Which ion is the primary phosphorus uptake form in acidic soils?

The primary phosphorus uptake form in acidic soils is the H2PO4- ion. In acidic conditions, the pH level is typically lower than 7, which affects the speciation of phosphorus in the soil. At these lower pH levels, phosphorus tends to exist mainly as the dihydrogen phosphate ion (H2PO4-). This ion is more soluble and readily available for plant uptake compared to other forms of phosphorus.

In contrast, the other options do not represent the form of phosphorus most available in acidic soils. The PO43- ion, or phosphate ion, is more dominant in neutral to alkaline conditions, where pH is above 7, making it less available in acidic environments. The HPO42- ion, or hydrogen phosphate, is also better suited for neutral to slightly alkaline soils. The term Pi typically refers to inorganic phosphorus in general, but does not specify the form that is most accessible for plants in acidic conditions.

Therefore, H2PO4- is the form that aligns with the chemical behavior of phosphorus in acidic soils, making it the primary form that plants utilize for their phosphorus needs.