O Level Notes : Geography - Ecosystems - Soil formation and factors influencing soil formation

FACTORS INFLUENCING SOILFORMATION Pedogenesis is the natural process of soil formation. It takes long periods of time for soil to form and there are various factors which influence soil formation. Jenny (1941) stated the soil forming factors in the formula:

Soil= F (C, T, O, P) t

Where;

F means the function of. Soil formation is the function of C, T, O, P and t

P is the parent rock

C is climate

T is topography

O is for organisms

Soil forming factors

Parent rock

The parent rock is the unconsolidated or fragmented deposits and minerals in which soils are developing. The mineral composition of the soil is derived from this parent rock. The physical characteristics of the soil are also determined by the parent rock. Soil texture, composition and colour also depend on the parent rock.

Organisms

Biota or living organisms such as fungi, bacteria, animals, humans and vegetation play a significant role in the formation of soil. Organisms such as fungi and bacteria are decomposers, they help in the decaying of organic matter. The organic matter forms humus and become an integral component of the soil. Soil profiles in hard woods are alkaline. This is so because the soils take up magnesium, calcium, and potassium from forest branches and trees that are used to develop organic matter. Soil profiles in coniferous forests tend to be more acidic.

Climate

Most soil formation processes depend on precipitation and rainfall. In areas where temperatures and rainfall are always high, the rate of chemical reaction is high. Decomposition rate of rocks to form soils is high, for example, in humid tropics. The process of leaching which is an important soil formation process is also high in the same regions. High temperature and rainfall also leads to rapid growth of vegetation. Dense forests in areas with high rainfall and temperature leads to accumulation of humus. Weathering of rocks by chemical decomposition is rapid in those regions hence they tend to have deep soils. Areas with low rainfall have slow rate of chemical decomposition and less organic composition in the soils. They tend to have thin infertile soils.

Relief

Relief is also referred to as topography. It is the configuration of the land surface. It is the shape of the land surface and the slope of the land surface. The elevation or gradient of the land affects the soil formation processes. On steep slopes there is no accumulation of weathered rocks to decompose into soil. Humus also do not accumulate on steep soils since it is washed down the slope by running water. Gentle slopes increase the rate of infiltration hence processes such as leaching are high.

Time

The process of soil formation takes a long time. This take hundreds or thousands of years. Most soils take more than 10 000 years to develop. Soils within this range are still relatively young. Time is needed for the other components of soil to interact so as to form soil.

UNIT 3.4: TROPICAL SOILFORMATION PROCESSES

The important soil formation processes in the tropical areas include organic accumulation, leaching, eluviation, illuviation, cheluviation, and gleization. 

Organic accumulation

Organic matter is derived from vegetation. The decayed organic matter accumulates on the surface. It decays due to activities of microorganisms to form humus. Humus is a major component of the top soil.

 Humus; an important component of the soil

Leaching

Leaching is another soil forming process in the tropics. It is the removal of soluble mineral matter in solution from upper layers to the lower layers of the soil. There are some mineral matters which are soluble, they easily dissolve in water, for example,  rock salt. Infiltration water reaches such minerals and cause them to dissolve. They are then transported down through the soil layers. This process is common in humid climates. The top soil is left without minerals. 

 Eluviation

Leaching

Eluviation is the process whereby fine mineral particles such as clay are washed down from the upper to the lower layers. The materials are moved in suspension by the percolating water. This process can also take place in the same layer when water move horizontally. The zone from which materials have been eluviated is called eluvial zone. 

Eluviation layer

Eluviation

The materials washed down from upper layers to the lower horizons by eluviation are deposited at the lower layers. This deposition is called illuviation. The deposited materials accumulate and are cemented together to form laterites.

Cheluviation

It is the downward movement of materials in the soil similar to leaching.

Gleization

Involves the gradual decomposition of peaty material. This decomposition leads to the reduction of ferric compounds to create gleys or hydromorphic soils which are structure less, sticky and boggy- blue in colour. In Zimbabwe this process is common in vleis and dambos. The wet conditions in these areas which are waterlogged make bacteria activity slow. Decomposition is slow. Bacteria and other microorganisms need moist or humid conditions to be more active and not the water- logged conditions of vleis and dambos.

SOILPROFILE

Soil profile is the sequence of layers of the soil from the top soil to the parent rock is called horizons. By digging pits, we are able to identify the different layers of the soil, made clear by differences in soil colour.

  The Soil Profile

Humus: this is found above Horizon A. It is made up of organic matter. This organic matter maybe partially decomposed. Twigs, leaves, grass and excreta from animals may be part of the organic matter.

Horizon A: This is the top soil. It is made up of humus, living creatures and inorganic materials. The top soil is dark coloured owing to the presence of organic matter. It is rich in nutrients which support plant growth. Living organisms such as earthworms, millipedes, centipedes, bacteria and fungi are found in this layer.

Horizon B: this Horizon is also referred to as the subsoil. It contains materials such as iron, aluminum and organic compounds. It is comparatively harder and compact than the top soil. It is lighter in colour than the top soil because there is less humus in this layer. Though it has less organic matter, it is rich in  minerals brought down from the top soil. It contains mineral salts, for example, iron oxide.

Farmers mix HorizonAand B during ploughing.

Horizon C: is a zone of no humus. It composes of unconsolidated parent material from which Horizon A and B have been formed. It lacks characteristic features of horizonAand B. It can be unweathered or deeply weathered.

Horizon D: This horizon is made up of the parent rock. It is the original rock from which soil is formed. Rocks such as granite, basalt, quartzite, limestone and sandstone can be the parent rocks of certain soils. The soils' characteristics are derived from their parent rocks.