Soil iron compounds

In kaolin (clay) processing, sulfur dioxide reduces colored impurities, eg, iron compounds. In the bromine industry, sulfur dioxide is used as an antioxidant in spent brine to be reinjected underground. In agriculture, especially in California, sulfur dioxide is used to increase water penetration and the avadabiHty of soil nutrients by virtue of its abiHty to acidulate saline—alkaH soils (327). It is also usefiil for cleaning ferric and manganese oxide deposits from tile drains (328). 

One advantage of steel as an anode is the low gassing at the electrode during operation, since the predominant reaction is the corrosion of iron. Thus, the problem of resistive polarisation due to gas blocking, as may be the case with more inert materials, does not occur. Iron compounds do, of course, form but these do not appreciably affect the anode/soil resistivity. Furthermore, the introduction of metallic ions, by anode corrosion, into the adjacent high resistivity soil is beneficial in lowering the resistivity. 

Whereas any N and K that is applied to fields is immediately available to the growing plant and then tends to be washed down out of reach of the plant roots, a large proportion of the P that is applied is rendered unavailable to the plant by being bound into aluminium or iron compounds if the pH of the soil is low, and by calcium compounds if the pH of the soil is high. Although it may seem rather an expensive waste of money, at the time, that as much as 40% of the P applied in farmyard manure should be unavailable to the crop, it does stay in the soil and will become available to the crop in time. 

The soil solution will contain numerous inorganic and organic compounds derived from the solid components making up the soil. Common compounds include oxides, particularly those of silicon, aluminum, iron, and titanium in low concentrations. These compounds move down the soil profile sometimes contributing to formations such as the spodic horizon, which can contain aluminum and iron oxides along with highly decomposed carbon. 

In the United States, severe iron deficiency in crop planls occurs most frequently on the alkaline soils of the western stales and on v ery sandy soils, although some plants, especially hroad-leaved evergreens, are sometimes iron deficient on many other kinds of soils. Iron deficiency is rarely due to t total lack of iron in the soil. It is nearly always due to die low solubility of the iron that is present. For example, some soils that arc red from iron compounds may contain too little available iron for normal planl growth. The relative susceptibilities of cultivated crops to iron deficiency arc listed in Tabic I. 

Fig. 1. Hypothetical soil profile chat has all principal horizons. Not all horizons shown are present in any given profile, but every profile has some of them. Terms used in diagram Eluviation is the downward movement of soluble or suspended material in a soil from the A horizon to the B horizon by groundwater percolation. The term refers especially, but not exclusively, lo the movement of colloids, whereas the term leaching refers lo the complete removal of soluble materials. Illuviation is the accumulation of soluble or suspended material in a lower soil horizon that was transported from an upper horizon by the process of eluviation. Gleying is soil mottling, caused by partial oxidation and reduction of its constituent ferric iron compounds, due to conditions of intermittent water saturation. Process is also called gleizalion (Adapted from USD A diagram)...

In addition to their use in the functional speciation role, selective extraction methods have been used to target element species in soil, or elements bound to, or associated with, particular soil phases or compounds. Examples include the use of extractants to release, for determination, metals on exchange sites, or metals bound or associated with soil iron or manganese oxyhydroxides or with soil organic matter. Most of these extractants are, however, less specific than intended and may extract species from other phases. Such extractants, however, are commonly, and conveniently, designated by their target species, e.g. extractable metal species or carbonate-bound species, but should more strictly be regarded as examples of speciation in which the species are operationally defined, i.e. by the method used to isolate them. 

Importantly, it also occurs naturally in several oxidation states and is, therefore, redox sensitive. Methylation and hydride formation are important, and sulfur and iron compounds play an important role in the cycling of selenium. Microbiological volatilization of organic selenium, particularly dimethyl selenide, is known to be an important factor in the loss of selenium from some selenium-rich soils and waters (Frankenberger and Arshad, 2001 Oremland, 1994). Phytoplankton can also promote the production of gaseous selenium compounds in the marine environment (Amouroux et aL, 2001). 

The behavior of selenium in soils mirrors that of the pure oxides (Goldberg, 1985). In acid soils, selenium is likely to occur mainly as Se(lV) strongly adsorbed to iron oxides. Less commonly, Se(IV) may form highly insoluble iron compounds such as ferric selenite (Fe2(0H)4Se03) or iron selenide (FeSe). In alkaline, oxidized and selenium-rich soils, most of the selenium is likely to be present as Se(Vl) which is very weakly adsorbed. Furthermore, there are no common insoluble selenate minerals. Hence, selenate accumulates in soluble form particularly in arid and semi-arid areas where evaporation tends to concentrate selenium along with other soluble salts (Deverel et al., 1994). 

Fitzpatrick, R, W. (1988) Iron compounds as indicators of pedogenic processes Examples from the southern hemisphere. In Stucki J. W. et al. (Eds.) Iron in soils and clay minerals. Reidel Publ. Co. Dordi echt Holland, NATO ASI Ser. 217 351-396. 

Nielands, J. B. (1981). Microbial iron compounds. Annu. Rev. Biochem. 50, 715-731. Nielson, N. E. (1976). The effect of plants on the copper concentration in the soil solution. 

Cleaning of an object might be necessary to remove encrustations, developed during exposure to soil, such as calcite, gypsum or silicates. Porous pottery might be stained with iron compounds or deposits, resulting from the content of vessels. Difficulties occur, when the dirt is soluble and has been carried into the pores of the body. 

Complexes with some soil colloids or on iron compound surfaces are, however, taken up to a lesser extent. 

Pure natural waters are usually colourless, or azure-blue in thick layers. The blue colour intensity increases with decreasing amount of smaller suspended matter present in water. The presence of finely-divided substances causes a change from blue into green colour. Greenish colour of water is also caused by the presence of calcium salts and the near-green colour of some lakes is due to the shape of the bottom. Yellow to brown colours of surface waters are caused by humic (soil) substances and three-valent iron compounds. 

Sulphates are transported into the seas and oceans directly from the atmosphere and from surface waters. One third of sulphates in surface waters comes from the soil and rocks, two thirds from the atmosphere. As compared to the amount in surface water, sulphur is present in sea water in concentrations higher by a factor of 240. In coastal waters, which are characterized by a remarkable input of sulphur, the biological reduction occurs with a simultaneous H2S release. This gas escapes into the atmosphere, when a mud layer is exposed in the course of the tidal variation. Since the process also occurs in the depths of seas, hydrogen sulphide reacts with the iron compounds present, producing sulphides, which are deposited continuously in sea sediments. 

Carbonic acid evolved from decomposing plant residues plays a role in iron availability. In acid soils this carbon dioxide forms bicarbonates which increase the solubility of iron compounds. In alkaline—calcareous soils, however, the carbon dioxide formed acts to keep the iron in insoluble forms and thus may increase chlorosis. Brown (1961) points out that green manure crops when incorporated into moist calcareous soil often cause severe iron chlorosis to develop in deciduous fruit trees. 

Oades, J.M., 1963. The nature and distribution of iron compounds in sods. Soil Fert. 27, 69-80. 

A major reason for an acid soil is the predominance of exchangeable hydrogen ions and a lack of exchangeable metal cations. Low pH soils are undesirable for plant growth, partly because soluble aluminum and iron compounds become available and tie up phosphate ions. The addition of calcium to the soil creates a condition where through cation exchange Ca+2 exchanges for some of the H+ ions absorbed on clay colloids or micelles in the soil. In this manner, calcium becomes available for plant uptake and at the same time the soil pH is raised. 

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