Soil: Factors and Process of soil formation, Soil texture, structure and pH

CONTENT

• Factors of Soil Formation
• Process of Soil Formation
• Soil Structure, Texture and PH
• Experiments on Soil

FACTORS OF SOIL FORMATION

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

1. CLIMATE: Climate is the average weather condition of a place over long period time. The elements of climate are: temperature, rainfall, wind and pressure.
2. Temperature: The alternating cooling and heating of  rocks results in continual expansion and contract which eventually make the rock to crack and breakdown  to form Soil.
3. Rainfall: The action of running water from rainfall causes gradual wearing away of rocks during erosion to form soil. Rain drops provide water for hydrolysis, rainfall also breaks down some parent rocks to form soil.
4.  Wind: Rocks collide during the time of high wind velocity in desert, this collision results in breakdown of rocks to form soil.
5. Pressure: High pressure on hanging rock may cause it to fail down and break in 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 characteristic and chemical composition of the formed.

• TOPOGRAPHY: This is the sharp of the ground in relation to the underlying rock of the earth’s surface. It affects soil formation in the following ways.
• The shape of the land influences the movement and amount of water in the soil.
• Sloppy surfaces support erosion which encourages soil formation.
• 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;
• Termites, earthworm and rodents mix with the mineral and organic matter together to form soil.
• They aerate the soil, making air to react with rocks to form soil.
• The activity of man during tillage.
• Microorganism causes decomposition of organic matter to form soil.
• The root of plants penetrates the rocks there by breaking them.
• Organism produces carbon dioxide which promotes weathering of rocks.
• Microbe helps in decomposition of organic matter.
• The leaves which fall from trees decay to increase the humus content of the soil.

• TIME: all the above factors takes a lot of time to inally give rise to soil.
• It takes a long time for small piece of rocks to disintegrate into grains of soil.
• It takes a long time for plants to decay to form soil.
• It takes short time to form immature soil.

EVALUATION

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

1. PHYSICAL PROCESS: The agents of physical weathering are temperature, ice, wind, water and pressure
2. Temperature: The alternating heating and cooling of rocks produce pressure within the rock which makes them to break into smaller pieces.
3. Wind: Solid materials carried by soil surfaces.
4. Ice: The conversion of water inside the cracks of rocks into ice results in increase in volume. This results in more pressure on the rocks which eventually break into smaller pieces.
5. Water: Running water carries some fragments of rocks in the river bed, thus breaking off small pieces of rocks.

• CHEMICAL PROCESS: Agents of chemical weathering include: solution, carbonation, hydration, hydrolysis and oxidation.
• Hydration: It occurs when water combines with or binds to some minerals e.g.
• Calcium Sulphate changes to gypsum CaSO₄+2H₂0 → CaSO₄.H₂O
• Red Haematite changes to hydrated yellow Haematite or Limonite Fe203 +3H20 = Fe203.H20
• CuS04 + 5H20 = CuS04.5H 20
• Carbonation: The carbondioxide released to the atmosphere combines with acid. This weak acid reacts with rocks to form soil.
• Oxidation/Reduction:  When minerals containing iron, manganese and sulphur are exposed to air and water, the famous iron is oxidized to the ferric state.

4F0C03 + O2         =          2FeO3 + 4CO2

(Iron II Carbonate)      (Iron Oxide)

• Solution: reaction of water with soluble particles/minerals present in the rock and eventually displacing them given the sloe of the soil.
• 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;
• The root of plants penetrates the rocks to form soil.
• Earthworm and termites burrow into the rocks and break off fragments.
• Activities of man during soil tillage causes the rock to breakdown to form soil.

EVALUATION

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

SOME PROPERTIES OF THE SOIL

1. Soil Structure: This refers to the ways in which the different particles of the soil are packed or arranged. Good Soil structure will promote good yield of crops. Soil Structure can be preserved in the following ways:
2. Planting of cover crops
3. Application of manure
4. Avoidance of overgrazing
5. Mulching
6. Avoidance of clean clearing

The types of soil structures are:

• Single grained structure
• Crumb Structure
• Plate-like structure
• Spherodial structure
• Prismatic structure
• Block-like structure

IMPORTANCE OF SOIL STRUCTURE

• It determines how fertile a soil is.
• A good soil structure supports aeration.
• It also prevents erosion and water logging.
• A good soil structure promotes the activities of soil microbes.
• A good soil structure supports the growth of crops.

2. Soil Texture:

This is the relative proportion of various particles of the soil. It also refer 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 are usually referred to as primary particles of the soil. The name and sizes of the various soil particles are shown in the table below;

METHOD OF SOIL TEXTURE DETERMINATION

• By feeling
• By mechanical analysis through sieving
• By sedimentation
• By moulding

IMPORTANCE OF SOIL TEXTURE

• It is useful in evaluation of the ability of soil to supply mineral nutrients.
• It supports soil microorganism essential for crop growth.
• It determines the type of crop to grow on the land.
• It enables the farmer to know the type of soil on his farm.
• 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

• It determines the rate of formation and decomposition of organic matter.
• High temperature will impede the activities of micro-organisms.
• Optimum temperature promoted seed germination.
• It determines the population of soil microbes.
• It determines 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 use to determine soil acidity as shown below.

Note that;

• PH2 is strongly acidic
• PH6 is slightly acidic
• PH7 is neutral
• PH8 is slightly alkaline
• PH13 is strongly alkaline

CAUSES OF SOIL ACIDITY

1. Leaching
2. Use of acid fertilizers
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. Dolonite or Calcite
7. Basic slay
8. Gypsum

SOIL PROFILE Soil profile is defined as the vertical section of the soil, showing 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 top soil, 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 represent the type of material from which top soil 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 preserve of microorganism 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.

READING ASSIGNMENT

Essential Agricultural Science for Senior Secondary Schools by O.A. Iwena, Chapter 11, pages 65-87

WEEKEND ASSIGNMENT

1. A soil with PH6 is ____ (a) neutral (b) strongly acidic (c) slightly acidic (d) alkaline
2. Which of the following is not a liming material (a) limestone (b) quicklime (c) slake lime (d) urea
3. The horizon which contain 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) sand (c) silt (d) gravel
5. The breaking down of rock into smaller particles is called ___ (a) weathering (b) cracking (c) breaking  (d) hut

SECTION B

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

            (b) Define soil PH.