Download Factors influencing soil formation

Factors influencing soil formation
List of factors influencing soil formation
(i) High rate of weathering of the parent rock.
(ii) Low temperature
(iii) Sloping landscape
(iv) Impermeable parent rock
(v) Destruction of soil by burning
(i) Wind
The force of wind carries along smaller pieces of rock which hit against the surface of parent rock causing
it to disintegrate.
(ii) Chemical
Rain water dissolves Carbon (iv) Oxide in the atmosphere forming carbonic acid which dissolves
limestone in the parent rock causing it to disintegrate. This is chemical weathering.
(iii) Water
Moving water or ice has a grinding effect against the surface of the parent rock. This causes the rock to
disintegrate into smaller particles.
This is physical weathering.
(iv)Termites
These are living organisms which burrow through the soil breaking and mixing the particles. This is
biological weathering.
(v) Man’s Activities
Man’s activities such as mining, earth moving, mechanized cultivation etc. break the rock into smaller
particles. This is biological weathering.
EDAPHIC FACTORS INFLUENCING AGRICULTURE
Lesson objectives:
By the end of the lesson you should be able to
(i) give the definition of soil,
(ii) describe the process of soil formation,
(iii) describe the soil profile ,(iv) classify soil by its characteristics.
Soil (edaphic) factors influencing agriculture include
Soil formation influenced by soil profile at sloping landscape
The carrying away of soil particles down the slope resulting to high fertility zones.
Soil profile can influence vegetation and crops to be grown in a given area
Burning during land preparation can destroy soil microorganisms and humus
Factors which Increase soil Formation
(i) High rate of weathering of the parent rock
(ii) Sloping landscape
Factors which reduce soil formation
(i) Low temperature
(ii) Impermeable parent rock.(iii) Destruction of soil by burning.
Soil Formation
Soil formation takes place during a process called weathering
There are three forms of weathering:
* Physical weathering,
* Biological weathering,
* Chemical weathering
Physical weathering
This involves the disintegration of rocks into smaller fragments by physical weathering
agents
Agents of physical weathering;
Wind
The force of wind carries along small pieces of soil which hit against the surface of parent rock causing it
to disintegrate into smaller soil particles.
Water
Moving water or rain has a grinding effect against the surface of the parent rock. This causes the rock to
disintegrate into smaller soil particles.
Man activities
Mans activities such as mining, earth moving, mechanized cultivation etc. break the rock into smaller
particles. This is biological weathering.
The amount of nutrient in the soil depend on the presence living organisms in the environment.
Biological weathering
This involves the action of living organisms, such as animals and plant materials (roots, leaves, stem)
The roots force their way through the rock crevice coursing them to split.
Upon decomposition of plant materials a weak acid called humic acid is formed, which reacts with rocks
weakening them.
Agents of biological weathering
Termites, moles and earthworms among others
These are living organisms which burrow through the soil breaking and mixing the particles.
Chemical weathering
Rain water dissolves carbon dioxide in the atmosphere forming carbonic acid which dissolves limestone
in the parent rock causing it to disintegrate. The chemical weathering involves the following process:
Hydration
Soluble minerals in the rocks absorb water and expand weakening rock structures
The rock becomes porous and eventually disintegrates into smaller soil particles
Hydrolysis/ solution formation
Water reacting with soluble minerals in the rocks loosening and breaking them into smaller particles.
Oxidation
Reaction of rock minerals with oxygen to form oxide compounds which easily break up
Carbonation
Carbon dioxide dissolves in water forming weak carbonic acid which reacts with rocks
This includes all elements of weather such as temperature, rainfall and wind
Time
Well mature soils have good structure and are less eroded, while soils formed over a period of time tend
to be shallow and have a less stable soil structure.
Parent rock
Materials with granite and quartz minerals contribute to sandy soils while volcanic materials lead to clay
soils.
Destruction of soil by burning
This is poor practice that accelerates materials breakdown and damaging soil structure.
Soil Profile
Soil profile refers to the vertical arrangement of different layers of the soil from
the top surface to the bedrock.
These layers are also called horizons
Layers of the soil profile
Superficial layer -Consist of dry decaying and decayed organic matter
Top soil - Well aerated, darker, contains active organism and has most of the plant roots.
Sub soil - Compact, less aerated and has an impermeable layer called hard pan.
