GEOGRAPHY A LEVEL(FORM SIX) NOTES – PHYSICAL GEOGRAPHY 1.5 -STUDY OF SOIL

PHYSICAL GEOGRAPHY 1.5 – STUDY OF SOIL

2.1 CONCEPT OF SOIL AND SOIL PROFILE

Soil appears very complex, and different scholars (soil scientists) have developed various definitions of soil based on its nature. Soil is defined as:

The thin uppermost part of the earth’s surface consisting of water, air, organic, and mineral matter made by accumulation of weathered materials on which plants and animals live.

Or

The uppermost surface layer of unconsolidated (loose) materials which overlie the crustal rocks and on which plants grow.

Or

The naturally occurring substance forming the thin uppermost part (layer) of the earth’s surface made by the accumulation of weathered materials on which plants and animals live.

Or

A natural body of adjoining horizons of the parent materials which have undergone to a greater or lesser degree a natural change, under the influence of water, air, various species of organisms, and dead matter. This definition was developed by soil scientist Dukucheiv.

Or

A natural body synthesized into a profile formed from the parent materials acted upon by climate and organisms and modified by relief over a considerable period of time.

Or

A natural body resulting from interrelationship and interaction of several physical, biological, and chemical processes, all of which vary according to different natural environments.

Or

It is a loose top layer of our planet’s crust on which we live.

Or

It is a natural body of organisms, minerals, and organic constituents differentiated into horizons of variable depth which differ from the materials below in morphology, physical makeup, chemical properties and composition, and biological characteristics. It is the most universally accepted definition developed by Joffe (1949).

The term soil is derived from the Latin word Solum, which means ground.

The scientific study of soil, its origin, characteristics, and distribution is called Pedology. Hence, Pedology is defined as the scientific study of soil, including its origin, properties, significance, and distribution. Soil scientists are called pedologists.

IMPORTANCE OF SOIL

1. Soil is the medium in which most plants grow or root, and thus soil is used by humans to grow crops. Soil acts as a medium for plant growth in varied ways as follows:
   • Soil provides mechanical support to plants as their roots are held within the soil body.
   • Soil is important for the foundations of engineering structures, sewage disposal, gardens, and lawns.
2. Soil is important for industrial purposes, i.e., it is extracted and used as raw materials to manufacture certain industrial goods. For example, pottery, tiles, and brick industries use soils as chief raw materials.
3. Soil forms the most important base for the life of organisms. Hence, it is among the elements of the ecological system for living organisms. For instance, soil supports the life of plants and other organisms feed on plants.
4. Soil is used to bury dead bodies of human beings.
5. Certain soils contain minerals which can be extracted commercially. For example, apatite mineral is extracted from lateritic soils and used as an important raw material to manufacture phosphate fertilizers. This is done near Tororo in Uganda.
6. Soil facilitates the drainage of the surface as water supplied by rainfall is absorbed into it.

SOIL COMPONENTS

Soil, being a natural body forming the thin loose uppermost part of the earth’s surface, contains numerous materials. The materials contained in soils are collectively referred to as soil components. Soil components are the constituents of the soil body or materials contained in the soil body.

The materials contained in the soil body vary in nature. Some are organic, others inorganic. Some are in liquid state, while others are gaseous. Thus, by considering the nature of the materials, the soil body is recognized to have four major components, differing in their abundance within the soil body.

The main components present in the soil body include:

1. Mineral matter
2. Organic matter and living organisms
3. Water
4. Air

1. MINERAL MATTER

Mineral matter in the soil body includes all inorganic substances (particles) found in it. These materials are mainly small pieces of rock of different sizes, derived from parent materials by weathering.

The mineral particles present in the soil vary in size from smaller to larger ones. By considering the size of these mineral particles, soils are categorized into different types including the following:

• Gravel: 20mm – 200mm
• Fine gravel: 2mm – 20mm
• Coarse sandy: 0.2mm – 2mm
• Fine sandy: 0.02 – 0.2 mm
• Silty: 0.002 – 0.02mm
• Clay: less than 0.002mm

Mineral matter, in relation to others, has a volume abundance of about 45% of the total soil components.

