GEOGRAPHY FORM THREE TOPIC 3: SOIL

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SOIL

Soil Formation

Soil

Define soil

The term soil is derived from the latin word “solum” which means ground. Therefore can be defined as the uppermost surface layer of loose or unconsolidated material which overlies the crystal rocks and on which plants grow.ORSoil is the natural body of mineral, organic and nutrient constituents which result from the interaction of the country rock with the environmental factors of climate, topography, plant and animal life.

Factors for Soil Formation

Describe factors for soil formation

Soil formation (pedogenesis) is principally initiated by the weathering of the parent rocks. Weathering can be chemical or mechanical. But the type of soil and rate of soil formation depend on a number of interacting factors (interplay of factors) in a particular environment, hence soil is the product of its own environment.

Soil continuously changes. The changes are generally slow but in certain circumstances, especially where human activities are involved, the changes can be rapid. The study of soil involves understanding the factors responsible for its formation. These factors are parent materials, climate, living organisms, topography and time:

1. Parent Material (Rock):This is the most important factor in soil formation since it determines the type of soil formed, soil colour, soil depth, the rate of soil formation, soil structure, soil texture, porosity and mineralogical composition or its fertility. It also influence soil maturity, such that if the rock is hard, It takes a long time for soil to mature while the rate of maturity is fast where the parent rock is soft. The fast maturity of soils, formed from soft rock is due to the fast rate of weathering process. Mature soil are deep and productive while immature soils are shallow and less productive.
2. Living Organisms (Biotic):These include the influence of plants, animals as well as human beings. Vegetation influence both chemical and mechanical weathering leading to the development of the soil profile. Also vegetation contribute to soil fertility by adding humus in the soil after dying and decomposing. Some plant roots (legumes) have nodules with bacteria that fix nitrogen into the soil. Plants roots modify the soil by increasing porosity, improving the soil depth and aeration.Micro - organisms are active in decomposition of the organic matter to form humus. Burrowing animals facilitate weathering process by loosening the soil particles. Lastly man’s activities like cultivation, break up the rocks into smaller fragments. Man also adds humus to the soil which contribute to soil profile development.
3. Climate: The important variables under climate include temperature, precipitation and wind. Temperature affects the rate of decomposition of organic matter, it contribute to soil profile development through weathering as well as the rate soil development. Where there is high temperature, soil development tends to be fast due to the fast rate of weathering, and where temperature is low, there is also a low rate of soil development due to the low rate of weathering process. Precipitation also affects soil profile development. In some areas soil is eroded, leading to soil profile destruction while in areas deposition leads to positive soil development due to accumulation of weathered materials and organic matter. Rainfall adds moisture, which facilitate both chemical and mechanical weathering and hence soil profile development. Wind has both positive and negative impacts on soil profile development. It can erode the soil through deflation leading to soil degradation or it can deposit some materials at the edge of the desert to form the soil called loess.
4. Relief (topography):The role of topography in soil formation is mostly indirect. It influences climate and vegetation. It controls the rate and nature of weathering, removal and deposition (redistribution) of the soil parent materials. The most important aspects of topography as a factor of soil formation are slopes; altitudes, aspects and location along the slope. Soil erosion is rapid on steep slopes and less on gentle slopes.Therefore on steep slopes soil profiles are shallow while on gentle slopes they are expected to be deeper. Also leaching are more pronounced in the upper-slope areas leading among other things to well-drained soil.Altitude affects soil mainly through the action of climate and vegetation. Altitude lowers temperature and increases precipitation. Thus leads to zonation of climate, vegetation and soil along hillsides. In terms of aspects, the side that receive more sunshine tends to have well developed soil than the side which receives low amount of sunshine since isolation accelerate plants growth and the weathering process.
5. Time: The time factor may be associated with the age of the country rock especially those on which soil have been directly formed. It may also be associated with the duration of the operation of soil formation process, that is whether the soil has sufficient time to form mature profiles and associated characteristics or not. When soil formation has taken a long and enough time, soil tends to be more mature, they are usually deep and well developed.

