Which factor plays a role in forming insoluble phosphorus in acidic soils aside from aluminum?

In acidic soils, the availability and solubility of phosphorus can be significantly affected by various elements and compounds present in the soil. Iron plays a crucial role in forming insoluble phosphorus compounds, primarily through the precipitation of phosphorus as iron phosphates.

When soils are acidic, the solubility of iron increases, promoting its reaction with phosphate ions. The formation of iron phosphate precipitates renders phosphorus unavailable for plant uptake since these compounds are not soluble in the soil solution. This interaction is especially significant in the context of nutrient management and can impact crop yield since phosphorus is essential for plant growth.

In contrast, calcium typically plays a more prominent role in alkaline soils, where it is involved in the formation of calcium phosphate, which can enhance phosphorus availability. Organic matter can affect phosphorus dynamics, but it does not directly form insoluble phosphorus compounds in the same way that iron does in acidic conditions. Therefore, iron is the primary factor among the choices provided that contributes to the formation of insoluble phosphorus in acidic soils.