showing anion exchange, reactions with hydrous oxides or ligand exchange, fixation by silicate clays, and precipitation. 

Following is an example of precipitation reactions under alkaline soil conditions  

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Adsorption and precipitation are the retention mechanisms for inorganic phosphorus. Adsorption initially happens very quickly, and then the rates decrease. Inorganic phosphorus sorption by soils is described by several models Langmuir, Freundlich, and linear... 

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