The Importance of Soil

Assess the importance of Soil

The following is importance of soil formation

1. Medium for plant growth.Soil is where most plants grow. Soil provides anchorage for the plants as well as protection of roots from damage.It is where or a medium through which water, air and nutrients are made available to plants. The well-aerated soil facilitates the absorption of water and nutrients from the soil by plants.
2. Soil support animal life.As soil support plant life it also support animal life because plants are the source of foods to animals and this is most for herbivores. Also some animals eat soil as food in form of salt licks for example pregnant women who lack some minerals in their bodies.
3. Soil provide habitat for living organisms.In the soil there are some animals living there example burrowing animals like rodents, earthworms and termites
4. Provide sites for agricultural activities.The fertile soil promotes agriculture activities, both animal husbandry and crop cultivation. This is because soil supports the growth of pasture for animals.
5. Provide settlement.Soil influences distribution of settlement for example the areas with good fertile soil are densely populated compared to the areas with poor soil.
6. Soil provide building materialsSoil is used in making bricks, tiles and white wash. All these materials are used in building houses, bridges etc. Also soil is used directly in road construction
7. Source of mineralsThere are some minerals found in soil that can be extracted for commercial purposes. Also it is used to manufacture fertilizers as it contain minerals for example the fertilizers that contain phosphate e.g. In Minjingu (Manyara) region.
8. It provides raw materials for pottery and ceramics.Soil is used in making pots and these help to provide income to those who engage in this activity.

Soil Composition and Properties

Soil Composition and Properties

Illustrate soil composition and properties

Soil is made up of the following components

• Organic matter (humus)
• Inorganic matters (minerals)
• Soil water and soil air

Organic matter (humus)

This form 5% of the total volume and is made up plant and animal remains. Humus is formed as a result of decomposition of animal/plants remains.

Importance of humus

• It improves the structure of the soil
• It regulates the soil temperature.
• It leads to higher agricultural production.

Inorganic matter (minerals)

This forms 45% of total volume and is made up of minerals from the parent rock. Minerals constitute some nutrients needed by plants for growth.

Soil water

This forms 25% of the total volume. It is one of the important soil constituents. It is derived essentially from rainfall especially from infiltration; Too much water in the soil leads to leaching and hence loss of nutrients.

Importance of water to soil

• It regulate the soil temperature
• It control the chemical process like chemical weathering as well as mechanical weathering
• It helps in the solution and transfer of nutrients in the soil.

Soil Air

This forms 25% of the total volume and constitutes the soil atmosphere from which plants and animals obtain oxygen for their metabolism. Air accelerates oxidation and biological activities and aerated soil lead to good production while poor aerated soil will lead to poor production.

Soil Profile and Characteristics

Soil Profile

Define soil profile

Soil profile is the vertical section of the soil from the surface to the parent rock characterized in distinct layers (horizons), usually of different textures and colours. Ideal soil profile has horizons A, B, C.

Soil Profile and its Characteristics

Illustrate soil profile and its characteristics

Physical properties of the soil include soil profile, soil depth, soil colour, soil texture and soil structure

Chemical properties of the soil includes soil P.H, carbon exchange and leaching.

• A – Horizon – Is the top most layer which include organic matter to form Ao. This horizon varies in colour from place to place example dark to grey. It is also called the zone of eluviation from which the materials are washed down through leaching.
• B – Horizon – Is also called the zone of accumulation or illuviation. In this layer materials washed from the A – horizon are deposited or accumulated. Accumulation of washed down materials lead to formation of another layer called Hard Pan.Horizon A and B are also referred to as Solum (soil) such that A – horizon is the top soil and B - horizon is the sub soil
• C – Horizon is the partially weathered parent rock from which the soil develops. It is underlain by D – Horizon which is a fresh non - weathered parent rock.

Simple Soil Classification

Soil According to Textural Groups

Classify soil according to textural groups

SOIL DEPTH

Soil depth is the variation of soil from place to place due to materials influenced by the nature of the rock as well as duration of the soil forming processes. Soil is shallow especially in steep slopes due to erosion and in areas where the underlying rock is hard. Other places have deep soil due to soft parent rocks.

