The Soil Constituents

XRF characteristics that can limit its usefulness are the surface area observed and surface contamination. In XRF, the surface area measured is small, meaning that a large number of determinations must be made in order to obtain a representative sample of the elements present. In addition, transport and storage of uncovered soil samples can lead to surface contamination that will subsequently appear as part of the soil constituents. 

The availability of inorganic electron acceptors, both in the soil constituents and added in mineral fertilizers. This affects the time course of soil reduction and hence the rate at which sufficiently reducing conditions for methanogen-esis develop. 

In the previous sections of this book, we focused on the nature of contaminants and the geochemical reactions that can occur in the subsurface environment. Chemical compounds introduced into infiltrating water or in contact with soil or rock surfaces are subject to chemically and biologically induced transformations. Other compounds are retained by the soil constituents as sorbed or bound residues. Thus, in terms of geochemical interactions and reactions among dissolved chemical species, interphase transfer occurs in the form of dissolution, precipitation, volatilization, and various forms of physicochemical retention on the solid surfaces. 

As with other pesticides, organic matter appears to be the soil constituent most important for basic pesticide sorption. Weber et al. (1969) showed that maximum sorption of several s-triazines on organic matter occurred at pH levels close to the pKa values of the compounds. The molecular structure of the pesticide and the pH of the sorbent strongly affected the degree of sorption. The pH-dependent sorption and the relationship between pH and dissocation constant with pH suggests an ion exchange mechanism (Saltzman and Yaron, 1986). 

In soils, mass transfer is usually considered to be sufficiently fast for local equilibrium to be maintained (this is essentially based on the assumption of extreme subdivision of particles). However, exceptions occur depending on the type of soil constituent one is dealing with, and, in particular, if the soil constituent is coated with anything that retards access into the solid phase (humic and/or colloidal substances). In such cases, mass transfer could play a relevant role in kinetic behavior. 

It is thus apparent that the distribution of organic compounds into soil is a complex process involving the interplay of the soil constituents, properties of the adsorbate and environmental factors relative humidity and temperature. Mechanisms can range from cation exchange to partitioning and a tme adsorptive... 

Hvorslev (1949) classified soil samples as nonrepresentative, representative, or undisturbed. This classification is made by comparing the state of the sampled soil to that as exists in situ. A nonrepresentative sample is one in which soils from different strata have been mixed, or some of the soil constituents of the sampled soil are missing. Samples that contain scrapings from the well of the borehole or samples with some of the fine particles washed out during sampling fall in this category. A representative soil sample is one in which there is no change in soil constituents but whose structure, water content, or void ratio has been altered. An xmdisturbed sample, ideally, is one that represents the in situ condition. 

The density of charge of the soil components was measured by a particle charge detector constructed by Miitek, FRG. The suspension was filled into a cylindrical testing apparatus and a large fraction of the solids was adsorbed on its wall. As a function of pH and characteristics of the soil constituents these solids are charged except at... 

Mossbauer spectroscopy has made a modest contribution to the study of composite samples and separated mineral phases from the moon. The constituents of the lunar soil are of a highly complex nature. The primary aim has been to study the amount of iron in the soil, the distribution of iron over the soil constituents, and the particular valence states. The soils at the moon landing sites do not represent averages of the collected rocks. The spectrum of the soil (Figure 9) is the superposition of patterns of T e in silicate minerals, silicate glasses, ilmenite, metallic iron, and troilite. 

Of the various inorganic soil constituents, smectites (montmorillonite clays) have the greatest potential for sorption of pesticides on account of their large surface area and abundance in soils. Weak base pesticides, both protonated and neutral species, have been shown to be sorbed as interlayer complexes. Sorption of atrazine on smectites ranges from 0 to 100% of added atrazine, depending on the surface charge density of the smectite (36). 

The alkahes do not sequester heavy-metal ions and have Httie soil-suspending effect. They are effective in maintaining a high pH and saponify the acidic constituents of soil and thus promote cleaning. In the cleaning of ceramics, glass, and metal surfaces, however, the alkahes act as primary detergents even in the absence of surfactants in these systems. 

Although most of the volatile components are released to the atmosphere, a small fraction is dissolved and/or carried away with the water in the soil matiix. Leached waters are carried with the water as it percolates through the underlying soil strata. Most of the organic constituents contained in the leachate receive additional treatment as they pass through the soil cohimn. Leached wastes can also be lost in surface rtinoff. 

The nuclei of iron are especially stable, giving it a comparatively high cosmic abundance (Chap. 1, p. 11), and it is thought to be the main constituent of the earth s core (which has a radius of approximately 3500 km, i.e. 2150 miles) as well as being the major component of siderite meteorites. About 0.5% of the lunar soil is now known to be metallic iron and, since on average this soil is 10 m deep, there must be 10 tonnes of iron on the moon s surface. In the earth s crustal rocks (6.2%, i.e. 62000ppm) it is the fourth most abundant element (after oxygen, silicon and aluminium) and the second most abundant metal. It is also widely distributed. 

The weathering process which eventually reduces the rock of the parent material to the inorganic constituents of soil comprises both physical and chemical changes. Size reduction from rocks to the colloidal state depends not only upon the mechanical action of natural forces but also on chemical solubilisation of certain minerals, action of plant roots, and the effects of organic substances formed by biological activity. 

In the soil, an imbalance of amino acids, the presence of D-amino acids derived from microbial metabolism, or analogs and homologs of constituent protein amino acids could produce a deviation in the... 

Studies on the Polysaccharide Constituents of an Acid Extract of a Fenland Muck Soil, P. Finch, M. H. B. Hayes, and M. Stacey, Trans. 9th Int. Congr. SoilSci., 3 (1968) 193-201. 

Foreword in The Chemistry of Soil Constituents, Ed. D. J. Greenland and M. H. B. Hayes, John Wiley and Sons, (1978). 

Fig. 8-3 Diagram illustrating the development of positively and negatively charged sites on surfaces of soil constituents, at low and high pH. (Reproduced with permission from R. L. Parfitt (1980). Chemical properties of variable charge soils. In "Soils with Variable Charge" (B. K. G. Theng, ed.), p. 168. New Zealand Society of Soil Science Offset Publications.)...

---