Practice exam #3 - QUESTION 3

Phosphate ions are PO43- and are a part of a family of different phosphate molecules (HPO42-, H2PO4-, H3PO4) that have different numbers of hydrogen atoms and vary at different charges. They also each predominate at different pHs. Phosphate ions, having the least number of hydrogens and the highest charge, predominate in highly basic conditions with a pH from 12-14. In contrast, the values for typical soil pHs range between 5 to 9, with soils most commonly being slightly acidic for a pH around 6-7 ish, where most plants grow optimally. At these pHs, the phosphate ion is not the dominant phosphate species and is only present as a very small proportion of the total phosphorus concentration of the soil. Additionally, PO43- is a very negatively charged anion, which means that it is very easily leached because it is strongly repelled from negatively-charged soil colloids.

1st consider the two most common forms of phosphate in soils (think about the pH range common in soils).
Although HPO₄^2- and H₂PO₄^- are anions, phosphate does not readily leach from soils. why? Consider Eakin's answer above as a starting point.