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Soil Chemistry (continued) + ward ' s science The elemental composition of soil varies with depth below the land surface because of percolating water, chemical processes, and biological activity. The principal chemical processes are: • Hydrolysis: reaction with water to form products containing hydroxide ions • Complexation: reaction of a metal with a ligand to form a product containing both metal and ligand • Oxidation-reduction: changes in the oxidation state of an element • Ion exchange: replacement of one ion by another on a solid surface • Hydration: reaction with water to form a product containing water molecules • Flocculation-dispersion: settling-resuspension of solid particles in water; this process affects soil particle removal by erosion or by translocation downward along soil pores. The principal effect of these processes is the appearance of illuvial horizons in which compounds (for example, aluminum and iron hydrous oxides, aluminosilicates, or calcium carbon- ate) have been precipitated from solution or deposited from suspension. Minerals The minerals in soils are the products of physical, geochemi- cal, and biologically driven (pedological) weathering (Fig. 2). Soil minerals may be either amorphous or crystalline. They may be classified further, approximately, as primary or secondary minerals, depending on whether they are inherited from parent rock or are produced by chemical weathering, respectively. The bulk of the primary minerals that occur in soil are found in the silicate minerals, such as the olivines, garnets, pyroxenes, amphiboles, micas, feldspars, and quartz. The feldspars, micas, amphiboles, and pyroxenes commonly are hosts for trace elements that may be released slowly into the soil solution as weathering of these minerals continues. Chemical weathering of the silicate minerals is responsible for producing the most important secondary minerals in soil. These are found in the clay fraction, sometimes in the form of coatings on other min- erals, and include aluminum and iron hydrous oxides, carbon- ates, and aluminosilicates. Ion exchange A portion of the chemical elements in soil is in the form of cations that are not components of inorganic salts, but that can be replaced reversibly by the cations of leaching salt solu- tions. These cations are said to be readily exchangeable, and their total quantity, usually expressed in units of centimoles of positive charge [cmol(+)] per kilogram (kg) of dry soil, is termed the cation-exchange capacity (CEC) of the soil. The cation-exchange capacity of a soil usually varies directly with the amounts of clay and organic matter present and with the distribution of clay minerals. The stoichiometric exchange of the anions in soil for those in a leaching salt solution is a phenomenon limited to chloride and nitrate in the general scheme of anion reactions with soils. Under acid conditions (pH < 5), exposed hydroxyl groups at the edges of the structural sheets or on the surfaces of clay-sized particles become protonated and thereby acquire a positive charge. The degree of protonation is a sensitive function of pH, the ionic strength of the leaching solution, and the nature of the clay-sized particle. The magnitude of the anion-exchange capacity (AEC) usually varies from near 0 at pH 9 for any soil colloid to as much as 50 cmol(−)/kg (centimoles of negative charge per kg) of allophanic clay at about pH 4. Soil solution The solution in the pore space of soil acquires its chemical properties through time-varying inputs and outputs of matter and energy that are mediated by several parts of the hydrologic cycle and by processes originating in the biosphere (Fig. 3). Thus, the soil solution is a dynamic and open natural water system whose composition reflects the many reactions that can occur simultaneously between an aqueous solution and an assembly of mineral and organic solid phases. This type of complexity is not matched normally in any chemical labora- tory experiment, but nonetheless must be amenable to analysis in terms of chemical principles. An understanding of the soil solution in terms of chemical properties has proven to be es- sential to progress in the maintenance of soil fertility and the quality of runoff and drainage waters. Fig. 2: Extensive chemical weathering can remove essential minerals and elements from soil. Typically, only the most insoluble compounds remain in the soil. For example, iron oxides may persist, imparting a reddish-brown color to the soil. (Credit: Stephen Reynolds)