SOIL COLOUR

Soil colour is determined by the mineralogy composition from which the soil is derived and organic matter content. It varies from one place to another. Soil colour can be used for classification and description of soil of a certain place. The colours are grouped into:

• Dark (Black, grey, dark grey and dark brown)
• Bright (yellow, orange, red, redish brown)
• Light (whitish – grey, white)

Some soil can have one colour throughout like red earths and yellow brown.

SOIL TEXTURE

This refers to the degree of coarseness or fineness of the soil materials especially soil mineral particles. It can also be referred to as variation in the particle size.According to soil texture, soil can be classified as

1. Course sand 2 to 0.2 mm diameter
2. Fine sand 0.2 to 0.02 mm diameter
3. Silt 0.02 to 0.00 mm diameter
4. Clay less than 0.002 mm
5. Loan soil i.e. mixture of sand, clay and silt

IMPORTANCE OF SOIL TEXTURE

1. It influences the soil porosity, permeability, compaction, and structure
2. It influences plant growth
3. It influence cultivation during the agricultural activities
4. It influences soil resistance against erosion
5. It influences soil fertility

SOIL TEMPERATURE

Soil has certain degree of temperature and this tend to vary from place to place due to the variation in the climatic conditions. In cold areas like Tundra regions soil is also cold this is due to the small amount of insolation received there. In tropical areas soil are warm due to high intensity of insolation heating the surface.

IMPORTANCE OF SOIL TEMPERATURE

It controls biochemical and chemical processes especially the decomposition of organic matter and plant growth.

• It determines the existence of micro- organisms in a certain area.
• It controls the amount of moisture in the soil

SOIL POROSITY

This refers to the total volume of pores or spaces between particles of the soil materials in undisturbed soil.

Soil porosity is influenced by

• Soil texture – The finer the particles the greater will be the total surface area. Hence the soil with fine particles like clay has greater porosity.
• Structure of the soil also influences permeability.

SOIL STRUCTURE

This refers to the arrangement of soil particles into aggregate compound particles. Individual undisturbed soil is called ped. The aggregation of soil particles produce peds of different shapes and sizes when aggregation is absent as in loose sand soil, the soil is described as structure less. Strongest of the soil is influenced by its organic matter.

IMPORTANCE OF SOIL STRUCTURE

• It determines water retention capacity and aeration
• It is an indicator of soil fertility or sustainability for agricultural activities
• It influences plant growth by influencing the root penetration and water retention

CHEMICAL PROPERTIES

SOIL REACTION (PH)

Soil reaction is the term used to describe the degree of acidity and alkalinity in the soil and it results mainly from climate. This degree of acidity and alkalinity in the soil is expressed in the PH.

PH is the value which is measured in terms of the hydrogen ion concentration.PH scale runs from 1 to 14.The PH 7 is neutral.Any condition below 7 is acidic and any condition above 7 is alkaline.

IMPORTANCE OF SOIL PH

• It helps in determining the selection of crops and agricultural distribution
• It affects plant growth such that where there is too much acidity there will be poor plant growth. This is because the increase of acidity leads to the increase in leaching which affects soil structure.

LEACHING

This is another chemical property of soil referring to the process in which nutrients are washed down in solution from the top- soil layer. During leaching process the bases are washed down leading to concentration of hydrogen ions which in turn cause the increased acidity in the top soil.Leaching is very effective in wet conditions.

SIMPLE HYPOTHETICAL PROFILE FOR MATURE SOIL

The soil profile varies from one place to another depending on the variation in environment conditions.

For example under deciduous forest, soil with little organic matter can be produced (brown earth or brown forest soil) while in mid latitude grasslands deep black earth soil (chernozem) is formed.Chernozem has a lot of organic matter. In the desert area the soil profile usually lack the Ao horizon due to scarcity or absence of vegetation.

