Soils clay fraction

Tiller, K. G., J. Gerth, and G. Brummer. 1984. The relative affinities of Cd, Ni and Zn for different soil clay fractions and goethite. Geoderma 34 17-35. 

Relative Degree of Soil Development Prominent Minerals in Soil Clay Fraction... 

Roberts DR, Scheideggee AM and Sparks DL (1999) Kinetics of mixed Ni-Al precipitate formation on a soil clay fraction. Environ Sci Technol 33 3749-3754. 

Jozefaciuk, G. and Sokolowska, Z. (2003). The effect of removal of organic matter, iron oxides and aluminium oxides on the micropore characteristics of the soil clay fraction. Pol. J. Soil Set 36, 111-118. 

Raychev, T., Jozefaciuk, G and Sokolowska, Z. (2003). Effect of organic matter removal on fractal dimension of soil clay fraction. Bulg. J. Ecol. ScL, 11, 73-74. 

FIGURE 7.12 X-ray powder diffraction patterns of a soil clay fraction untreated (top) and treated (bottom) with dithionite-citrate-bicarbonate attack, which removes all secondary Fe oxides Cn = a-AljOj (corundum) Gt = goethite Hm = hematite PS = phyllosilicates and Qz = quartz. Hematite identification is uncertain because of strong overlap with phyllosili-cate and goethite peaks, hence the question marks. Samples contained a-Al203 as an internal standard. (Reprinted from Schulze, D. G., Clays Clay Miner. 34, 6, 681-685, 1986. With permission from the Clay Minerals Society.)... 

Clay is the most important component of soil liners because the clay fraction of the soil ensures low hydraulic conductivity. In the United States, however, there is some ambiguity in defining the term clay because two soil classification systems are widely used. One system, published by the American Society for Testing and Materials (ASTM), is used predominantly by civil engineers.18 The other, the U.S. Department of Agriculture s (USDA s) soil classification system, is used primarily by soil scientists, agronomists, and soil physicists.19... 

Crust formation thus is attributed to the disaggregation of the uppermost soil layer, initiated by the mechanical impact of the rain drops, and the subsequent dispersion of the clay fraction facilitated by the high ESR of the soil and by the low electrolyte content of rain water (Shainberg, 1990). Upon drying, the dispersed clay is responsible for the formation of the hard crust and decreases in infiltration rate. 

Cavallaro and McBride (1984a) observed that the removal of Fe oxides from two clay soils reduced Zn adsorption. Shuman (1976) reported that the removal of Fe oxides resulted in an increase or decrease in Zn adsorption, but later in another similar study (1988) he found that the removal of either amorphous or crystalline Fe oxides increased Zn adsorption capacity and decreased Zn-bonding energy. The author explained that adsorption sites on the Fe oxide coatings were not as numerous as those released when the coatings were removed. Elliott et al. (1986) observed that DCB extraction of Fe oxides from two subsoils of the Atlantic Coastal Plain increased heavy metal adsorption. Wu et al. (1999) found that Cu adsorption on the fine clay fraction increased after dithionite treatment with possible exposure of much more high-affinity sites for Cu on the fine clay. 

Methods and Results At 15 sites along a 1000 m transect across the Talbot deposit, soil profiles were sampled in 10 cm depth intervals, up to 50 cm. The <250 pm (silt + clay) and <2pm (clay) fractions of the soil samples were analysed using an aqua regia digestion and ICP-MS measurement of 53 elements. 

Most elements of interest in the Talbot 250 pm and <2pm separates are well above analytical detection limits, except PGEs. Clay separates have, on average, 50% higher concentrations of most elements than silt + clay fraction of the soils. 

The clay fraction, which has long been considered as a very important and chemically active component of most solid surfaces (i.e., soil, sediment, and suspended matter) has both textural and mineral definitions [22]. In its textural definition, clay generally is the mineral fraction of the solids which is smaller than about 0.002 mm in diameter. The small size of clay particles imparts a large surface area for a given mass of material. This large surface area of the clay textural fraction in the solids defines its importance in processes involving interfacial phenomena such as sorption/desorption or surface catalysis [ 17,23]. In its mineral definition, clay is composed of secondary minerals such as layered silicates with various oxides. Layer silicates are perhaps the most important component of the clay mineral fraction. Figure 2 shows structural examples of the common clay solid phase minerals. 

Many submerged soils are developed in recent in alluvium and are often young or only weakly weathered (Section 1.3). The overall composition of the clay fraction is therefore often close to that of the parent sediment. Hence the following generalizations can be made for rice soils in the humid tropical lowlands (Kyuma, 1978 Binkman, 1985)... 

In neutral soils with smectite or vermiculite in the clay fractions, the changes in redox and bases status following soil flooding may cause synthesis of materials similar to smectite with Fe + in the octahedral sheet. Other cations, e.g. Zn +, may also become entrapped. 

Brinkman R. 1985. Mineralogy and surface properties of the clay fraction affecting soil behaviour and management. In Soil Physics and Rice. Manila International Rice Research Institute, 161-182. 

Overland Runoff The fraction of rainfall or irrigation water that flows over a land surface from higher to lower elevations, known as overland runoff, is an additional pathway for contaminant transport. Runoff occurs when the amount of rain or irrigation water is greater than the soil infiltration capacity. The formation of a crust on the soil surface is a major contributor to runoff formation in arid and semiarid zones, because it decreases the infiltration capacity. The soil crust is a thin layer (0-3 mm) with a high density, fine porosity, and low hydraulic conductivity compared to the underlying soil. This skin forms as a result of falling raindrops or sodification of soil clays. 

Phenolic compounds have also been oxidatively polymerized to humic substances by clay minerals (29) and by the mineral fraction of a latasol (66). After a 10-day equilibration period, montmoril-lonite and illite clay minerals yielded 44 to 47% of the total added phenolic acids as humic substances whereas quartz gave only 9%. Samples of a latasol yielded over 63% of the total amount, from mixtures in varied proportion, of mono-, di- and trihydroxy phenolic compounds as humic substances (66). Extractions of the reaction products yielded humic, fulvic, and humin fractions that resembled soil natural fractions in color, in acid-base solubility, and in infrared absorption spectra. Wang and co-workers (67) further showed that the catalytic polymerization of catechol to humic substances was, enhanced by the presence of A1 oxide and increased with pH in the 5.0 to 7.0 range. Thus the normally very reactive products of Itgnin degradation can be linked into very stable humic acid polymers which will maintain a pool of potentially reactive phytotoxins in the soil. 

Fig. 12.30 Relationship between the fraction of oxidic Fe dissolved by dithionite/citrate/bicarbonate in 30 min and the proportion of Fe in hematite in hematitic/goethitic clay fractions of some Spanish soils (Barron Torrent, 1987, with permission).

Kaiser, K. Zech, W. (1998) Soil dissolved organic matter sorption as influenced by organic and sesquioxide coatings and sorbed sulfate. Soil Sci. Soc. Am. J. 62 129-136 Kaiser, K. Zech, W. (1999) Release of natural organic matter sorbed to oxides and a subsoil. Soil Sci. Soc. Am. J. 63 1157-1166 Kaiser, K. Zech,W. (2000) Dissolved organic matter sorption by mineral constituents of subsoil clay fractions. J. Plant Nutr. Soil Sci. 163 531-535... 

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