Changes of pH across the rhizosphere induced by roots

Summary Plants that absorb nitrogen as NO₃ ^– tend to raise the pH in the rhizosphere. Those absorbing nitrogen as NH₄ ⁺ or N₂ lower the pH. The change in pH near the root surface may be calculated approximately from the H⁺ or HCO₃ ^– efflux and radius of the root; and the pH buffering capacity, moisture content, initial pH and p_{CO 2} of the soil. An accurate equation, solved numerically, also takes account of root hairs, mass flow and slow acid-base reaction in the soil. The pH at the root surface will often differ from the pH a few mm away by 1–2 units.