Practice exam #3 - QUESTION 3

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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.

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.

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.

The solubility of phosphorus is very low at extreme pHs (on either side) it is most available around 6 or 7. Only the ionic form of phosphate is available to plants.

First of all, we need to know that the content of p is originally low in the soil (< 0.01%). Although we increased phosphate content by fertilizing, the nutrient would quickly became unavailable due to phosphate fixation.