2. ORGANIC MATTER AND LIVING ORGANISMS

Organic matter and living organisms together have a volume abundance of about 5% of the total soil components.

ORGANIC MATTER

Organic matter includes the remains of dead organisms (plants and animals) that have been fully or partially decomposed and mixed with the soil mass. Part of the soil body largely consisting of organic matter is known as humus.

Soil is supplied with organic matter from organic manures applied through agronomical practices. Other sources include dead plants, dead animals, and industrial organic waste products.

Soil organisms

The soil body contains living organisms. Life in the soil includes plants and animals. Plants are known as flora, while animals are known as fauna.

Fauna is categorized into micro and macro fauna depending on the size of animal organisms present in the soil body. Micro fauna include the smallest animals that cannot be seen by the naked eye unless a microscope is used, e.g., protozoa. Macro fauna include organisms which are relatively large in size and can be seen by the naked eye, e.g., insects and worms.

Similar consideration applies to flora as they are also of varied size and categorized into micro and macro flora. Micro flora include plant organisms which are quite small in size and not seen by the naked eye, while macro flora are plant organisms relatively large in size and easily seen by the naked eye.

IMPORTANCE OF ORGANIC MATTER AND LIVING ORGANISMS IN THE SOIL

1. Organic matter influences soil moisture by retaining water in the soil in varied ways, including:
   • Reducing the rate of evaporation from the soil.
   • Limiting the water percolation process.
2. Organic matter and living organisms influence various physical, chemical, and biological processes taking place in the soil body. For instance:
   • Physical processes like physical weathering done by the penetration of plant roots, especially of big trees.
   • Chemical processes like decomposition of materials in the soil body by soil living organisms.
   • The presence of organisms and organic matter in the soil forms biological roles.
3. Organic matter helps the process of soil aggregation to bind soil particles together. The remains of dead organisms act as a glue to bind soil particles together to form aggregates, hence improving soil structure.
4. Organic matter reduces the plasticity of the soil. Certain soils readily turn plastic once excessively saturated with water, and such soils pose a number of disturbances. But soils with organic matter have much minimized plasticity.
5. The remains of dead organisms provide good habitable environments for the life of soil organisms. For instance, earthworms make soil a habitable environment with organic matter.
6. Organic matter adds more plant nutrients to the soil body released from tissues of dead plants. For instance, nitrogen, sulphur, magnesium can be supplied and mostly act as the storehouse of exchangeable cations. The process for nutrients being released into the soil from the broken tissues of organic matter is known as mineralization.
7. Organic matter regulates the chemical condition of the soil through the release of minerals from their broken tissues.

3. SOIL AIR

The soil body contains air, which forms 25% of the total soil components. Air in the soil occurs in pore spaces (open spaces) of both micro and macro pores. The amount of air in the soil body depends on two determinant factors:

• The size of the soil particles. Usually, soil bodies with large size particles have more air than those with fine particles.
• The amount of water present in the soil body. When water occupies a pore space, it reduces the amount of air in the soil body because water also occurs in pore spaces. Hence, there is an inverse relationship between the amount of water and air in the soil body.

The kinds of air present in the soil body include:

• Oxygen: 20.25%
• Carbon dioxide: 0.25%
• Hydrogen and others: 79.5%

Importance of soil air

Air is needed to enable plants to manufacture their own food by photosynthesis. In the process of photosynthesis, water absorbed by plants is broken down by sunlight into hydrogen and oxygen, then hydrogen combines with carbon dioxide in a series of reactions to manufacture carbohydrates.

+ O sunlight carbohydrates + Oxygen.

Chlorophyll

4. SOIL WATER

The soil body contains water which is derived from rainfall, stream flow, and irrigation practices. The amount of water present in the soil is about 25% of the total soil components.

