Inorganic forms of phosphorus in soils and sediments

Inorganic P exists in soils and sediments both as P containing minerals and in various amorphous forms. To understand the fate of P under various environmental conditions, it is necessary to characterize pools of P in soils and sediments. This is generally accomplished with sequential extraction schemes, which extract different phases or pools of P rather than specific minerals. Common inorganic P pools identified in sequential fractionation procedures include (1) loosely bound (also referred to as labile or exchangeable P), (2) fractions associated with Al, Fe, and Mn oxides and hydroxides, (3) the Ca- and Mg-bound fraction, and (4) minerals and organic material resistant to previous extractants. The procedures often incorporate estimates of organic as well as inorganic P. Generally the objectives of P fractionation are to (1) provide insight into the fate and transformation of P added to soils as fertilizer or manure, (2) estimate the availability of P to plants for agronomic purposes, (3) estimate the potential for P movement from (erosion) and through (leaching) soils, and (4) provide information regarding the interaction between P in the sediments and the overlying water in the case of aquatic systems. Examples of P fractionation data applied to Florida's subtropical soils, wetlands, and aquatic systems are presented to illustrate the fate and transport of P in these ecosystems. It was found that a large amount of P (up to 80% of total P) has the potential to leave heavily manure-impacted upland regions although less than 10% of the total P was likely to leave a low manure-impacted pasture soil. Most of the P in the wetland soils and sediments were associated with inorganic Fe and Al, and are assumed to be stable with little possibility of being desorbed except under extended water-logged conditions. Concentrations of readily available P in sediments depended on sediment type. Littoral and peat sediments, rather than mud or sand/rock sediments, are more likely contributors to internal P cycling of lakes. In addition to P fractionation schemes designed to extract somewhat specific forms of soil P, several extractants have been used routinely to extract a representation of "available P" for soil testing purposes. These extractants are generally designed to provide a measure of P (and in some cases, other elements) that can be correlated with crop response. Recently, traditional agronomic soil tests have been proposed for environmental purposes as well. From an environmental standpoint, results from these soil test procedures may be used to predict parameters such as equilibrium P concentration (EPC), labile P (resin-P), and algae-available P. The concept of the degree of P saturation (DPS) has recently been introduced as an environmental measure of soil P available to be released to surface and subsurface runoff. The DPS relates extractable P to the P sorbing capacity of a soil. Although the methodology has been used successfully in the Netherlands and in parts of the United States, there is a need to identify methodologies which allow simple, inexpensive measurements of DPS under various environmental conditions.