Soil structure, formation and type

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Soil structure, formation and type

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.


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.

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:

  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.


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.

Brown earth