Types of soil water

1. Gravitational water: It is the amount of water that enters the soil and passes out vertically through the soil body by gravity. It normally causes the occurrence of leaching.
2. Field capacity soil water: It is the percentage of water remaining in the soil body after all gravitational water has been removed, i.e., water retained in the soil despite the force of gravity.
3. Wilting point soil water: It is the amount of moisture remaining in the soil when the soil reaches a point where its moisture content is such that plants fail to absorb enough moisture and start to wilt.
4. Available soil water: It is the amount of water held in the soil between the field capacity and the wilting point levels. This water can be absorbed by plants.
5. Unavailable soil water: It is the amount of water in the soil body below the level of wilting point. This water cannot be absorbed by plants and eventually causes them to die.

Importance of water in the soil body

1. Water acts as a solvent of various minerals in the soil body, making it easier for plants to absorb minerals from the soil in solution form.
2. It accelerates the process of weathering.
3. Soil water is used by plants to manufacture their own food by the photosynthesis biological process.

+ O Sunlight carbohydrates + oxygen.

Chlorophyll

4. Water regulates soil temperature.

5. It is needed for the activities of soil organisms to decompose the remains of dead organisms, i.e., water enhances biochemical processes taking place in a soil body.

NOTE: Water is inversely related to the amount of air in the soil.

5% – Organic matter, 45% – Mineral matter, Water – 25%, Air – 25%.

2.2 SOIL FORMATION (SOIL DEVELOPMENT)

Soil largely consists of mineral particles derived from parent rocks by weathering. Soil formation is defined as the evolution (genesis) of soil from parent rocks under the control of both active and passive factors through a number of processes. The whole process for soil development is known as pedogenesis and it is continuous.

Exposed surface rocks are broken down by weathering to produce simpler unconsolidated materials known as regolith. The regolith further breaks up into simpler materials known as mineral substrates which then mix with other soil-forming materials of water, air, organic matter, and organisms to form a thin loose layer of soil supporting plant growth.

Supported plants help further development of soils by attracting animals. When plants and animals die, decay, and add organic matter into the soil. Over decades or centuries, soils get fully developed and support a wide range of plant species.

Soil formation is a complex process involving a wide range of physical, chemical, and biological processes.

STAGES IN THE DEVELOPMENT OF SOIL

Critical points of soil development are known as stages of soil formation.

The first stage is the accumulation of a layer of loose, broken, unconsolidated parent materials called regolith.

Regolith can be derived from in situ or transported materials.

Regolith in situ results from exposed surface rocks broken into simpler materials by weathering.

Transported regolith is brought into the area by running agents including alluvium by fluvial action, till by glacier, loess by wind, or volcanic ash.

The second stage is the formation of true soil or topsoil resulting from the addition of water, gases, organisms, and dead matter.

PROCESSES FOR SOIL FORMATION

Natural activities involved in soil formation are broadly categorized into simple and complex processes.

• Simple processes organize on their own and play particular roles in soil formation, sufficient to perform a role without involving others. Some result in soil horizons.
• Complex processes involve combinations of other processes, producing distinctive soil types and operating in varied climatic regions.

Simple processes for soil formation

1. Weathering

Gradual weakening, disintegration, and decomposition of rocks into simpler particles under weather forces like rainfall and temperature changes. Weathering produces regolith and mineral substrates, preparing materials for other processes. It is the most fundamental process in soil formation.

Weathering is categorized into:

a) Mechanical weathering

Disintegration of rocks into fragments without chemical change, through exfoliation, frost action, salt crystallization, and slacking.

b) Chemical weathering

Decomposition of rocks by chemical reactions involving water and atmospheric gases like oxygen and carbon dioxide, through carbonation, hydrolysis, oxidation, hydration, and solution.

c) Biological weathering

Disintegration of rocks by organic activities like root wedging.

2. Leaching

Removal of materials in solution or suspension downwards as water moves vertically through the soil by gravity. It involves eluviation (washing out) and illuviation (accumulation) of materials, contributing to soil horizons.

3. Humification

Process by which remains of dead plants and animals accumulate, decompose, and mix with soil to form humus, aided by soil organisms. Occurs rapidly in tropical humid areas and gradually in cool regions.

4. Organic sorting

Reorganizing mineral particles and organic matter to form soil aggregates, improving soil structure.

