In flooded soils, anaerobic conditions prevail, which can significantly affect the chemical behavior of nutrients. Under such conditions, the solubility of certain micronutrients, particularly iron and manganese, tends to increase. The reduced oxygen levels facilitate the conversion of iron from its insoluble ferric form (Fe³⁺) to its more soluble ferrous form (Fe²⁺). Similarly, manganese can undergo reductions that increase its solubility as well. This enhanced solubility allows these nutrients to be more readily available for plant uptake, which can be advantageous in flooded conditions where other nutrients might be less accessible.
The other nutrient pairs listed do not experience a similar increase in solubility under flooded conditions. Copper and zinc might not be as significantly affected by reduced oxygen levels. Calcium and potassium are primarily in soluble form already, and their availability is less impacted by flooding. Nitrogen and phosphorus may be affected by the flooding conditions, but their solubility changes are not as pronounced as that of iron and manganese in anaerobic soil conditions. Thus, the primary nutrients that see increased solubility in flooded soils are iron and manganese.