Which statement about iron and manganese availability as soil pH changes is correct?

• A. With higher pH iron and manganese availability increase
• B. In alkaline soils, iron and manganese availability is high
• C. In neutral pH soils, manganese is not available
• D. Iron and manganese availability increases in more acidic soils

Answer: (D)

Iron and manganese become more soluble as soils grow more acidic, so their availability to plants increases when pH is lowered. In acidic conditions, the hydrogen ions in the soil help dissolve iron and manganese compounds, keeping more of these micronutrients in a form that roots can absorb. As pH rises toward neutrality and especially toward alkalinity, iron and manganese tend to precipitate as insoluble hydroxides and oxides, removing them from the soil solution and making them less available. This is why iron nutrition often worsens in alkaline soils, leading to chlorosis, and why manganese behaves similarly with respect to solubility as pH changes. The statement that availability increases in more acidic soils captures this solubility relationship, so it is the best answer. The ideas that higher pH increases availability, that alkaline soils keep iron and manganese highly available, or that manganese is unavailable at neutral pH, do not fit the established pattern of micronutrient solubility with pH.