AGE 202 — Introduction to Soil Science: Lecture Note

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AGE la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS

AGE la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Course code: AGE 202 Course title: Introduction to Soil Science Course credit load: 2 Units Series 1: Origin and Formation of Soils • Part 1.1: The Nature and Importance of Soil o Sub Part 1.1.1: Definition and importance of soil o Sub Part 1.1.2: Components of soil o Sub Part 1.1.3: Soil as a dynamic system • Part 1.2: Soil-Forming Factors o Sub Part 1.2.1: Parent material o Sub Part 1.2.2: Climate and organisms o Sub Part 1.2.3: Topography and time • Part 1.3: Soil-Forming Processes o Sub Part 1.3.1: Weathering of rocks and minerals o Sub Part 1.3.2: Transformation, translocation, and accumulation o Sub Part 1.3.3: Humification and organic matter incorporation • Part 1.4: The Soil Profile o Sub Part 1.4.1: Concept of the soil profile and horizons o Sub Part 1.4.2: Description and characteristics of major soil horizons o Sub Part 1.4.3: Reading a soil profile in the field

AGE la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Introduction Soil is the thin layer of material covering the Earth's surface that supports plant life, filters water, cycles nutrients, and sustains the ecosystems on which agriculture depends. Yet soil is not simply ground-up rock: it is the product of millions of years of interaction between geological, climatic, biological, and topographic forces, and it continues to form and change today. Understanding how soils originate and develop is the foundation for everything else in soil science — from reading a soil profile in the field to selecting the right management practice for a specific soil type. The aim of this Series is to introduce you to the nature and importance of soil, the five major soil-forming factors, the processes by which those factors transform parent material into soil, and the concept of the soil profile as the record of soil formation and the primary tool for field soil description and classification. We shall commence this Series by examining what soil is and why it matters. Next, we will study the five factors that control soil formation. Following that, we will cover the weathering, transformation, and organic matter processes that build soil. Finally, we will wrap up with the soil profile — its horizons, their characteristics, and how to describe a profile in the field. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1 define soil and explain its importance to agriculture and the environment (Linked to Part 1.1), • SLO 1.2 identify and describe the four major components of soil (Linked to Part 1.1), • SLO 1.3 explain how soil functions as a dynamic natural system (Linked to Part 1.1), • SLO 1.4 describe the five soil-forming factors and explain the role of each in determining soil properties (Linked to Part 1.2), • SLO 1.5 explain physical and chemical weathering processes and their role in soil formation (Linked to Part 1.3), • SLO 1.6 describe the processes of transformation, translocation, and accumulation in the soil profile (Linked to Part 1.3),

AGE la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS • SLO 1.7 explain humification and the role of organic matter incorporation in soil formation (Linked to Part 1.3), • SLO 1.8 define the soil profile and describe the major soil horizons and their characteristics (Linked to Part 1.4), and • SLO 1.9 conduct and record a basic soil profile description in the field (Linked to Part 1.4). 1.1 The Nature and Importance of Soil 1.1.1 Definition and importance of soil Soil is the naturally occurring, unconsolidated surface layer of the Earth that has been altered by physical, chemical, and biological processes to support plant growth. It differs from the underlying parent rock or unweathered geological material by having a distinct composition, structure, and biological community. The definition of soil in the pedological (soil science) sense emphasises its role as a medium for plant growth; in the engineering sense, soil refers more broadly to any loose surface material regardless of biological activity. The importance of soil to human society is difficult to overstate. It is the foundation of food production, supporting 95 per cent of global food supply through crop and animal production. It is the primary water filter and reservoir, purifying water as it percolates through the soil profile and releasing it slowly to streams and aquifers. It cycles carbon, nitrogen, phosphorus, and other nutrients, regulating the chemical composition of the atmosphere and water. It provides physical support for infrastructure and buildings. And it is a reservoir of biodiversity, containing approximately a quarter of all living species on Earth. Soil degradation, through erosion, salinisation, compaction, and pollution, is one of the most serious environmental challenges facing Nigeria and the world. Fill in the blank: Soil is the naturally occurring, unconsolidated ________ layer of the Earth that has been altered by physical, chemical, and biological processes to support plant growth.

AGE la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Figure 1.1: Diagram showing the multiple roles of soil in food production, water filtration, nutrient cycling, and biodiversity support 1.1.2 Components of soil Soil is composed of four major components whose relative proportions determine most of its physical and chemical properties. Mineral matter (typically 45 per cent of the volume of a typical agricultural soil) consists of weathered rock and mineral particles of varying sizes (sand, silt, and clay); it provides the structural framework of the soil and is the primary reservoir of most plant nutrient elements. Organic matter (approximately 5 per cent) consists of living organisms, fresh plant residues, and decomposed humus; it profoundly influences soil fertility, structure, and water-holding capacity despite its small proportion. Soil water (approximately 25 per cent) occupies the pore spaces between solid particles; it is the medium through which dissolved nutrients are transported to plant roots and through which most chemical reactions in the soil occur. Soil air (approximately 25 per cent) also occupies pore spaces; it provides the oxygen required for root respiration and aerobic soil microorganism

AGE la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS activity. The proportions of water and air are inversely related and vary constantly with rainfall, drainage, and plant uptake. The mineral and organic fractions are relatively stable over time, while water and air contents are highly dynamic. Fill in the blank: The four components of a typical agricultural soil are mineral matter (45%), organic matter (5%), soil ________ (25%), and soil air (25%). Figure 1.2: Pie chart or diagram showing the four components of a typical agricultural soil by volume percentage

AGE la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS 1.1.3 Soil as a dynamic system Soil functions as a dynamic natural system — a continuously changing body in which physical, chemical, and biological processes interact and influence one another. Energy inputs (solar radiation, rainfall) drive the physical and chemical weathering processes that release mineral nutrients. Biological activity (plant root growth, microbial decomposition, earthworm burrowing) transforms organic materials, creates pore structure, and mediates nutrient cycling. Chemical reactions (mineral dissolution, ion exchange, precipitation, oxidation-reduction) control the availability of nutrients and toxic elements. The dynamic nature of soil means that its properties respond to management: tillage changes soil structure; organic matter additions improve fertility and water retention; drainage alters the chemical environment; and erosion removes the most fertile surface horizons. This responsiveness to management is both the opportunity and the challenge of soil science — soils can be degraded rapidly through poor management but can also be improved through informed intervention. Understanding the interconnected processes operating in the soil system is essential for making management decisions that sustain productivity without long-term degradation. Fill in the blank: Soil functions as a ________ natural system in which physical, chemical, and biological processes interact continuously and respond to environmental inputs and management practices.

AGE la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Figure 1.3: Diagram showing the interactions between the physical, chemical, and biological components of the soil system Pilot questions 1.1 1. Define soil in the pedological sense and explain how it differs from unweathered parent material. 2. State four roles of soil in the natural environment and explain the significance of each. 3. Name the four components of a typical agricultural soil and state their approximate percentage volumes. 4. Explain why soil water and soil air are considered inversely related. 5. Explain what is meant by describing soil as a dynamic natural system.

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