Organic Soils Peat

Peat is an accumulation of partially decomposed and disintegrated plant remains that have been fossilized under conditions of incomplete aeration and high water content (Hobbs, 1986). Physico-chemical and biochemical processes cause this organic material to remain in a state of preservation over a long period of time. 

Macroscopically, peaty material can be divided into three basic groups, namely, amorphous granular, coarse fibrous and fine fibrous peat (Landva and Pheeney, 1980). The amorphous granular peat has a high colloidal fraction, holding most of its water in an adsorbed rather than free state. In the other two types, the peat is composed of fibres, these usually being woody. In the coarse variety, a mesh of second-order size exists within the interstices of the first-order network, and in fine fibrous peat, the interstices are very small and contain colloidal matter. 

The volumetric shrinkage of peat increases up to a maximum and then remains constant, the volume being reduced almost to the point of complete dehydration. The amount of shrinkage that can occur generally ranges between 10 and 75% of the original volume of the peat, and it can involve reductions in void ratio from over 12 down to about 2. 

Depth (m) Moisture content (%) pH Organic content (%) Buik unit weight (kN m 3) Dry unit weight (kN m 3) Specific gravity initiai void ratio (6o) Coefficient of voiume change mv (m2 MN 1) Compression index, C  

If the organic content of a soil exceeds 20% by weight, consolidation becomes increasingly dominated by the behaviour of the organic material (Berry and Poskitt, 1972). For example, on loading, peat undergoes a decrease in permeability of several orders of magnitude. Moreover, residual pore water pressure affects primary consolidation, and considerable secondary consolidation further complicates settlement prediction. 

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Histosol Gr. histos, tissue Bog soils, organic soils, peat, and muck. No climatic distinctions... 

In this system, the soils are divided into groups (a) coarse soils, (b) fine soils (silts and clay) and (c) highly organic soils (peat). 

High rainfall areas over 700mm annual rainfall, or over 250mm excess winter rainfall Deep clayey soils Deep sUty soils Organic soils Peat soils... 

McGrath, D. (1997) Extractability, chemical composition, and reactivities of soil organic matter of Irish grassland soils. In Hayes, M.H.B. and Wilson, W.S. (eds) hfumic Substances in Soils, Peats and Waters. The Royal Society of Chemistry, Cambridge, UK, pp. 31-38. 

Stable radicals were formed during diagenesis of the organic sediment, and they have persisted ever since. The detailed mechanism of enzymatic reactions is not well understood, but it is known that stable free radicals can be produced. Steelink (18) has found that humic acids from soils, peats, and lignites have moderately high concentrations of free radicals, and since the... 

Clapp, C. E., Dawson, I. E., and Hayes, M. H. B. (1979). Composition and properties of a purified polysaccharide isolated from an organic soil. In Proceedings, International Symposium on Peat in Agriculture and Horticulture, Schallinger, K. M., ed., Special Publication 205, Agriculture Research Organization, Bet Dagan, Israel, pp. 153-167. 

Peatlands, or organic soils, are water-logged deposits of partly decomposed plant debris. Ecological variation and diversity in extent of domination by water influence the degree of humification in peatlands. Humification in peatlands does not coincide with decomposition as well as in mineral soils. Measurement and extraction of peatland humus are hampered by the presence of large proportions of unhumified material. Therefore, most studies on the characterization of peatland humus have focused on humic acids, or on pyrophosphate extracts of the organic soils. Pyrophosphate extracts contain less unhumified materials than alkali extracts but are far less effective in extracting the peat humic substances. 

The formation of humic substances in a peatland environment is a complex humification process which is principally due to certain enzymatic and microbial activities. These organic matter transformation processes are influenced by the nature of the peat-forming plants and certain physical and chemical properties within a particular peatland. In very acidic or low nutrient peatlands a very different microflora may exist than in a more eutrophic and less wet situation. In the former situation humification may be retarded and the peatland plants will be preserved and thus accumulate. On the other hand, in less acid environments with moderate amounts of nutrients and periodic water-table fluctuations humification proceeds relatively rapidly and leads to decomposed organic soils such is the case in drained and cultivated organic soils. 

Day, J. (1968). The classification of organic soils in Canada. Proc. Srdint. Peat. Congr., pp. 80-84. 

Levesque, M. P. (1981). Characterization and differentiation of peat materials in the context of peat soils classification. Proceedings of the Organic Soils Mapping and Interpretations Workshop, Fredericton, N. B. Res. Br. Agric. Can. L.R.R.I. Monogr. 82-44, 74-104. 

Mathur, S. P. (1982b). The inhibitory role of copper in the enzymic degradation of organic soils. Proceedings of the International Peat Symposium. Bemidji State University, pp. 191-219. 

Mathur, S. P. and Sanderson, R. B. (1980b). Relationships between copper contents, rates of biodegradation, and degradative enzyme activities of some cultivated organic soils in Ontario, Canada. Proc. 6th Inti. Peat Congr., 628-632. 

Figure 2.1 is a pictorial representation of the void ratio. Void ratios for granular soils range from 0.4 to 0.8. Clays and clayey soils will range from 0.5 to 1.5 or 2.0. Soils with high organic content, peats and mucks, may have void ratios as high as 4 or 5. 

The adsorption process also proceeds in organic soils, but the situation is more complicated and not completely clear. Bipyridylium herbicides are strongly adsorbed and thus deactivated by organic soil. However, in peat soils, for example, adsorption is often slow or incomplete, so that residual action must be reckoned with (OToole, 1965). 

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