Figure 2 Root-induced changes in the rhizosphere of rice in iron toxic soil. (a) Leaf bronzing and stunted growth due to Fe toxicity. (b) Leaf bronzing and oranging due to Fe toxicity with Mg deficiency. (c) Root effects on the soil: (1) oxidation of ferrous iron by O2 released from the roots; (2) venting of soil CO2 into the root aerenchyma and associated changes in soil carbonate equilibria; (3) excess intake by the root of nutrient cations (particularly ammonium, NH4+) over anions and associated release of H+; (4) in the zone where there is no O2 release, re-reduction of ferric iron fuelled by organic substrates (represented as CH2O) diffusing across leaky root membranes. Note the protons (H+ ions) consumed or produced in these reactions will be buffered by proton-donating or -accepting groups in the soil solid. (d) Calculated pH profiles in the rhizosphere for a given flux of H+ due to the above root-induced changes at different initial soil pH values (dotted lines) (Kirk, 2004). (e) Sensitivity of pH changes at the root surface to important variables, varied individually from standard values as in Panel (d).