Iron is a fundamental element found widely in soil, primarily existing in different chemical forms based on the soil's oxygen levels. It is the fourth most common element and the second most common metal in the Earth's crust, making it a ubiquitous component of soils derived from crustal materials. Iron is also essential for plants and animals, playing vital roles in various biological processes.
In cultivated soils, iron's presence is significantly influenced by the environment, particularly the availability of oxygen. Iron primarily exists in two main oxidation states:
Key Forms of Iron in Soil
The specific form of iron found in soil largely depends on whether the environment is rich in oxygen (oxic) or deficient in oxygen (anoxic).
Ferric Iron (Fe³⁺)
- Conditions: Found predominantly in oxic environments, where oxygen is readily available.
- Characteristics: Fe³⁺ is less soluble and tends to precipitate as oxides, oxyhydroxides, and hydroxides, often contributing to the reddish or yellowish color of soils.
Ferrous Iron (Fe²⁺)
- Conditions: Primarily present in anoxic environments, where oxygen is limited or absent (e.g., waterlogged soils).
- Characteristics: Fe²⁺ is generally more soluble than Fe³⁺, making it more available in the soil solution under reducing conditions.
The reference states that in cultivated soils, iron is mostly present in the Fe³⁺ and Fe²⁺ forms under oxic and anoxic environments, respectively.
Here's a simple overview:
| Iron Form | Environment | Solubility (Generally) |
|---|---|---|
| Fe³⁺ | Oxic (Oxygen-rich) | Lower |
| Fe²⁺ | Anoxic (Low-oxygen) | Higher |
While these are the predominant forms, iron can also be associated with soil minerals (like silicates, oxides), organic matter, or present in the soil solution in dissolved forms or complexed with organic ligands. Its high abundance in the Earth's crust ensures its presence as a basic constituent of soil-forming materials.
Understanding the forms of iron in soil is crucial as it affects its mobility, bioavailability to plants, and interaction with other soil components.
