Which combination correctly matches soil pH with phosphorus binding?

• A. Low pH binds with calcium; high pH binds with iron and aluminum.
• B. Phosphorus does not bind with any elements.
• C. Low pH binds with potassium; high pH binds with magnesium.
• D. Low pH binds with iron and aluminum; high pH binds with calcium.

Answer: (D)

Phosphorus availability in soil is controlled by how phosphate ions interact with minerals depending on pH. In acidic soils (low pH), phosphate tends to bind with iron and aluminum oxides/hydroxides, forming insoluble ferric and aluminum phosphates that are not easily taken up by plants. In alkaline soils (high pH), phosphate bonds with calcium to form calcium phosphate minerals, which are also relatively insoluble. This makes phosphorus less available at both low and high ends of pH, while mid-range pH often allows greater P solubility. Therefore the correct pairing is that low pH binds with iron and aluminum, and high pH binds with calcium.