5. Mineralization

Breaking down dead plants and animals to release mineral nutrients into the soil for plant uptake.

6. Cheluviation

Dissolution and downward transport of minerals under the influence of chelating agents (organic acids from decomposed organic matter).

7. Capillary action

Upward movement of water causing deposition of materials to form layers depending on bedrock nature.

Complex processes for soil formation

Operate in different geographical regions with varied climates, forming distinctive soil types:

1. Podzolization
2. Laterization/ferralization
3. Calcification
4. Salinization
5. Gleization

1. Podzolization

Leads to podzol soils in cool wet climates. Involves humification, severe leaching of basic oxides, and limited leaching of organic materials to form acidic ash-grey soils called podzols.

2. Laterization / ferralization

Leads to lateritic soils in humid tropical and subtropical regions. Rapid organic matter decomposition and severe leaching produce reddish soils with little organic matter near the surface.

3. Calcification

Leads to calcific (calcareous) soils with calcite deposits in dry semi-arid areas. Limited leaching and upward movement of calcium carbonate by capillary action cause calcite accumulation near the surface.

4. Salinization

Salts are drawn upwards to the surface by capillary action forming saline soils in hot desert areas where evaporation exceeds precipitation, leading to poor plant growth and hard crust formation.

5. Gleization

Leads to immature soils (glei soils) in poorly drained areas where organic and inorganic matter are incompletely decomposed. Common in cold climates with frost, gentle slopes with impermeable rock, high water tables, and heavy rainfall.

FACTORS OF SOIL FORMATION

Soil formation is influenced by variables known as factors of soil formation:

Soil = f (PCROT) where:

• P – Nature of parent rocks
• C – Climate
• R – Relief
• O – Organisms
• T – Time

Factors are categorized into active (climate, organisms) and passive (relief, time, parent materials) factors. All factors are interdependent.

1. The nature of parent rocks

• Influences weathering speed; acidic rocks like granite resist weathering more than basic rocks like gabbros.
• Influences physical soil properties like texture, structure, permeability, and porosity.
• Determines mineralogical composition of soil.
• Influences occurrence of certain soil types, e.g., limestone leads to calcareous soils.

2. Climate

• Influences weathering and soil formation through temperature and rainfall.
• Favors vegetation growth, which contributes organic matter and biological weathering.
• Determines soil types globally, e.g., podzols in cool humid climates.
• Causes soil horizons through leaching and capillary action.

3. Relief (topography)

Relief affects soil formation by slope, aspect, and drainage:

• Steep slopes have slower soil formation and shallow soils due to erosion.
• Gentle slopes have faster soil formation and deeper soils.
• Level lands may be poorly drained, limiting soil maturity.
• Aspect affects moisture and temperature, influencing soil formation rates.
• Relief variation causes soil catena, a sequence of soil types down a slope.

4. Organisms

• Plants cause biological weathering by root penetration.
• Soil animals decompose materials by burying litter, consuming it, secreting enzymes, and producing carbon dioxide that forms weak acids aiding decomposition.

5. Time

Longer time allows soil to mature. Soil may take up to 400 years to form 10mm and 3,000–12,000 years for mature soil depth suitable for farming.

SOIL PROFILE

A soil profile is a vertical section of the soil body from the surface to underlying bedrock, showing horizontal layers called horizons.

The hypothetical mature soil profile has the following horizons: O, A, B, C, and D.

• O – Horizon: Organic horizon
• A – Horizon: Eluviation horizon
• B – Horizon: Illuviation horizon
• C – Horizon: Regolith horizon
• D – Horizon: Bedrock horizon

O – Horizon (Organic horizon)

Uppermost layer formed by accumulation of plant and animal materials, rich in organic matter, subdivided into recognizable undecomposed material and humus.

A – Horizon (Eluviation horizon)

Zone of depletion where materials are leached downward, containing organic matter and lying below O horizon.

B – Horizon (Illuviation horizon)

Layer of accumulation receiving materials from above horizons, with little organic matter.

