As the new soil develops higher amounts of surface area per unit volume, minerals that were formed under very different temperature, pressure, and chemical conditions than exist on the surface of the Earth are often quite unstable. For instance, the three minerals in granite (silica, mica, and feldspar) are crystallized below the Earth's surface under much higher temperature and pressure than exist on the surface of the Earth. In addition, there is no water or molecular O2 present under the conditions of granite rock formation. Such changes in temperature, pressure, and chemistry make the minerals in granite thermodyna-mically unstable, and the high surface areas exposed by physical weathering allow chemical changes to proceed at a faster rate. Rock minerals that may have been protected for hundreds of millions of years can weather rapidly when exposed to the vastly different conditions of oxidation, reduction, oxidation-reduction shifts, hydration, dissolution, and precipitation present in a forming soil.
For instance, an example of oxidation might include the oxidation of Fe(ii) sulfide to sulfate and soluble iron, a reaction common when sulfide minerals created beneath the surface of the Earth are exposed to molecular oxygen and water:
The Fe2+ may also be oxidized to Fe3+ in the presence of oxygen and water. Common Fe3+ oxide minerals are reddish brown in color, and are often used as an indicator of well-weathered oxic soil horizons.
When soils are waterlogged and oxygen consumption exceeds the ability of atmospheric O2 to replenish molecular O2, then chemical reduction reactions predominate, with denitrification shown as a potential example:
Under these conditions, the lack of oxidized iron will result in blue-gray soil colors, sometimes referred to as 'gley' colors. Some soils have fluctuating conditions of oxidation/reduction, and the soil profile can develop distinctive zones that reflect these conditions. Soil mottling, where the horizons contain zones indicating reducing conditions adjacent to zones ofoxidized iron represented by reddish brown colors, are common in soil profiles of wetlands.
Hydration occurs when water or hydroxyl groups associate with soil minerals, forming new and different minerals. An example would be the hydration of the mineral anhydrate to form gypsum:
The sorption of water and hydroxyl molecules on mineral surfaces, including silicate and aluminum minerals, is a common series of chemical reactions during soil formation. Some soil types, notably shrinking/swelling Vertisol soils, are characterized by such reactions.
Water is not normally present during the formation of igneous rock minerals. When water is exposed to such primary minerals during soil formation, hydrolysis reactions can cause minerals to chemically weather to other forms. An example is the hydrolysis of microcline to kaolinite, with the release of soluble potassium and silica:
Precipitation is basically the opposite of dissolution, and is very common in lower soil horizons, particularly where there is not a large excess of precipitation to move minerals completely through the soil profile, or where the chemistry ofpercolating water changes in such a way as to exceed the saturation capacity of a particular mineral. An example is the formation of calcium carbonate in the lower soil horizons of a caliche layer in an Aridosol, common in desert environments:
Though abiotic chemical reactions do take place in soil and are important, the role of organisms in shaping soil formation and chemical reactions is critical. The decomposition of organic matter, for example, produces CO2. In the presence of water, CO2 forms carbonic acid, which is an aggressive chemical weathering agent:
Additionally, the organic acids that are constantly exuded into the soil by biological processes are also important in mineral weathering/dissolution. Once solubilized, the metal ions weathered out of primary minerals can be complexed by organic acids and transported through the soil profile to be deposited at depth. This interaction between organic acids and metals gives Spodosol soils their characteristic pattern of horizonation.
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