3 Major Types of Weathering and Soil Profile

Introduction

We tend to think of the landscape as unchanging. However, it is slowly and continuously being reshaped. Usually, this reshaping of the landscape results into the production of soils. Soil is formed from the break-down of the parent materials by the process known as weathering. This is the result of climatic factors such as wind, water, temperature and sunshine. It is also due to action by chemicals. In this section, we shall discuss the various types of weathering processes.

WEATHERING

Exposure to wind, rain and frost causes rocks to break apart and crumble. This is known as weathering. There are two main types of weathering: physical and chemical.

1. Physical weathering

Physical weathering includes the actions of wind, water and temperature. Rain water can collect in cracks in the rocks and freeze as the temperature drops. This causes expansion as the ice forms, forcing the crack apart. When the temperature rises, the ice melts, and again water fills the crack. 

This process of freeze-thaw weathering can be repeated over time, making the crack steadily wider until a boulder falls. Temperature changes also produce stresses in the rock because the different minerals present expand (in hot sunshine during the day) and contract (in the shade and at night) at different rates. Rocks and boulders will detach from a slope, causing scree.

As seen above, in physical weathering the agents are: water, wind, temperature changes and ice.

Water: Running water carries away loose particle of rocks of different sizes and deposits them elsewhere. As the rock fragments are carried, they constantly rub against each other and the ground of the river bed. This wears away the ground and also the fragments themselves which become smaller.

Wind: Wind carries away dust particles, which are blown against rock surfaces, causing them to wear away. Frequently, wind removes loose material leaving the rock exposed to further weathering.

Temperature: Temperature changes can cause the expansion (on heating) and contraction (on cooling) of the various minerals, which the rock consists. Each of the minerals expands and contracts at different rates of different amounts according to their physical properties. This causes fractures to occur in the parent materials, which over a long period of time disintegrates.

Ice: As the temperature reach freezing point, water in the crevices and cracks of rocks freezes. The force of expansion of the ice causes the rock to crack further the shatter. When the temperature rises again, the ice melts and the pieces of rock shattered by the ice are carried away by the melting Ice. The movement of these pieces of rock causes further disintegration.

2. Chemical weathering

Natural rain water contains dissolved carbon dioxide and is therefore slightly acidic. This rain ‘water will very slowly dissolve rocks such as limestone:

limestone + rain water → calcium hydrogencarbonate

CaC0₃(s) + H₂0(1) + C0₂(aq) → Ca(HC0₃)₂(aq)

This process will be increased by ‘acid rain’, which contains dissolved pollutants such as sulphur dioxide and oxides of nitrogen. Some minerals take up water and become hydrated. This causes expansion, and therefore stress, in rock formations. Fragments of rock are again broken off.

The material broken from rock structures by weathering can then be carried away by erosion.

Both the physical and chemical weathering, can be initiated by living organisms either in search for food or habitation (shelter). This form of weathering is sometimes referred to as biological weathering.

3. BIOLOGICAL WEATHERING

Plants and soil organisms also cause weathering. Plant roots grow into minute cracks in rocks and the chemical they produce result in some breakdown of the rocks.

Soil organisms such as termites, earth warms and bacteria have a considerable effect on soil formation.

Characteristics and types of Soil

One useful product of reshaping the landscape is soil. Soil is formed from fine particles of eroded debris mixed with humus. Humus is decayed organic material from plants and animals. It takes about 400 years to produce 1 cm of soil. Most soils are a mixture of particles of different sizes. They vary in the amounts of salts, water and humus present. The types of soils range from sandy to clay soils.

Soils can be classified according to the proportion of sand, clay and humus which they contain. Thus, a soil containing a large quantity of sand is called sandy soil. A soil containing a large quantity of clay particles is called clay soil. A soil containing a mixture of about one-half sand, one third clay and one sixth humus is known as loamy soil, and it is the most suitable for agriculture.

The proportion of the different sizes of particles in a soil determines the texture of a soil. Soil texture refers to the percentage of composition in a soil of sand, clay and silt particles. A soil in which sand particles predominate shows a coarse texture while a soil in which clay particles predominate has a fine texture. 

Thus a soil may be termed gravelly, sandy, silt or clayey depending on the proportion of the particular particles in the soil. As different sized particles have different physical properties, the proportion of them in a soil will therefore influence the soil properties; physical and chemical. For example, gravel particles are rather large and also heavy due to their high content of iron. 

