Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: June 13, 2006
Publication Date: N/A
Interpretive Summary: At soil pH values, both organic and inorganic phosphate ions are anionic. At the molecular level, they each have a strong net negative charge. The most abundant mineral in soils is iron and soluble iron can be ferric (=+3 charge) or ferrous (=+2 charge). Although it is well known that inorganic iron and phosphate form an insoluble salt, the same molecular forces are also attractive in soils with phytate. Organic phosphates increasingly appear to be an important, but neglected sink and source of phosphate in the environment. A major organic phosphate molecule is myo-inositol hex kis phosphate or more commonly called phytate. Phytic Acid has six phosphate groups per molecule and twelve acidic protons. The acidic protons are removed from the molecule over a pH range of 1.5 to 9.5 causing the charge associated with the molecule to be highly dependent upon and sensitive to small changes in the pH of the system. The combination of the large number of phosphate groups per molecule and the pH dependent charge variability make phytic acid a potential sink for phosphate and alternatively a source depending on the pH and other chemical species present in the terrestrial system. Association constants were made at mole ratios of one phytate molecule per one soluble iron molecule. Ferrous phytate forms more readily than, and is more soluble than, its ferric phytate counterpart.
Understanding the fate and transport of organic forms of phosphate requires in the case of myo-inositol hex kis phosphate (phytate) knowledge charge speciation as a function of pH and affinity of mineral cations such as soluble iron for phytate. Twelve acidity constants exist for phytic acid because each of the six phosphate groups on phytate can have a unit anionic charge of one, two, or three. The high anionic charge density and multiplicity of phytate species at normal soil pH values highlights the potential affinity of the anionic species for cationic species. Ferric and ferrous ions, because they are often the most abundant soil cations present, are also the most likely to form phytate complexes. Affinity for ferrous ions is stronger and the complex more soluble than the ferric acid counterpart. The speciation of phytate as metal complexes is an essential component in explaining any soil phosphorous dynamics involving microbes and physical or physical chemical systems.