SIMPLE SOIL CLASSIFICATION

Soil classification refers to the grouping of soils according to specific characteristics, such as properties or factors like climate also soil can be classified according to the age, texture and colour. One common classification is that based on texture.

According to the soil texture triangle, there are three main textures namely sand, silt and clay. This is based on the size of their particles as discussed earlier. The percentage content of each one of these determines the type of soil according to texture. Note that sandy soil have sand content of over 45%, clay soil have above 27% while silt soil have silt content of above 40%.

Soil Texture Triangle

• SAND:This soil consists mainly of coarse and fine sand and contain very little amount of clay such that it is not sticky when wet and is loose when dry, percentage of sand is above 85, that of clay is up to 10 and silt is up to 15. When such soil is rubbed, it does not leave any film on the fingers.
• LOAMY SAND:This consist most of sand but with sufficient clay such that it gives it a slight plastic quality when it is very moist. When it is rubbed between fingers it leaves a slight film of fine material, sand particles account for 70% to 90%, clay up to 15% and silt up to 30%
• SANDY LOAM:This soil has high percentage of sand between 43% and 85% with clay content of up to 20% and silt up to 50%. It moulds easily when it is sufficiently moist but does not stick easily to the fingers.
• LOAM:In this soil, sand and silt dominate with an average of 40% each while clay account for about 20% on average. It moulds easily when it has sufficient moisture and does stick to the fingers to some extent.
• SILT LOAM:It has a high percentage of silt of between 50% and 87%, sand between 13% and 50% and clay up to 27%. It is moderately plastic and not very sticky it has a smooth soapy feeling due to high content of silt.
• SAND CLAY LOAM:This has over 45% sand, up to 28% silt and clay between 20% and 35%. It can be a bit sticky because of the clay content but quite porous because of the sand.
• CLAY LOAM:Sand content between 20% and 45% silt between 15% and 53% clay between 27% and 40%. This one has sticky distinction when moist because of clay.
• SILT CLAY LOAM:The amount of sand is between 27% and 60%, silt between 40% and 78% and clay between 27% and 40%. The high silt content makes it smooth and has a soapy feeling. It is less sticky than clay loam or silt clay.
• SILT:This have over 80% silt particles, up to 20% sand and less than 12% clay. It is predominantly smooth and has a typical soapy feeling of silt.
• SANDY CLAY:Sand between 45% and 65%, silt up to 20% and clay between 35% and 55%. In the presence of sufficient moisture this soil is plastic and sticky clay and sand are dominant.
• CLAY:The proportion of sand goes up to 45%, while that of silt is up to 40% clay account for above 40%. The soil is sticky when moist and has a plastic feel. It can be rolled into threads when moist and can be moulded into different shapes. And can retain fingerprints.
• SILT CLAY:Sand up to 20%, silt between 40% and 60% and clay between 40% and 60%. This soil is composed of almost fine particles throughout. It is smooth and has to some extent the soapy feel of silt but has a degree of stickiness because of the high proportion of clay.

Soil Erosion

Soil Erosion

Define soil erosion

In this subtopic there are various concepts to be discussed, these are definition of the term, agents of soil erosion and how they work, types of soil erosion and its effects on social and economic activities. Also, demonstrating ways of controlling soil erosion through the application of various conservation techniques.

Soil erosion is the wearing away, detachment and removal of soil materials from one place to another place through agents like water, wind and ice.

How Agents of Soil Erosion Work

Examine how agents of soil erosion work

There are several agents of soil erosion these are water, wind and ice.

WATER

Water is the most important agent of soil erosion , the erosion by water involves the following:

• Splash erosioncaused by moving water from rain, this tends to remove some of the materials from one place to another.
• Sheer erosionwhich involves the removal of the uniform cover of the soil by surface run-off on gentle slopes.
• Rills erosionthat leads to the formation of small channels called rills on the surface.
• Gully erosion,that leads to the formation of deep troughs called gullies due to severe under cutting
• River erosion,takes place in the specific Chanel called river valley

WIND

Wind is another agent of soil erosion. It takes place in arid and semi-arid or where soil is loose. The soil in such areas is dry, loose and unprotected because of the scarcity of vegetation. It is turned into dust which is then blown away by wind.