Weathered rock - Has rocks being broken down and has no humus
Parent rock - Has intact rocks and ponds of water
Physical properties of soil
These properties are recognized by seening or feeling as explained below
Clay: - This soil is smooth and it is made of small particles.
Sandy: This soil is coarse and is made of large particles.
It cannot stick together even when wet.
By feeling the two types of soil, clay and sandy when wet
you will tell that soil is made up of particles which have different sizes and are arranged in different ways
to stick together or to remain loose.
The particles that make soil are: *
Clay particles,
* Sand particles,
* Silt particles,
* Vegetative matter.
SOIL STRUCTURE
Soil structure refers to the physical appearance of the soil according to the way the individual soil
particles are arranged packed or aggregatated. It is a term used to describe the overall arrangement or
grouping of soil particles or aggregates.
The soil structure type is determined by general shape aggregate.
Soil structure type is determined by general shape of aggregate. Soil structure class is determined by the
size of the aggregate and
the soil structure grade is dependent up the stability or cohesiveness of the aggregates.
Type of soil structure is influenced by climate, living organism, topography, parent material and time.
Clay particles and humus influence soil structure by the way they cement or
build up the different particles into bigger and more stable aggregates. Secretions from plant roots may
influence soil structure.
TYPES OF SOIL STRUCTURE.
There are many types of soil structure. The most common include:
Single - grained Structure.
This is an elementary structure which forms no aggregates meaning that particles are not cemented
together.It is relatively non- porous with small or spherical particles mostly found in the topsoil or sandy
soil, arid climates and alkaline soils.
Crumby/ Crumb Structures
This soil consists of small, soft porous aggregate of irregular shape. The type of structure does not
closely fit together and such soils and water will pass through the pores quickly.
However, a good structure should not only have enough pore spaces for water and air to enter, but
should also be able to retain sufficient water in the soil for plant roots to use.
Granular Structure
This is the type of soil structure made of friable rounded aggregates of irregular shape called granules.
This structure can be found in the topsoil horizon 0f cultivated soils and in the sub-soil horizons of sols
under grass or bush. Granular structure is the only arrangement influenced by practical methods of tillage
and trampling
Prismatic and Columnar Structures
These soils structures have aggregates arranged in vertical, cylindrical columns or pillars. The pillars vary
in length and may reach a diameter of 15cm or more. When the tops of the pillars are rounded the
structures is referred to as columnar but the pillars have flat, level tops the structural pattern is
prismatic.The columnar structures are found in middle horizons of soils with poor drainage while prismatic
structures are common from the middle to the lower horizons of fine textural soils.
The platy structures is mostly found in top horizon of soil in the forest and in clayey soils. Usually in layers
on top of each other as illustrated below:
Blocky Structure
This structure resembles the blocks used in construction. The aggregates easily fit together along vertical
edges with their dimensions or less equal.
Influence of Soil Structure to Crop Production
The soil structure influences the pore space in the soil. When the particles are closely packed together,
the soil has very few pore spaces. The amount of air and water present in soil depend on the pore space
available. This implies that soils with closely packed particles are poorly aerated and drained. Most crops
do well in well aerated and drained soils except a few such as rice, which do well in water logged soils.
Soils with good structure ensure a balance between soil, air and water and are good for crop production.
The soil structure should allow free circulation of air by having enough pore spaces which can be
occupied by air as in granular or crumby structures.In such soils the plan roots and micro-organisms can
get the oxygen they need and carbon dioxide is expelled easily. The soil structure influences the water
holding capacity of soils. A good soils structure should hold enough water for plants use. Using heavy
machinery on wet soils destroys the structure, thus decreasing permeability and aeration. This may result
into high incidence of surface run off and erosion.
N/
B
Soil structure can be used to classify soils. It refers to be general arrangements of the soil particles or
aggregates. Depending on the aggregation, soil could have granular, clamby, prismatic, blocky or platy
structures.
SOIL TEXTURE
The term soil texture refers to the relative proportions of the various sized of minerals particles in a
sample of soils. These mineral particles include gravel sand slit and clay. It is also defined as the
coarseness or fineness of the soil when felt between the fingers. The texture of soil determines its ability
to absorb and retain water and soil nutrients.
SOIL PARTICLES AND THEIR SIZES.