Soil P exists in various chemical forms including inorganic P (Pi) and organic P (Po). These P forms differ in their behavior and fate in soils (Hansen et al., 2004; Turner et al., 2007). Pi usually accounts for 35% to 70% of total P in soil (calculation from Harrison, 1987). Primary P minerals including apatites, strengite, and variscite are very stable

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Note added at 4 days (2012-01-24 15:59:53 GMT) Post-grading
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المقصود هو الصيغ الكيمياوية التي تحتوي على الفسفور في التربة مثل المركبات العضوية و المركبات الغير عضوية