Objectives: By the end of this subtopic learners should be able to:
Explain the types of rocks from which soil is formed and describe agents of weathering.
Explain the role of weathering in soil formation.
Define soil profile.
Explain the appearance and composition of each horizon.
Point out the significance of each horizon to crop growth.
State the importance of soil profiling.
Soil Soil is the loose layer of material covering the earth’s surface, supplying anchorage and nutrients to plants. It is formed mainly from weathered rocks.
Functions of the soil
Soil anchors plants, meaning it supports plant roots and other parts of the plant.
Stores water for plant use.
Soil provides plant food in the form of nutrients that are required by plants for growth.
Soil is a habitat for soil organisms such as earthworms, millipedes, ants and snails.
Weathering is the breaking down of rocks into smaller particles to form soil.
Breaking down of rocks into soil occurs in three ways: physical weathering, chemical weathering and biological weathering.
Parent material refers to the underlying geological material from which soil is formed.
This parent material is commonly referred to as rocks or bedrock.
Types of rocks:
The three main types of rocks from which soil is formed are: Igneous, Sedimentary and Metamorphic rocks.
Igneous rocks are formed through the cooling and solidification of magma or lava.
Solidification into rock occurs either below the surface as intrusive rocks or extrusive rocks.
Slow cooling gives large crystals and large grained rocks.
Rapid cooling gives small crystals and fine grained rocks.
Examples of igneous rock are:
Granite - which is a large grained rock (Felspars, Mica and Quartz),
Basalt - found in the Lowveld
Pyroxenite and Norite - found in the Great Dyke.
Sedimentary rocks are formed by the destruction and weathering of igneous rocks.
Weathering can be by the action of frost, temperature changes, wind and water or by dissolving effect of rainwater which is slightly acid.
Coal is an example of a sedimentary rock formed from organic matter.
Other examples are sandstone and limestone.
Metamorphic rocks are formed from other rocks such as igneous or sedimentary rocks that are changed by great pressure and heat.
Metamorphic rocks include gneiss, slate and marble.
Weathering is the breaking down of rocks into smaller particles.
A. Physical weathering This is a mechanical process which results in breaking down of rocks by the action of:
Freezing water/frost action.
i. Frost action:
This occurs when water collects in rock crevices as liquid and temperatures become low to freezing point, the water freezes thereby occupying more space thus enlarging the crack.
Water will change to liquid, the rock will contract.
The expansion and contraction process will cause the rock to peel off (exfoliate).
ii. Running water:
Flowing water carries with it some rocks which will continuously hit and rub against each other. This rubbing against each other causes the rock to break and form soil.
Blowing wind carries with it some sand particles which hit on solid rock causing them to break to form soil.
iv. Temperature changes:
High temperatures during the day cause rock mass to expand. When it is cold during the night, the rocks contract causing the rock to peel off.
B. Chemical weathering
This is when the rock breaks down by the action of corrosive effect of substances which might have come in contact with the rock.
Acids and salts are some of the chemical substances which dissolve in water leading to the weakening of rocks.
This results in the breaking of the rock to form soil.
C Biological weathering
The breaking down of rocks due to living organisms such as animals, earthworms, ants or plant roots.
Plant roots grow between cracks and as they grow big they cause the rock to break forming soil particles.
Factors influencing soil formation
a. Parent rock: If the parent rock is hard it takes more years to weather compared to soft parent. b. Temperature: Changes in temperature from high to low result in expansion and contraction of rocks causing them to break down to form soil. (A very hot day followed by a very cold night influence soil formation). c. Water: Rocks which are always in water quickly breakdown, whereas rocks not exposed to water take time to form soil. d. Plants and animals: Continuous interaction of rocks with movement of animals, earthworms and ants result in breaking down of rock to form soil.
Soil profile is a vertical cross section of soil when cut through from the surface to the underlying rock.
Horizons of the soil profile
Soil profile has four main horizons, namely Topsoil (A),Subsoil(B), Partly weathered rock( C ) and Solid rock (Unweathered rock) (R or D)
Characteristics of each horizon
Horizon A: Top soil:
High microbial activities.
Plant roots are mainly found here.
Rich in plant food.
Rich in organic matter and is darker in colour.
Horizon B: Subsoil:
Subsoil is useful for farming if mixed with top soil.
Contain little plant nutrients.
Some plant roots get to this zone.
Lighter in colour because there is less organic matter.
Horizon C: Weathered rock
Not used by farmers.
Limited weathering takes place and it is mainly gravel.
Horizon D: Parent rock or Bedrock
Zone of hard rock.
Found several meters under earth.
Importance of soil profile
Soil profiling gives an understanding of the soil structure and its influence to plant growth.