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Old 9th June 2009, 05:06 PM   #1 (permalink)
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Geography A-level: Soils

Source taken from:http://www.s-cool.co.uk/alevel/geogr...formation.html

There are numerous different types of soils with distinctive profiles. Their formation in influenced by climate, geology, topography, and biological organisms. They are the thin surface layer of the Earth's crust.

Soil formation

Regolith: Weathering of parent material to give a layer of loose broken rock. True soil is topsoil and involves the addition of water, air, living organisms (biota) and humus.

Parent material: The supply of Minerals comes from underlying rock, which have different rates of weathering. It controls depth, texture, drainage, and quality of the soil.

Climate: This influences the rate of weathering of the parent rock. Precipitation affects the type of vegetation that grows in an area and then provides humus. Rainfall may lead to leaching if it is very heavy. If rainfall is light or evapotranspiration is greater than precipitation capillary action begins to operate where water and minerals are drawn to the surface.

Topography: This is the relief of land. On higher land precipitation, cloud and the wind increase but temperatures decrease which influence soil formation. Steeper slopes encourage throughflow and Surface run-off making mass movements more common. Soils are often thin and of a poor-quality. Soil Catenas such as the one below show how soil varies along a slope. Rock type is constant.


Time: It can take 3000 to 12000 years to have sufficient depth of soil for farming.

Organisms (biota): Plants and bacteria, fungi, and animals all interact in the nutrient cycle.

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Old 9th June 2009, 05:07 PM   #2 (permalink)
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Re: Geography A-level: Soils

These show a vertical cross-section through a soil and have a number of different horizons as shown in the idealised diagram below. Each profile is a result of inputs and outputs and three horizons exist:


A - upper layer, here biological activity and humus content is at its maximum. Leaching may occur.

B - zone of accumulation or illuviation, clays etc. taken from A are redeposited. A and B account for the true soil.

C - recently weathered material.

It is possible for a surface horizon to exist. This is when humus is slow to decompose.

Layers between horizons may not always be clear, depth can vary, and humus can mix in the soil or be a separate layer.

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Old 9th June 2009, 05:08 PM   #3 (permalink)
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Re: Geography A-level: Soils

Soil structure (properties)

Soils have four main parts: water, air, minerals and organic matter, their relationships produce a number of properties found in soil which are outlined below.

Refers to the shape of peds (individual soil particles) and their grouping. The shape of peds, their alignments, along with particle size/texture determines the size and number of pore spaces. Structure can be improved by ploughing.

Soil texture:

How fine or course the mineral matter is in the soil that is dependent on the amount of sand, silt and clay particles in the soil.

The textures of different soils are shown below:


Soil nutrients (bases):

Chemical elements in the soil are vital for plant growth and soil fertility. They come from rainwater, fertilizer, parent rock, decaying organic matter and clay. Minerals from decaying organic matter are positively charged ions (cations), those from clay are negatively charged ions (anions).

Exchanges that occur are known as cation exchange and this is where bases are made available for plants.

Soil moisture:

This is important because it influences upward and downward movement of nutrients and water in the soil.

Other properties of soil include temperature, acidity, biota, air, organic and inorganic matter.

Soil formation processes

Translocation

Includes numerous processes but is primarily the downward movement of water or materials in soil. The main features of translocation are shown below:


Processes that come under the heading of translocation include:

1. Leaching: Where soluble material is removed in solution.


2. Cheluviation: When iron and aluminum sesquioxides are removed due to chelating agents.

3. Illuviation: Material re-deposited in the lower horizons.

Soil types and examples

Zonal soils

These are classified on a global scale and have climate as the major determining factor. They are mature, have distinct profiles and clear horizons. They are strongly tied to climatic regions as shown in the chart below:

Climate Zone: Soil Type:
Arctic Tundra
Mediterranean Mediterranean
Desert Red yellow desert
Equatorial Latosols
Azonal soils

Far more recent, and soil forming processes have not been in operation for long. Horizons are unclear and they are not linked with climate and vegetation. Their immaturity is a result of high altitudes, low temperatures and slow decay of organic matter. Examples include scree, till and volcanic soil.

Intrazonal soils

Soils found within the climate belt are different from normal, they are a result of a dominant local factor, for example, parent rock. Three types exist:

Calcomorphic/calcareous soils: which develop on limestone.
Hydromorphic soils: where water content is always high.
Halomorphic soils: saline due to high salt level.
Podsol

Develops if precipitation exceeds evapotranspiration. Often soils are sandy, coniferous forest exists and the climate is cool. A typical podsol is shown below:


Leaching is intense, humus acidic, and horizons are bleached as iron, aluminium and organic matter is removed. Horizons are clear as the cool conditions lead to acidic soils, which do not encourage earthworms.

Brown earths

These are moderately or well-drained soils, found where precipitation exceeds potential evapotranspiration, and particles move downward through the soil. They are the main soil in the UK due to the warm temperate climate. Annual leaf fall contains nutrients and mull develops which is fertile humus. Top horizons are dark and become lighter in the B horizon as leaching and eluviation occur. The soil is reasonably fertile.

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Old 9th June 2009, 05:09 PM   #4 (permalink)
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Re: Geography A-level: Soils

The main issue is that soils take thousands of years to develop but are very quickly ruined by human actions. Natural loss occurs via leaching, erosion and mass movements but today the natural balance of soil is being upset far more frequently. Human impact alters nutrient content, leads to soil erosion, compaction and salinisation.

Soil erosion

The problem was first recognized in the UK in the 1970's when water erosion of upland peat and wind erosion on large open fields was prevalent. A large quantity of soil was removed, much of which was the organic rich topsoil. The more topsoil that is removed the more erosion that occurs, as there are fewer roots to bind the soil.


Causes of erosion

More intensive agriculture.
Larger and more powerful agricultural machinery.
Increase compaction of soil.
Inappropriate cultivation of steeper slopes.
Larger fields.
Year round agriculture.
Decrease in hedgerows.
Population pressure.
Development.
Soil erosion leads to a decline in productivity, a reduction in organic content of soils, and more minerals and silt in rivers. Once topsoil is removed it is very difficult to replace.

Managing soils

Soils vary naturally in their fertility, and their ability to produce high or low crop yields depends on nutrient content, structure, drainage, local conditions of climate and relief, acidity and soil texture. Crop harvesting removes soil nutrients resulting in a poorer quality soil. Soil management aims to reduce soil erosion and nutrient losses in a variety of ways:

Method: Actions:
Limitation of wind erosion. Preserve moisture in the soil, plant windbreaks.
Control of slope run off. Terrace slopes, apply humus, develop ditches that traverse hillslopes to intercept run-off, contour ploughing, limit field width.
Management of crops. Cover crops, introduce crop rotation.
Limit gulley enlargement. Plant trailing plants, construct dams.
Re-vegetate areas. Limit grazing to allow re-growth, deliberate planting of vegetation.

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