Introduction to Soils and Soil Formation

8/6/2019 Introduction to Soils and Soil Formation

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MOSES LUTTA: BRAESIDE HIGH SCHOOL 2011

Introduction to Soils and Soil Formation

There are numerous different types of soils with distinctive profiles. Their formation ininfluenced 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.


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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.

Soil profiles

These show a vertical cross-section through a soil and have a number of different horizons asshown 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 thesoil or be a separate layer.


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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 decayingorganic 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 andwater in the soil.

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


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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.


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

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


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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 ofhigh 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 dominantlocal factor, for example, parent rock. Three types exist:

1. Calcomorphic/calcareous soils: which develop on limestone.

2. Hydromorphic soils: where water content is always high.

3. 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:


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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 notencourage 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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Human impact on soils

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

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

1. More intensive agriculture.

2. Larger and more powerful agricultural machinery.

3. Increase compaction of soil.

4. Inappropriate cultivation of steeper slopes.

5. Larger fields.

6. Year round agriculture.

7. Decrease in hedgerows.

8. Population pressure.

9. 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:

Introduction to Soils and Soil Formation

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