At high soil pH, phosphorus commonly binds with what?

• A. Iron
• B. Calcium
• C. Aluminum
• D. Magnesium

Answer: (B)

Phosphorus availability in soil is highly pH dependent. In alkaline (high pH) soils, calcium ions dominate and readily react with phosphate to form calcium phosphate minerals. These compounds are sparingly soluble, so the phosphorus becomes fixed to soil minerals and is less available to plants. This is why limed or calcareous soils often show reduced plant-available P unless more P is added. In acidic soils, phosphorus tends to bind with iron and aluminum instead, forming insoluble Fe–P and Al–P, which is the reason those forms are more limiting under low pH. Magnesium can form phosphate minerals too, but calcium is the primary binder at high pH, making calcium the best answer.