Practice exam #3 - QUESTION 3

Phosphate ions in soil solution / plant available phosphate (H2Po4- or HPo42-). Plant available phosphate can come from the weathering of primary or secondary minerals (apatite, or Fe, Al, Ca phosphates), or can come from organic phosphate. Most phosphate in any of these scenarios has very low solubility and therefore does not enter the soil solution as readily. Weathering of these minerals can also take a very long time in order to realease those phosphate ions. Phosphate in solution is also heavily affected by a soils pH.