C – Horizon (Regolith horizon)

Consists of weathered parent materials where soil formation begins, with no organic activity.

D – Horizon (Bedrock horizon)

Consists largely of unweathered bedrock.

Note: O, A, and B horizons form the proper soil, while C horizon forms subsoil.

SOIL CATENA: Sequence of soil types down a slope related to topography.

DEVELOPMENT OF SOIL PROFILE

Soil profile development is influenced by climate, especially precipitation and temperature. Regolith breaks down into mineral substrates which mix with water, air, and organic matter to form topsoil. Leaching causes eluviation and illuviation forming A and B horizons. Organic material accumulation forms O horizon.

CLIMATE AND SOIL PROFILE

Climate influences soil profile types:

a. Cool humid climate

Produces podzolic soil with a well-defined O horizon due to slow organic matter decomposition.

b. Humid tropical and subtropical climate

Produces lateritic soil with little organic matter near the surface due to rapid decomposition and leaching.

c. Hot desert climate

Produces saline soil profiles due to upward movement of water and salts (salinization).

d. Polar climate

Produces glei soils with incomplete decomposition due to poor drainage and frost.

e. Dry and semi-arid climate

Produces calcareous soils with calcite deposits due to limited leaching and capillary action (calcification).

Importance of soil profile

• Determines depth of topsoil important for plant growth and microorganisms.
• Influences root penetration.
• Affects drainage and aeration.
• Determines water holding capacity.
• Influences soil fertility.

SOIL PROPERTIES

Soil properties are broadly categorized into physical, chemical, and biological properties.

A. PHYSICAL SOIL PROPERTIES

Include texture, structure, colour, temperature, porosity, density, depth, and others.

1. SOIL TEXTURE

Fineness or coarseness of soil determined by relative proportion of soil particles of different diameters.

Assessed by:

• Sense of feel
• Particle size analysis

Sense of feel method

• Gritty feel: sandy soil (coarse textured)
• Flour feel: silt soil (medium textured)
• Plastic feel: clay soil (fine textured)

Particle size analysis method

IMPORTANCE OF SOIL TEXTURE

• Determines root penetration resistance.
• Determines water infiltration rate.
• Influences soil erosion susceptibility.
• Influences soil fertility and nutrient holding capacity.
• Influences permeability, compaction, structure, porosity, and water retention.

2. SOIL STRUCTURE

Arrangement (aggregation) of soil particles into shapes.

Two kinds: single grained (particles independent, e.g., sandy soil) and massive (particles cemented into clumps).

Clumps are called aggregates (large) or peds (small). Fine textured soils form aggregates more easily.

Shapes include:

Platy Aggregates

More horizontal than vertical dimension.

Prism Aggregate

More vertical than horizontal.

Brocky Aggregate

All dimensions nearly equal.

Sphere Aggregate

Nearly round.

IMPORTANCE OF SOIL STRUCTURE

• Influences aeration and drainage.
• Influences seed emergence and plant growth.
• Influences cultivation ease.
• Indicates soil fertility.

3. SOIL COLOR

Soil colour varies widely, influenced by organic matter, mineral composition, leaching, and climate.

• High organic matter: dark brown soil.
• Hydrated iron minerals: reddish soil.
• Salt minerals: lighter soil.
• Manganese oxide: black soil.
• Glauconite: greenish soil.
• Calcite: white soil.

Leaching causes lighter colours; illuviation causes darker colours. Climate influences colour patterns.

Significance of soil colours

1. Indicate soil fertility.
2. Indicate soil moisture.
3. Indicate climate patterns.
4. Indicate mineral content.

4. SOIL POROSITY

Total space not occupied by solids, filled with water and air.

Types of pores:

• Macro pores: large, allow free air and water movement.
• Micro pores: small, hold water.

Factors influencing porosity:

1. Particle size.
2. Organic matter content.

Importance:

• Affects drainage, aeration, and fertility.

5. SOIL TEMPERATURE

Degree of heat in soil, influenced by solar radiation, soil colour, and ground cover.

