Ferrous Ions and Manganous Ions

Abstract

It has been seen in the previous chapter that, when organic reducing substances are in contact with soils, iron and manganese oxides of the soil can be reduced, producing ferrous and manganous ions. This reduction process is of particular significance for variable charge soils. This is because the contents of iron and manganese oxides in these soils are higher than those in constant charge soils, and therefore there is the basis for the production of large amounts of ferrous and manganous ions. On the other hand, the low pH of variable charge soils is one of the favorable factors in the reduction of iron and manganese. Hence, the possibility exists for the production of the reduced form of these two elements exists even under well-aerated conditions, provided that the soil contains organic matter. It is for these reasons that iron and manganese can actively participate in chemical reactions in variable charge soils in the form of ions, and they are of great significance in both soil genesis and plant nutrition. As a matter of fact, many reports have shown that the amount of manganous ions in strongly acid soils is so high that the toxicity of manganese may be one of the causes for the poor growth of plants. In this chapter, after the elucidation of the reduction and dissolution of iron and manganese, emphasis of discussions will be placed on the chemical equilibria of the two kinds of ions. Finally, the regimes of the two ions in variable charge soils under field conditions will be described. The solubility of iron oxides in soils is extremely low. The solubility product of Fe(OH)3 is about 10-38. Therefore, the concentration of Fe3+ ions in solution is only 10-8 M even at a pH of 4, if the concentration is determined solely by the solubility of the oxides. Similarly, the quantity of Mn4+ ions due to the dissolution of manganese oxides is also very small. On the other hand, because the solubility products of Fe(OH)2 and Mn(OH)2 are about 10-16 and 10-13, respectively, after reduction, ferrous and manganous (in this chapter the latter is simply referred to as manganese) ions can exist stably in soils.