Organic components in the soil

Humic substances are typical products of the humification process. They are the basic component of the humus, occurring in the humus material to the extent of 80 to 90%. They consist of a group of acid, yellow to dark brown polymeric substances of a heterogeneous aromatic and polydisperse character. Their acid nature allows them to react with mineral substances to form stable organomineral complexes sometimes referred to as humines. They differ from the remaining organic material in the soil by various typical features as follows  

Due to a high content of acid carboxy and phenolic groups, the humic substances may be dissolved in alkaline solutions. In the soil, the humic substances behave as natural ion exchangers, molecular sieves and as material able to accumulate mineral substances, particularly cations. At the present time it is assumed that macromolecules of humic substances, in spite of being a chemically very heterogeneous mixture, have in common a certain general molecular structure. The basic scheme and approximate molecular structure of humic substances are shown in Fig. 7.8. 

In spite of the heterogeneity, the humic substances can be grouped into three different types on the basis of certain characteristics as follows  

Humic acids are soluble in weak alkaline solutions and are essentially insoluble in water and mineral acids. They may be precipitated from solution by the action of mineral acids and bivalent or trivalent cations, however, they are fairly resistant to the acid hydrolysis. They are dark spherocol-loids with a cross-linked structure which plays a part in their high sorption capacity. They exhibit different degrees of a tendency to aggregation and very different degrees of dispersion. In comparison with other types of natural organic substances, the humic acids are characterized by their extraordinary stability in the soil. This stability is due to their ability to form organomineraJ complexes, particularly with clay minerals and with aluminium and iron hydroxides. 

Humins (humus coal) are soluble neither in alkalies nor in acids. This group in humic substances have been very little investigated. They are characterized as an insoluble form of humic acids. Humins are considered as the oldest, and ultimate products of the humification process. They are assumed to be strongly carbonized substances, where the high stability is increased by a strong bond to the mineral fraction. Due to their high chemical stability or even inert character, humins do not participate in the soil formation process and they do not fulfil the function of the true humus. 

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Penicillium expansum—which are widely distributed soil fungi and may have entered the lake from run-off—were capable of bringing about association of tetrachloroguaiacol with organic components in the aqueous phase, so that this material could subsequently enter the sediment phase (van Leeuwen et al. 1997). 

Assumptions inherent in the use of a Koc (or Kom) are that sorption is exclusively to the organic component of the soil all soil organic carbon has same sorption capacity per unit mass equilibrium is observed in the sorption-desorption process and the sorption and desorption isotherms are identical (Green and Karickhoff, 1990). Both Koc and Kd have units of L/kg or cm3/g. 

Humus2 Total of the diverse organic components in the environment exclusive of undecayed plant and animal tissues, their partial decomposition products, and the soil biomass. 

The principle of PICT that has been noted in Section 7.3.1.2 for algae has been applied to bacteria in a terrestrial system (Rutgers et al. 1998). Determination of the community structure based on the taxonomy of the components is based on their nutritional demands. Metabolic activity was assessed using 95 substrates, which do not have any structural relation either to the organic components of the soil or to the toxicant that is being examined. 

Butachlor is a pre- and early postemergence selective herbicide for the control of grass weeds and a few broad-leaved weeds. Depending on the organic components of the soil, it is used at a rate of 1.5-3 kg active ingredient/ha on seeded and transplanted rice in Asia, and in Central and South America. 

In the soil it is degraded fairly rapidly into very polar compounds, which are bound by the organic components of the soil. Degradation proceeds in 30 days under aerobic conditions. 

All components used in preparation of cast films were non-toxic and either natural renewable or biodegradable synthetic polymers. Fmthermore, all ingredients are environmentally compatible and can be utilized for application as agriculture mulch. The presence of natural fillers will help maintain organic matter in the soil and urea would act as an additional source of fertilizer nitrogen. 

Figure 12.3 Simplified model for the various processes governing the Cu and Zn isotopic composition collected in the Talbot area of Canada [33], The ore zone is redox-active, with interactions between microbes and fluids on the sulfide ores mobilizing Cu and Zn as aqueous and gaseous complexes into the overlying sandstone and carbonate, which are about 100 m thick in total. Migration of these components occurs through micro- and macrofractures and then they are trapped in the near-surface environment on clay minerals in the till or incorporated into the biosphere, including trees, moss, and organic matter in the soils.

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