Changes in Forms and Distribution Pattern of Soil Iron Oxides Due to Long-Term Cropping in the Northwest of Iran

Iron oxide is an important component of many natural and artificial systems and is widely found in soils, sediments and rocks under various environmental conditions. Fe oxide found in soil and sediment is a very small crystal (diameter 5 to 100 nm) [1] where the reactive surface can adsorb large amounts of inorganic matter and organic matter from solution [1]. Common sorbate is primarily cations such as phosphate, silicate, arsenate and humic compounds, as well as heavy metals.

The soil color is mainly affected by soil mineralogy. Many soil colors are caused by various iron minerals. The color development and distribution of the soil profile is caused by chemical and biological weathering, in particular redox reactions. As a major mineral in soil infertility, these elements bind to new and colorful compounds. Iron forms yellow or red secondary minerals, which are broken down into black and brown humic compounds, and manganese and sulfur form black ore deposits. These pigments produce various colored patterns in the soil. Under aerobic conditions, a uniform or gradual color change occurs, and due to environmental degradation (anaerobic) rapid color flow, complicated patterns and shade of color are generated.

The Hawthorn Plato is known for its "red clay". The unique red color comes from long-term weathering process leaving the iron oxide. Mixtures of red soil silicon, aluminum and other elements (including iron oxide) are called saprolite and easily corrode when trees and other vegetation are removed. Northern Piedmont has steep terrain and thin soil, but in the southern area there is a nutrient rich soil suitable for crop cultivation.

In this soil, rainfall is large and under wet condition, wet climate and dry climate alternate. Due to the large amount of rainwater leaching, most of the silica is washed away, leaving a mixture of hydrated oxides of aluminum and iron. The soil consists of a large amount of iron oxide in the form of small clumps which turn black when exposed to rain. Nitrogen, phosphoric acid, potash, lime, magnesia are usually poor in laterite soil. Therefore, its fertility is low, but it is easy to adapt to various crops, especially rice, rasi and sugarcane. Laterite soil occurs in the Deccan Plateau, the Western Ghats Mountains, the East Gatts Mountain Range, the Vindyas, the Raj Mahal Hills.