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Ferrous is the is the ionized form of Iron with charge of 2+ and this is the form primarily needed by plant life.
Another and more present form in oxidizing enviroment is Ferric Iron ion of 3+ charge which is not very soluble in normal soils pH states.
When flooded for more than a day soils often use all oxygen in soil water and their redox potentials sunk enough to reduce Ferric(3+) iron to Ferrous(2+) Iron.