The concentration of Fe3+ in soil solution ([Fe3+]) is far below the Fe content required for plant growth. Although the soil redox potential ( \({E}_{h}\) ) favours Fe3+ reduction, the concentration of ferrous iron ([Fe2+]) is generally neglected with respect to ferric and chelated iron. Our objective was to assess the contributions of free Fe3+ and Fe2+ to plant iron uptake. Iron hydroxide dissolution kinetics and reduction of ferric iron were formalized in two root uptake models based on the diffusion and convection of both ions towards the root surface. The simulations were compared to the iron uptake of Noccaea caerulescens grown in three soils. The modelling was used to evaluate the influence of pH and \({E}_{h}\) . Even in case of a strong acidification of the rhizosphere down to pH 4, [Fe3+] is much too low to satisfy plant iron needs. [Fe2+] dramatically changes with pH and \({E}_{h}\) . Although [Fe2+] is about 3 to 6 orders of magnitude higher than [Fe3+] in aerated soils, it is far from sufficient to satisfy the plant requirements in Fe when pH > 4. For pH < 4 and \({E}_{h}\) < 0.5 V, uptake of Fe2+ can cause excessive accumulation of Fe by plants. The concentration of Fe2+ dominates that of Fe3+ in soil solutions. Both vary strongly with soil pH and \({E}_{h}\) . However, in most soil conditions, they are largely insufficient for optimal plant growth, even when the rhizosphere is strongly acidified.
