Roots can excrete organic acids, and either H+ or HCO3 minus to balance charge associated with ion uptake
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.
The resulting H+ fluxes depend upon a range of internal, regulatory processes and external constraints: cell elongation, mineral nutrition, respiration and exudation, redox activity, environmental stresses such as nutrient deficiencies and metal toxicities. Those fluxes vary considerably, both in direction and intensity, according to the physiological status of the plant and the location along the root.
Root exudation of high concentrations of organic acid anions as a result of P deficiency does lower rhizosphere pH, making P and micronutrients such as Mn, Fe and Zn to be more available in calcareous soils.
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