FACTORS OF SOIL FORMATION

The five major factors which control soil formation are: climate, parent materials, topography, biotic factors and time.

Climate

Climate is the average weather condition of a place over a long period. The elements of climate are: temperature, rainfall, wind and pressure.

1. Temperature: The alternating cooling and heating of rocks results in continual expansion and contraction, which eventually causes the rock to crack and break down to form Soil.
2. Rainfall: The action of running water from rainfall causes the gradual wearing away of rocks during erosion to form soil. Raindrops provide water for hydrolysis, rainfall also breaks down some parent rocks to form soil.
3. Wind: Rocks collide during the time of high wind velocity in the desert, and this collision results in the breakdown of rocks to form soil.
4. Pressure: High pressure on the hanging rock may cause it to fall and break into pieces, resulting in soil formation.

PARENT MATERIALS:

These are the materials (previously existing rocks) from which the soil is formed. The parent materials are igneous, sedimentary and metamorphic rocks.

They determine the type, physical characteristics and chemical composition of the formed.

TOPOGRAPHY:

This is the shape of the ground concerning the underlying rock of the Earth’s surface. It affects soil formation in the following ways.

1. The shape of the land influences the movement and amount of water in the soil.
2. Sloppy surfaces support erosion, which encourages soil formation.
3. More soils are formed in the valleys than on the hills.

BIOTIC FACTORS

The activities of soil living organisms help to speed up the process of soil formation in the following ways;

1. Termites, earthworms and rodents mix with the mineral and organic matter together to form soil.
2. They aerate the soil, allowing air to react with rocks to form soil.

The activity of man during tillage.

1. Microorganism causes the decomposition of organic matter to form soil.
2. The root of plants penetrates the rocks there by breaking them.
3. Organism produces carbon dioxide, which promotes the weathering of rocks.
4. Microbe helps in the decomposition of organic matter.
5. The leaves which fall from trees decay to increase the humus content of the soil.

TIME:

All the above factors take a lot of time to finally give rise to soil.

1. It takes a long time for a small piece of rock to disintegrate into grains of soil.
2. It takes a long time for plants to decay to form soil.
3. It takes a long time to form immature soil.

EVALUATION

1. List the factors of soil formation.
2. Explain how (i) topography, (ii) wind and (iii) temperature result in soil formation.

PROCESS OF SOIL FORMATION (WEATHERING)

Weathering is defined as the disintegration or breakdown of rocks into tiny particles called soil.

The processes of soil formation include:

1. Physical process
2. Chemical process
3. Biological process

PHYSICAL PROCESS: The agents of physical weathering are temperature, ice, wind, water and pressure

1. Temperature: The alternating heating and cooling of rocks produces pressure within the rock, which causes it to break into smaller pieces.
2. Wind: Solid materials carried by soil surfaces.
3. Ice: The conversion of water inside the cracks of rocks into ice results in an increase in volume. His results in more pressure on the rocks, which eventually break into smaller pieces.
4. Water: Running water carries some fragments of rocks in the river bed, thus breaking off small pieces of rock.

CHEMICAL PROCESS: Agents of chemical weathering include: solution, carbonation, hydration, hydrolysis and oxidation.

1. Hydration: It occurs when water combines with or binds to some minerals, e.g.
2. Calcium Sulphate changes to gypsum, CaSO₄+2H₂0 → CaSO₄.H₂O
3. Red Haematite changes to hydrated yellow Haematite or Limonite Fe203 +3H20 = Fe203.H20, CuS04 + 5H20 = CuS04.5H 20
4. Carbonation: The carbon dioxide released to the atmosphere combines with acid. This weak acid reacts with rocks to form soil.
5. Oxidation/Reduction: When minerals containing iron, manganese and sulphur are exposed to air and water, the iron is oxidised to the ferric state.

4F0C03 + O2         =          2FeO3 + 4CO2

(Iron II Carbonate)     (Iron Oxide)

6. Solution: reaction of water with soluble particles/minerals present in the rock and eventually displacing them, given the slope of the soil.

7. Hydrolysis involves the breaking down of chemical bonds in rock minerals by water.

CaSiO₃                        + 2H₂O                        → H₂SiO₃                   +          Ca(OH)₂

Calciumsilicate                  water                                 silica                                calcium

(Wollastonite)                                                                                                      silicate hydroxide

BIOLOGICAL PROCESS: This involves the activities of plants and animals in the breaking down of rocks to form soil. It can happen in the following ways;

1. The root of plants penetrates the rocks to form soil.
2. Earthworms and termites burrow into the rocks and break off fragments.
3. The activities of man during soil tillage cause the rock to break down to form soil.

