Practice exam #3 - QUESTION 3

Phosphate ions are low in the soil solution due to relatively low levels of Total soil P! I can't really elaborate much further than that, but if I were to hazard a guess I'd reckon its because there aren't too many/abundant natural inputs of P, so without enough going in we won't have much remaining?

This is my attempt on this question.

Phosphate ions in soil solution are susceptible to fixation at every level of pH. In acidic soils, soluble compounds and hydrous oxides of metals (Fe, Al, and Mn) can transform P ions into unavailable forms. In alkaline soils, available P can be lost when taken by soluble calcium compounds. And although available P is relatively high in neutral soils, clay minerals can absorb/fix phosphate ions in the soil solution with double bonds that are difficult to break. Because of these processes, the amount of plant-available P that is left in the soil solution is very low.

Jacob and Eakin, both raise good points.