At low soil pH, phosphorus tends to bind with which elements, reducing its availability?

• A. Potassium
• B. Magnesium
• C. Iron and aluminum
• D. Calcium

Answer: (C)

In acidic soils, phosphorus tends to bind with iron and aluminum in the soil minerals, forming insoluble iron phosphate and aluminum phosphate compounds. These compounds don’t dissolve readily, so the phosphorus becomes fixed and unavailable to plants even if total soil P is high. This is why low pH soils show reduced P availability. Potassium and magnesium don’t form the same insoluble phosphate complexes under acidic conditions, and calcium phosphate is more associated with higher pH (alkaline) soils, not strongly with low pH.