EVALUATION

1. What is weathering?
2. List and explain the three processes of weathering.

SOME PROPERTIES OF THE SOIL

Soil Structure:

This refers to how the different particles of the soil are packed or arranged. Good Soil structure will promote a good yield of crops. Soil Structure can be preserved in the following ways:

1. Planting of cover crops
2. Application of manure
3. Avoidance of overgrazing
4. Mulching
5. Avoidance of clean clearing

The types of soil structures

1. Single-grained structure
2. Crumb Structure
3. Plate-like structure
4. Spheroidal structure
5. Prismatic structure
6. Block-like structure

IMPORTANCE OF SOIL STRUCTURE

1. It determines how fertile a soil is.
2. A good soil structure supports aeration.
3. It also prevents erosion and waterlogging.
4. A good soil structure promotes the activities of soil microbes.

A good soil structure supports the growth of crops.

Soil Texture:

This is the relative proportion of various particles of the soil. It also refers to the degree of fineness or coarseness of the various soil particles.

The particles which make up a soil sample include: gravel, sand, silt and clay, and are usually referred to as primary particles of the soil. The names and sizes of the various soil particles are shown in the table below;

 METHOD OF SOIL TEXTURE DETERMINATION

1. By feeling
2. By mechanical analysis through sieving
3. By sedimentation
4. By moulding

IMPORTANCE OF SOIL TEXTURE

1. It is useful in the evaluation of the ability of soil to supply mineral nutrients.
2. It supports soil microorganisms essential for crop growth.
3. It determines the type of crop to grow on the land.
4. It enables the farmer to know the type of soil on his farm.
5. It determines the movement of air and water in the soil.

 SOIL TEMPERATURE:

This refers to the hotness or coldness within the soil.

IMPORTANCE OF SOIL TEMPERATURE TO CROP GROWTH

1. It determines the rate of formation and decomposition of organic matter.
2. High temperatures will impede the activities of microorganisms.
3. Optimum temperature promoted seed germination.
4. It determines the population of soil microbes.
5. It determines the maturity and ripening of fruits.

SOIL PH

This is the measure of the degree of acidity or alkalinity of the soil. It can also be defined as the measure of the concentration of hydrogen ions (H+) in the soil. A PH scale is used to determine soil acidity as shown below.

Note that;

1. PH2 is strongly acidic
2. PH6 is slightly acidic
3. PH7 is neutral
4. PH8 is slightly alkaline
5. PH13 is strongly alkaline

CAUSES OF SOIL ACIDITY

1. Leaching
2. Use of acid fertilisers
3. Presence of acid parent materials
4. Nutrient uptake by plants
5. Presence of sulphur in the soil

REMOVAL OF SOIL ACIDITY

Soil acidity can be removed by the application of liming materials which are rich in calcium. Examples of liming materials are:

1. Slaked lime Ca(OH)2
2. Quick lime Cao
3. Calcium bicarbonate Ca(HCO3)2
4. Wood ash
5. Limestone CACO3
6. Dolomite or Calcite
7. Basic slay
8. Gypsum

SOIL PROFILE

Soil profile is defined as the vertical section of the soil, showing a series of horizontal layers of different types of soil. The horizontal layers are called HORIZONS

HORIZONS OF SOIL PROFILE

1. The A-Horizon: It is also called the topsoil, it is rich in organic materials; most soil organisms reside here.
2. The B-Horizon: It is rich in minerals, which are carried or leached down by percolating water.
3. Horizon C: It represents the type of material from which topsoil and subsoil are derived.
4. The D-Horizon: It is called the bedrock.

EXPERIMENTS ON SOIL

1. Sedimentation experiment.
2. Experiment to determine the moisture content of the soil.
3. Experiment to determine the porosity/water holding capacity of the soil.
4. Experiment to determine the capillarity of the soil.
5. Experiment to demonstrate the preservation of microorganisms in the soil.
6. Experiment to determine the percentage of organic matter in a soil sample.

GENERAL EVALUATION

1. What is soil?
2. List 5 factors of soil formation.
3. What is weathering?
4. Explain the processes of physical weathering.
5. Define (i) Soil acidity (ii) Soil structure (iii) Soil texture.

WEEKEND ASSIGNMENT

1. A soil with pH 6 is ____ (a) neutral, (b) strongly acidic,c (c) slightly acidic, (d) alkaline
2. Which of the following is not a liming material: (a) limeston,e (b) quicklime,e (c) slake lime, (d) urea
3. The horizon which contains organic matter is _____ (a) A (b) B (c) C (d) D
4. The soil with particle size 0.002 to 0.02mm is _____ (a) clay, (b) san,d (c) sil,t (d) gravel
5. The breaking down of rock into smaller particles is called ___ (a) weathering,g (b) cracking, (c) breaking, (d) hut

SECTION B

1. Describe the sedimentation experiment with the aid of an appropriate diagram.
2. (a) List five ways of removing soil acidity. (b) Define soil PH.

See also

SOIL

AGRICULTURAL INSURANCE

AGRICULTURAL FINANCING

ARTIFICIAL INSEMINATION

ANIMAL IMPROVEMENT