Soil formation

Soil formation is the process by which soil comes into existence or is made.Soil is formed by the breakdown of rocks in a process called weathering.

Soil genesis;  the weathering of rocks and the conversion of weathered rock material into soil.

Weathering; is the breakdown or disintegration of rocks into smaller particles to form soil.

NB:   Soil formation occurs in the following four (4) processes;

Weathering

Decoposition

Translocation

Deposition

Types of weathering

  • Chemical weathering
  • Biological weathering
  • Physical weathering

physical weathering

This is the mechanical breakdown of rocks into small fragments (particles) with no change in their chemical composition.

Processes of physical weathering

Differential expansion and contraction of rocks; when the sun heats up the rocks during day, the rock expand and when they cool at night they contract.

This alternate expansion and contraction cracks the rocks and eventually they breakdown.

Exfoliation; during day, upper layers of the rock expand faster than the lower layer. This brings about separation and disintegration of the upper layer to the lower layer.

Frost action/freeze and thaw; In cold region when the water in the crack freezes, it expands. This increases the size of the cracks and eventually the rocks break off.

Break down by living organisms; As plants extend their roots, the pressure they exert leads to breakdown of rocks. The burrowing animals also breakdown the rocks through their weight and their activities.

Pressure release; hard old rocks may be covered with softer recent sedimentary material making them to experience a lot of pressure due to the weight of the overlying blanket of sedimentary materials when their material is removed by erosion or glaciations, the underlying rock expands rapidly hence cracking.

Chemical weathering

This is the breakdown of rocks with a change in their chemical composition.

It is mainly because of the reaction of minerals in the rocks with atmospheric gases e.g. Oxygen, carbon dioxide and water vapour

process of chemical weathing

  • Oxidation; this involves addition of oxygen to the rock minerals to form metallic oxides weaker than original minerals e.g. in tropical areas with high temperatures and high humidity.

E.g.  Iron (II) carbonate + Oxygen                  Iron oxide + carbon dioxide

     4 FeC03  +  02                          2Fe203 + 4C02

                                                           (haemite)

  • Hydration; this involves the combination of water with minerals to form hydrated compounds which are weaker than the original minerals.

Iron (II) oxide    +   water                  hydrated iron oxide (goethite)

Fe203     +   H20    Fe203.H20

  • Carbonation; this involves the reaction of C02 with bases in rocks to form carbonates and hydrogen carbonates e.g.

Calcium hydroxide + Carbon dioxide          Calcium hydrogen carbonate + water

Ca (0H) 2   +   C02          CaHC03 + H20

Potassium hydroxide + carbon dioxide                Potassium carbonate + water

2K0H   + C02                          K2C03 + H20

The carbonates formed are soluble in water and are easily washed away.

  • Reduction; this involves removal of oxygen molecules from mineral compounds. This leads to decomposition and crumbling of the mineral compounds especially in poorly drained conditions.
  • Hydrolysis; This occurs when metallic cations of many compounds are replaced by hydrogen ions (H+ ions) from water.

E.g. Calcium silicate + water                      Silisic acid + calcium hydroxide.

CaSi03 + 2H20               H2Si03 + Ca (0H) 2

  • Acidification; Acids e.g. carbonic acid formed between atmospheric C02 and rain water reacts with rocks.

H20 + C02             H2C03

The carbonates formed are soluble in water and are easily washed away.

  • Reduction; this involves removal of oxygen molecules from mineral compounds. This leads to decomposition and crumbling of the mineral compounds especially in poorly drained conditions.
  • Hydrolysis; This occurs when metallic cations of many compounds are replaced by hydrogen ions (H+ ions) from water.

E.g. Calcium silicate + water                      Silisic acid + calcium hydroxide.

CaSi03 + 2H20               H2Si03 + Ca (0H) 2

  • Acidification; Acids e.g. carbonic acid formed between atmospheric C02 and rain water reacts with rocks.

H20 + C02             H2C03

Carbonic acid

Biochemical weathering

This is the type of weathering where the rocks are broken down by living organisms e.g.

Micro organisms e.g. fungi and bacteria decompose organic materials to form humus.

Termites, earth worms and rodents feed on organic materials and add humus to the soil when they die and decompose.

Lichens, fungi, bacteria colonise the rocks and produce organic acids which breakdown the rocks.

Roots of huge trees force their way into rocks forcing them to split.

Animals’ hooves break the rocks by movement on the rocks.

Accumulation of calcium carbonate from shells of molluscs (e.g. snails) and fish to form rocks e.g. limestone.

Some roots produce organic substance at the root caps and their dissolve rock particles.

Man’s activities e.g. cultivation break rock particles.