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Genesis / Classification
The soil is at the interface between the atmosphere and lithosphere (the mantle of rocks making up the Earth's crust). It also has an interface with the hydrosphere, i.e. the sphere describing surface water, ground water and oceans. The soil sustains the growth of many plants and animals, and so forms part of the biosphere. A combination of physical, chemical and biotic forces acts on organic and weathered rock fragments to produce soils with a porous fabric that contain water and air (pedosphere). We consider soil as a natural body of mineral and organic material that is formed in response to many environmental factors and processes acting on and changing soil permanently.
Figure 1.1.1. Compartments of a landscpape.
Because soil is important for cultivation and agricultural production, soil fertility and productivity are important issues to address. Detailed pedological knowledge is useful for land evaluation purposes, i.e. the classification in fertile productive soils and less valuable soils. Soils are an integral part of landscapes and the knowledge of the distribution of different soils helps to preserve a high standard in environmental quality. For example, site specific management cannot be developed without detailed knowledge of soils. Critical sites, e.g. shallow hillslope soils prone to erosion and leaching of nutrients, can be identified using pedology. Soil surveys furnish basic inputs to soil conservation planning and provide information used in equations for predicting soil loss and water pollution under various management practices on different soils.
Some definitions are given, which are closely related to soil science and pedology:
Pedology: The interpretation, mapping, and classification of soil based on soil genesis.
Pedon: A pedon is the smallest volume that can be recognized as a soil individual. It has three dimensions and its area ranges from 1 to 10 square meters, depending on the variability in the horizons. The shape of the pedon is roughly hexagonal. A soil volume that consists of more than one pedon is termed a polypedon.
Ped: Peds are soil aggregates which are recognizable in the field because they are separated by voids and natural planes of weakness. They should persist through cycles of wetting and drying, as distinct from the less permanent aggregates.
Pedogenesis: The process of soil formation as the integral result of the combination of soil forming factors (climate, parent material, relief, organisms, time).
Soil classification: The grouping of different soils into classes on the basis of one or more criteria. A class is a group of individuals or units similar in selected properties and distinguished from all other classes by differences in these properties.
U.S. Soil Taxonomy: Classification of soils developed by the USDA Soil Survey Staff with the support of and contributions from land-grant university participants in the National Cooperative Soil Survey and colleagues in other countries. It was formally released and published by the USDA in 1975 (Soil Survey Staff, 1975). In this course we follow the sixth edition of the Keys to Soil Taxonomy (Soil Survey Staff, 1994).
The following particle size ranges are used for clay, silt, sand and gravel following the USDA system [units: mm] (a detailed description of soil texture is given in chapter 9) Soil Morphology:
xxclay: < 0.002xx 
xxsilt: 0.002 - 0.05xx
xxsand: 0.05 - 2.0xxx
xxgravel: > 2.0xx
Soil Survey Staff. 1975. Soil Taxonomy. U.S. Dept. Agric. Handbook 436. U.S. Govt. Printing Office, Washington D.C.
Soil Survey Staff. 1994. Keys to Soil Taxonomy USDA - Soil Conservation Service. 6th ed., Washington D.C.
Recommended Reading:
Soil Science of America. 1997. Glossary of Soil Science Terms.
Weblink:
SSSA: Glossary of Soil Science Terms
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Genesis
The vertical dimension, exposed by excavation from the surface to the parent material, constitutes the soil profile. Layers in the soil, distinguished on the basis of color, soil texture, soil structure, and other visible properties, are called horizons. Several horizons make up the soil profile.
Next, a description of a soil profile developed in a temperate humid environment is given showing all of the principle soil horizons (Figure 1.2.1): The upper layer, from which materials are generally washed downwards, is described as eluvial. Lower layers in which these materials accumulate are called illuvial. The eluvial horizon is denoted by the notation E and an illuvial horizon by the notation B. The B and E horizon plus the tophorizon denoted by an A comprise the solum. The unconsolidated parent material in which the soil is formed is denoted by a C. If unweathered rock (consolidated material) exists below the parent material it is labelled with a R. Organic litter on the surface, which is not incorporated in the soil is designed the O layer.
Fig. 1.2.1. A hypothetical soil profile showing all of the principle soil horizons.
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Genesis / Classification
Soils are made of four main components, i.e.
mineral
matter (40 - 60 %),
soil
water (20 - 50 %),
soil
air (0 - 40 %) and
organic
material (small percentage).
The relative proportions of the four major components may vary widely, but generally lie within the ranges listed.
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Genesis / Classification
Classification of natural phenomena like soils is typically done for the following purposes:
Organize knowledge about the
subject to enable investigation and communication to be both logical
and comprehensive (structure/organization for scientific/technical
development).
Provide a framework for
establishing relationships among soils and their environment that
leads to advancement of theoretical and experimental aspects of soil
and related science (academic focus).
Establish groupings of soils for
which usefule and reliable interpretations can be made (utilitarian
focus). For example: optimal use(s), hazard/limitation/remediation
assessment, potential productivity, framework for technology
transfer/information dissemination.
There are two different classification approaches, i.e. technical vs. natural classification. Technical classifications are designed for specific applied purposes (e.g. , soil engineering classifications, based largely on physical properties), whereas natural classifications attempt to organize the divisions of soils from a more holistic appraisal of soil attributes. The general principles of natural soil classification systems are:
A natural system of
classification should express general or universal relationships that
exist in nature. One should be able to understand, remember,
generalize, or predict from information obtained.
The scheme should be based on
characteristics or attributes of things classified as related to
their genesis. It should place similar things together on the basis
of their properties.
It is technically impossible to
use all of the properties of soils to classify them. Judgement based
on existing knowledge must be used to determine which properties are
most important.
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