Significance:

• Determines existence of soil organisms.
• Controls biochemical processes.
• Controls soil moisture.
• Influences horizon formation.
• Influences plant growth.

6. SOIL DENSITY

Weight per unit volume of soil.

(i) Particle density: weight per unit volume of soil solids.

= gm/cc

(ii) Bulk density: weight per unit volume of whole soil sample.

= gm/cc

Bulk density affected by organic matter, granulation, compactness, and cultural practices.

Note: Other physical properties include aeration, soil water, and soil depth.

B. CHEMICAL SOIL PROPERTIES

Include soil reaction, leaching, cation exchange, soil colloids, and soil nutrients.

1. SOIL REACTION

Degree of acidity, alkalinity, or neutrality based on hydrogen (H⁺) and hydroxyl (OH⁻) ion concentration, expressed as pH.

• More H⁺: acidic soil.
• More OH⁻: alkaline soil.
• Balanced: neutral soil.

Assessment methods:

• Electrometric method (pH meter)
• Colorimetric method (dyes)

Causes of soil acidity

• Leaching of basic oxides (Ca, Mg, K, Na).
• Microbial activities producing organic acids.
• Use of acidic fertilizers like ammonium sulphate.

Causes of soil alkalinity

• Presence of basic oxides.
• Insufficient rainfall preventing leaching.

Agronomical significance

• Determines suitability for plants and microbes.
• Influences organic matter decomposition.
• Helps select crops based on tolerance.
• Affects nutrient availability.
• Guides soil amendment needs.

Amendments

Liming to reduce acidity; eradication and conversion to reduce alkalinity.

2. LEACHING

Washing out of minerals downwards by water percolation, dominant in humid regions.

Influenced by particle size, climate, slope, and vegetation.

Effects

• Decline in fertility.
• Increased acidity.
• Soil colour changes.
• Formation of horizons.
• Reduced microbial activity.
• Poor plant growth.

3. CATION EXCHANGE

Exchange of cations (Ca, Mg, K, Na) with hydrogen ions in soil, influenced by ion concentration and reactivity.

4. SOIL COLLOIDS

Minute substances that remain dispersed in liquid, negatively charged, attracting cations, exhibiting ion exchange.

5. SOIL NUTRIENTS

Chemical elements essential for plant growth, sourced from weathering, organic matter decomposition, fertilizers, and rainwater reactions.

C. BIOLOGICAL SOIL PROPERTIES

Include soil organisms and organic matter.

1. SOIL ORGANISMS

Soil supports organisms of various sizes, including micro and macro flora and fauna.

IMPORTANCE OF MICROBIAL ACTIVITIES

• Nutrient supply: Mineralization of dead tissues.
• Nitrogen balance: Nitrogen-fixing bacteria and algae supply nitrogen.
• Organic matter supply: Breakdown of residues.
• Soil structure improvement: Organic by-products bind particles.
• Symbiosis: Microorganisms benefit hosts, e.g., legume bacteria.

2. ORGANIC MATTER

Remains of dead plants and animals partially or fully decomposed and mixed with soil.

SOURCES

• Organic manures (compost, farmyard manure).
• Plant remains (roots, shoots, grasses).
• Industrial organic waste.
• Dead animals.

FACTORS INFLUENCING ORGANIC CONTENT

1. Aeration: Better aeration speeds decomposition.
2. Moisture and temperature: Adequate moisture and 20–29°C favor decomposition.
3. Topography: Level soils richer than slopes.
4. Fertility: Fertile soils have more organic matter.
5. Soil reaction: Slightly acidic to alkaline soils favor microbes.
6. Ecological system: Forests and grasslands add more organic matter.

IMPORTANCE

• Retains moisture.
• Helps aggregation.
• Reduces plasticity.
• Provides habitat for organisms.
• Adds nutrients.

2.3 SOILS CLASSIFICATION

Science of grouping soils by properties and factors.

EMPIRICAL SYSTEMS

Based on properties like texture and colour.

INTEGRATED CLASSIFICATION SYSTEMS

Considers soil development and orders.

