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 differ ent growth media, siliceous or calcareous sand. Different nutrient sol utions were supplied in separate experiments, but in all cases, nitrog en was in the form of nitrate (NO3-), and iron chelates were present i n solution. In siliceous sand the pH generally increased, but acidific ation appeared with low NO3- nutrition. Roots released H+ and HCO3- si multaneously, and these ions partially reacted to form H2CO3 The pH va riations depended on the balance of the released ions and on the low b uffer 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 san d without plants. Even though HCO3- was released in great quantities b y plants, the pH of the solutions did not become alkaline because of t he 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 p oor physicochemical conditions, HC was released from the corn roots, a nd this H+ efflux was correlated to the total alkalinity of the soluti on.