The influence of the growth media and mineral nutrition on corn root hydrogen / bicarbonate releases and rhizosphere pH

A study was made of the influence of substrate on the root releases of hydrogen ions (H+) and bicarbonate ions (HCO₃ ^‐) by corn (Zea mays, cv.Dea) grown between the 5/6 leaf and the 9/10 leaf stage in two different growth media, siliceous or calcareous sand. Different nutrient solutions were supplied in separate experiments, but in all cases, nitrogen was in the form of nitrate (NOg"), and iron chelates were present in solution.

In siliceous sand the pH generally increased, but acidification appeared with low NO₃ ^‐ nutrition. Roots released H⁺ and HCO₃ ^‐ simultaneously, and these ions partially reacted to form H₂CO₃. The pH variations depended on the balance of the released ions and on the low buffer capacity in this slightly acidic pH range. The algebraic sum of the ion effluxes was approximately equal to the sum of the ion uptakes; no stoichiometric coupling between the total H⁺ effluxes and the NO₃ ^‐ or potassium (K⁺) uptakes was recorded.

In calcareous sand HCO₃ ^‐ was released by the roots, but the H⁺ seedling effluxes always acidified the solutions with regard to the reference solutions in calcareous sand without plants. Even though HCO₃ ^‐ was released in great quantities by plants, the pH of the solutions did not become alkaline because of the high buffer capacity of the solution in contact with the calcareous medium. In this environment the plants reacted to the high levels of HCO₃ ^‐ and showed symptoms of lime‐induced chlorosis. To overcome the poor physicochemical conditions, H⁺ was released from the corn roots, and this H⁺ efflux was correlated to the total alkalinity of the solution.