A. CLASSIFICATION BY TEXTURE

Engineers use the Unified Soil Classification System (USCS).

I) SANDY SOILS

• Easy to cultivate
• Well drained and aerated
• Vulnerable to erosion
• Limited organic matter

II) SILT SOILS

• Retain moisture
• Vulnerable to erosion
• May cement in rain

III) CLAY SOIL

• High nutrients and organic matter
• Difficult to plough
• Prone to waterlogging
• Difficult root penetration
• Expand and shrink

IV) LOAM SOILS

20% clay, 40% sand, 40% silt for balanced properties.

B. CLASSIFICATION BY ORDER

Based on dominant factors, devised by USDA:

• Zonal soils
• Intrazonal soils
• Azonal soils

I. ZONAL SOILS

• Influenced by climate and vegetation
• Mature with distinctive profiles
• Include tropical, temperate, and tundra soils

a. Tropical Soils

• Ferralitic (Lato) soils: reddish, deep, silica leached
• Ferruginous soils: lateritic, dark brown, harden seasonally
• Desert soils: alkaline, grey, low moisture

b. Temperate Soils

• Podzols: grey acidic soils in cold coniferous forests
• Brown earths: rich organic matter in deciduous forests
• Chernozem: black earth in grasslands
• Prairie soils: transitional dark brown soils
• Chestnut soils: alkaline in arid climates

c. Tundra soils

Little humus in cold climates.

II. INTRAZONAL SOILS

• Dominated by local factors like parent rock or drainage
• Include calcareous, hydromorphic, and halomorphic soils

III. AZONAL SOILS

Young soils without well-developed characteristics, often on steep slopes.

• Lithosols (stony soils)
• Rendosols (sandy dunes)
• Mountain soils
• Volcanic soils

SOIL EROSION

Wearing, detachment, and removal of soil by water, wind, and ice.

TYPES

By occurrence:

• Geological erosion
• Accelerated erosion

By agents:

• Wind erosion
• Water erosion

Geological erosion

Natural erosion before human activity.

Accelerated erosion

Caused by human activities like deforestation and overgrazing.

Wind erosion

Removal of topsoil by wind in dry areas.

Water erosion

1. Splash erosion: Soil dislodged by raindrops.
2. Sheet erosion: Uniform removal of surface layer.
3. Rill erosion: Small channels formed by runoff.
4. Gully erosion: Enlarged gullies from rill erosion.

FACTORS INFLUENCING SOIL EROSION

NATURAL

1. Climate: Rainfall and wind intensity.
2. Ground cover: Vegetation protects soil.
3. Topography: Slope steepness.
4. Soil characteristics: Texture, structure, organic matter.

MAN MADE

• Overgrazing
• Monoculture
• Burning
• Deforestation
• Growing crops in low rainfall areas
• Ploughing along slope
• Mining
• Engineering works

EFFECTS OF SOIL EROSION

1. Loss of fertility
2. Low crop yields
3. Shallow soils
4. Decrease in forestland
5. Destruction of houses
6. Difficult cultivation
7. Hinders navigation
8. Floods
9. Waterborne diseases

CONTROL OF SOIL EROSION

• Crop rotation
• Contour farming
• Terracing
• Planting trees and grasses
• Mulching
• Hillside ditching
• Cover cropping
• Green manuring
• Controlled grazing

2.4 SOIL FERTILITY

Ability of soil to support plant growth by supplying nutrients, water, and air in balanced ratios.

FACTORS INFLUENCING SOIL FERTILITY

SOIL TEXTURE

Influences water and nutrient holding capacity and aeration.

DEPTH OF SOIL PROFILE

Determines root penetration and supply of nutrients.

POSITION OF GROUNDWATER TABLE

High water table causes poor drainage and air exclusion.

SOIL STRUCTURE

Affects moisture and aeration.

SOIL REACTION

Influences organic matter decomposition rate.

ORGANIC MATTER

Acts as conditioner and nutrient source.

COMPOSITION OF PARENT MATERIALS

Influences supply of inorganic nutrients.

2.5 LOSS OF SOIL FERTILITY

Decline due to degradation, pollution, and erosion.