Types and Effects of Erosion to Social and Economic Activities

Describe types and effects of erosion to social and economic activities

There are main two types of soil erosion which are normal geological erosion and accelerated soil erosion

NORMAL GEOLOGICAL EROSION

Is the wide spread type of erosion that occurs wherever there is a natural flow of energy and matter on the earth’s surface without man’s influence. It is fortunately very slow and so not normally injurious to the soil cover of the world.

ACCELERATED SOIL EROSION

Is the type of erosion associated with man’s activities (man included). It is spectacular (very destructive) therefore it has attracted man’s attention. Its side effects include physical loss arising from the reduced crop yield and total crop failure and or wasted efforts and money spent on unsuccessful soil conservation projects.

EFFECTS OF SOIL EROSION

Soil erosion leads to the followings effects both socially and economically:

• Pollution of water bodies due to the introduction of materials eroded from the surrounding areas.
• Loss of fertility which in turn causes the reduction in yields or total crop failure
• Migration of people from areas affected to the areas which have not been affected by erosion.
• Over flooding of the rivers as a result of the creation of the small channels leading to the river system.
• Deforestation as a result of the death of plants due to the loss of soil
• Loss of habitat as a result of deforestation caused by the loss of soil
• Soil erosion can lead and accelerate rock weathering by exposing the underlying rock to the weathering agents like temperature
• It leads to the cost incurred in during the process of conserving the soil, which has been eroded.
• Soil erosion can destroy transport and communication systems like roads, railway lines and telephone posts
• It can lead to the destruction of houses, rendering people homeless.

Population Growth and Rate of Soil Erosion on the Quality of Life

Relate population growth and rate of soil erosion on the quality of life

As discussed above we can see that soil erosion can affect the quality of life of the people positively and negatively.

• When the region is severely affected by the soil erosion, where crop production is impeded, when useful soils are carried away, the region experiences shortage of food. This causes famine and malnutrition. With inadequate nutrition child mortality rate goes up and population growth is impeded.
• When the foundation of existing buildings and roads are eroded. Accessibility to areas is made difficult. Such areas are isolated in terms of social services such as hospitals and education. The general health and welfare of the people become poor leading to increase in mortality and lowering of population.
• When life becomes unbearable in the rural areas because of severe soil erosion, able-bodied persons especially men migrate to urban areas to other better areas in search of employment. This reduce population in the affected areas as well as the required man power to develop the areas.

The ways of Controlling Soil Erosion through the Application of Various Conservation Techniques

Demonstrate ways of controlling soil erosion through the application of various conservation techniques

The following are the some of the central measures that can be taken to control erosion:

• Afforestation and Reforestation:Afforestation is the planting of trees where no forest has been known to exist.Reforestation is the planting of trees on land that previously had a forest.
• Control of bush fires:When open grassland are burnt the soil is directly exposed to agents of erosion.
• Controlled open grazing:Overgrazing which is another cause of soil erosion should be avoided by mulching the number of livestock kept on any piece of land.
• Erecting brushwood barriers:On land where gullies have developed, barriers of brushwood or even stone walls can be constructed across the gullies to help trap the soil.
• Construction of cut-off drains:Cutoff drains are open trenches which are dug across the slope and soil is heaped on their down – slope sides to form a kind of ridges. These drains prevent large amount of water that might have resulted in formation of rill, gully and sheet erosion from down the slope.
• Constructing of dams and weirs:These are structure that are built across river’s valley for the purpose of controlling a river flow or for retaining water in a large reservoir within the valley. They may be made of earth’s (earth dam) and stone (rock fill dam).
• Use of artificial waterways:An artificial waterway is a small channel that is constructed down a slope and into which surface run-off collects. Normally water from cut-off drains as well as from terraces should be discharged into rivers or into non-erodible areas such as stony grounds.

• READ TOPIC 4: Elementary Survey And Map

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