Phosphorus forms in water column and soil

The speciation of phosphate ions in solution is dependent on pH and is characterized by three equilibrium constants  

FIGURE 9.6 Relative distribution of phosphate species as a function of pH. 

Phosphine is not stable under aerobic environments. The aerobic soil layer can function as the sink for PH3 through abiotic oxidation PHj + 2O2 = H3PO4. Trace amounts of PHj have been detected in wetlands, paddy fields, and sewage treatment systems (Devai et al., 1988 Devai and DeLaune, 1995 Morton and Edwards, 2005). 

Phosphorus solubility is also directly affected by the changes in redox potential (Patrick, 1964). In well-drained mineral soils, some of the inorganic P is bound to oxidized forms of iron such as iron oxyhydroxides. Ferric phosphate is a common phosphate compound in these systems. Similarly, in oxidized portions of mineral wetland soils, some of the inorganic phosphorus is also present as ferric phosphate. The mineral form of FeP04 is called strengite. 

We know that under anaerobic soil conditions, oxidized forms of Fe function as electron acceptor and are reduced to ferrous iron. 

---