Causes

1. Soil erosion
2. Leaching
3. Waterlogging
4. Flooding
5. Burning
6. Weeding
7. Harvesting
8. Mass wasting

Effects

• Threat to marginal areas
• Lower productivity
• High agricultural costs
• Land conflicts
• Downstream flooding
• Economic slump
• Over cultivation
• Biodiversity loss

METHODS OF MAINTAINING SOIL FERTILITY

• Agronomical practices
• Addition of organic matter
• Addition of inorganic fertilizers
• Control of erosion

USE OF AGRONOMIC PRACTICES

Good farming methods maintain organic matter.

Main practices:

CROP ROTATION

Growing different crops in cycles to maintain nutrient balance.

Principles:

• Different crops each season.
• Rotate shallow and deep-rooted crops.
• Include different families.
• Legumes followed by valuable crops.
• Include fallow periods.
• Use annual crops.

SEASONAL CROP ROTATION TABLES

Advantages

• Prevents erosion.
• Maintains fertility.
• Controls weeds, pests, and diseases.
• Provides diversification.
• Balances nutrient use.

2. CONTOUR FARMING

Planting and ploughing along contour lines to reduce runoff and erosion.

• Checks runoff speed.
• Retains soil moisture.
• Improves productivity.

Disadvantage: heavy labor.

3. TERRACING

Making embankments across slopes to control runoff.

• Checks soil washout.

Disadvantage: heavy labor.

4. MULCHING

Covering soil with plant remains to conserve moisture and reduce erosion.

Advantages

• Conserves moisture.
• Reduces erosion.
• Adds organic matter.
• Improves structure.
• Controls weeds.

Disadvantages

• Labor intensive.
• Fire risk.
• May compete for nitrogen.
• Requires land for mulch crops.

5. DESTOCKING

Reducing livestock to prevent overgrazing.

6. AFFORESTATION AND REFORESTATION

• Prevents floods.
• Slows runoff.

7. FERTILIZATION

Adding organic or inorganic manures to improve fertility.

Organic manures advantages

• Add nutrients.
• Improve structure.
• Conserve organic matter.
• Encourage microbes.

Limitations

• Storage and transport issues.
• Variable composition.
• Unbalanced nutrients.

Inorganic fertilizers include sulphate of ammonia, ammonium nitrate, urea, superphosphates, potassium sulphate, and NPK.

Limitations

• Do not conserve organic matter.
• Do not improve structure.
• Do not encourage microbes.

8. IRRIGATION TECHNOLOGY

• Maintains soil moisture.
• Renews vegetation.

9. COVER CROPPING

Growing cover crops, often legumes, between rows to maintain moisture and organic matter.

Qualities

• Non-competitive with main crops.
• Grow on poor soil.
• Drought resistant.
• Not alternate pest hosts.

Advantages

• Protect soil from evaporation and erosion.
• Control weeds.
• Improve nitrogen content.
• Add organic matter.

Problems

• May compete for water and nutrients.
• May host pests.

10. STRIP CROPPING

Planting crops and trees in alternate strips to prevent bare fields and reduce erosion.

11. FALLOWING

Leaving land to rest to restore fertility.

12. GREEN MANURING

Growing and incorporating green crops to add organic matter and nitrogen.

Qualities

• Grow on poor soil.
• Drought resistant.
• Fast growing.

Advantages

• Improves fertility.
• Brings nutrients from subsoil.

Problems

• May use excessive water in low rainfall areas.

LAND CULTIVATION BEFORE RAIN SEASON

• Reduces runoff and erosion.
• Maintains moisture.

14. HILLSIDE DITCHING

Making ditches along contours to control runoff.

• Controls erosion.
• Maintains moisture.

Disadvantage: labor intensive.

15. INTERCROPPING

Growing leguminous crops to fix nitrogen and improve soil quality.

16. AGRO-FORESTRY

Planting trees within farms as windbreaks to reduce erosion.

17. GOVERNMENT POLICY

Formulating policies for community participation, proper land use, and enforcement.