Which substance forms insoluble phosphorus specifically in acidic soils?

The formation of insoluble phosphorus in acidic soils primarily involves iron precipitates. In such soils, where the pH is low, iron (Fe) can react with dissolved phosphorus (P) to form insoluble compounds such as ferric phosphate (FePO4). This reaction significantly reduces the availability of phosphorus for plant uptake, as the formed compound does not dissolve in the acidic environment.

In acidic soils, the high concentration of iron ions facilitates this precipitation process. Consequently, iron precipitates effectively tie up phosphorus, rendering it unavailable to plants, which can lead to deficiencies despite potentially adequate total soil phosphorus levels.

Other options such as calcium precipitates or organic matter do not typically play the same role in acidic conditions. Calcium tends to mobilize in basic (alkaline) soils and is less effective in acidic ones for phosphorus binding. Organic matter can bind phosphorus but generally does not precipitate it in a manner that makes it insoluble under acidic conditions. Reactive P is not a substance that precipitates; rather, it refers to forms of phosphorus that are readily available for plant use.