In very acidic soils, which nutrient tends to become less available?

• A. Nitrogen
• B. Potassium
• C. Phosphorus
• D. Magnesium

Answer: (C)

Phosphorus availability drops in very acidic soils because phosphate ions readily react with iron and aluminum oxides present in acidic conditions, forming insoluble iron phosphate and aluminum phosphate compounds. These fixed, insoluble forms aren’t accessible to plant roots, so even if phosphorus is present, it can’t be taken up effectively. Phosphorus is most available around neutral pH (about 6–7), and its availability declines as soil becomes more acidic due to this fixation process. Nitrogen and magnesium or potassium don’t show this same consistent fixation under acidity, though their behavior changes with pH and leaching, making phosphorus the nutrient that tends to become less available in very acidic soils.