Consequently, gravelly soils contain large spaces between the particles and these spaces allow water to drain off very quickly. Because of this gravelly, soils have very poor water holding capacity (water retention). Such soils are also poor in plant nutrients. By comparison, clay particles exhibit a large surface area and therefore have a considerable greater capacity for holding nutrients and water than either gravel or sand. On soil texture, aeration, drainage, nutrient and water-holding capacity of the soil, depends on penetrability by the roots, etc.

Lastly, the ease of working or cultivating a soil will very much be influenced by its texture. Thus clay soils, having a fine texture, are described as heavy soils and sand soils as light soils. Soil structure is the way the soil particles in a soil are packed. If the soil particles are closely packed, a soil has a solid structure e.g. in a clay soil. If the soil particles are loosely packed, a soil has a loose structure, e.g. in a sandy soil. If the soil particles in a soil appear like the inside of a loaf of bread, then structure is crumb or friable, e. g, in a loam soil.

Table 3.2 Different soil types

Soil profile

Soil profile refers to the arrangement of layers of soil in a profile. This arrangement can only be seen in a dug out pit. It is therefore important for you to carry out practical works to identify the different soil layers. This is because the arrangement of soil, provides a guide in the choice of what crops to grow on a piece of land.

It is important for you to realise that as soil-forming processes continue, the soil develops in depth resulting in the formation of a distinct sequence of soil horizons. A soil horizon is a layer parallel to the soil surface, whose physical characteristics differ from the layers above and beneath. The vertical arrangement of the various soil layers or horizons, are called the soil profile. The different horizons in the profile can easily be seen. The distinct horizons in the profile represent soil layers at different stages of development. 

Thus, one horizon will differ from the other in one or more properties, e.g. colour, chemical composition, size and the arrangement of the soil particles. Where the soil-forming processes have been active for a long time, then there would be the development of mature, deep soils with a well-developed soil profile, provided there has not been much erosion. 

Conversely, youthful shallow soils with a poorly developed profile result where erosion has been active and the soil forming processes have not been in action for a long enough. Such a soil will show under decomposed weatherable minerals in its profile. A deep soil having a well-developed profile has great potential for agriculture as it is able to hold more moisture for plant use than a shallow soil

Soil Profile

O – Horizon:  Organic matter: Litter layer of plant residues in relatively undecomposed form.

A – Horizon:  Surface soil/ Topsoil: Layer of mineral soil with most organic matter accumulation and soil life. This layer eluviates (is depleted of) iron, clay, aluminium, organic compounds, and other soluble constituents. When eluviation is pronounced, a lighter coloured “E” subsurface soil horizon is apparent at the base of the “A” horizon. A-horizons may also be the result of a combination of soil bioturbation and surface processes that winnow fine particles from biologically mounded topsoil. In this case, the A-horizon is regarded as a “biomantle“.

B – Horizon:  Subsoil: This layer accumulates iron, clay, aluminium and organic compounds, a process referred to as illuviation.

C – Horizon:  Parent rock: Layer of large unbroken rocks. This layer may accumulate the more soluble compounds .

R – Horizon:  bedrock: R horizons denote the layer of partially weathered bedrock at the base of the soil profile. Unlike the above layers, R horizons largely comprise continuous masses (as opposed to boulders) of hard rock that cannot be excavated by hand. Soils formed in situ will exhibit strong similarities to this bedrock layer.

The soil horizons down the profile are named in the following order; the topsoil, subsoil and the substratum. They can also be named respectively as A, B, C and D/R. Of these the topsoil is usually the more fertile, more subject to weathering and cultivation (tillage) operations. The topsoil is also usually better aerated and has therefore the more active soil microorganisms which decompose vegetable matter into organic matter and humus. The topsoil therefore is the horizon in which organic matter accumulates. Further breakdown of the organic matter releases organic acids which react with various mineral constituents in the soil to produce nutrients which are in turn usable by the plants.

The subsoil is usually more compact and less well aerated than the topsoil. An impermeable layer called a ‘hard pan’ may be found in some layers of the subsoil. The hard pan impedes drainage and root penetration and in practice, it has to be broken down by cultivation so as to facilitate water percolation as well as root growth. Sometimes minerals may accumulate in the subsoil as a result of leaching from the topsoil.

Plants are also responsible for moving mineral nutrients from one layer to another through their litter and dead remains. Also involved in this process are various soil- inhabiting organisms, e.g. earthworms and termites. These may interfere with the development of distinct soil horizons by mixing materials from one horizon with another through their various activities, e.g. burrowing, movement, etc. The substratum is also called the bed rock and represents the original soil parent material which is still intact and unweathered.