Soil Science Notes NMSS125 PDF

Soil Science Notes Valerie Emma Grobler C23046 Diploma in Nature Management Soil Science (NMSS125) Ms. Cecile-Mari Meyer 23 July 2024 Glossary A-horizon: The mineral horizon with a defined structure and is mostly made up of humus. Additions: Organic material deposited on the soil from above or below the surface. Adhesion: The sticking together of particles of different substances. Aggregates: Arrangement of primary soil particles that cohere around one another more strongly than other surrounding particles. Anions: A negatively charged ion. Apedal: A soil with no distinguishable peds when moist. B-horizon: Otherwise known as subsoil, which accumulates minerals, clays, and other materials leached from the overlying horizons. Catena: The sequence of different soil profiles that occur down a slope. Cations: A positively charged ion. C-horizon: This horizon represents the unaltered parent material from which the soil developed, exhibiting minimal evidence of soil-forming processes. Clastic: Rocks composed of broken pieces of older rocks. Cohesion: The sticking together of particles of the same substance. Core: The innermost part of a terrestrial planet. Crust: The outermost shell of a terrestrial planet. Desertification: The process by which natural or human causes reduce the biological productivity of drylands. Desorption: The release of ions or molecules from soil solids into solution. E-horizon: The eluviation horizon, characterised by the loss of fine particles, organic matter, and soluble minerals through leaching processes. Erosion: The process in which rock, soil, and mineral particles are carried away by wind and water. Evapotranspiration: Water transfer from the ground to the atmosphere in the form of evaporation and by transpiration from plants. Flocculation: The process in which soil particles dispersed in a solution contact and adhere to each other. Geology: The science that deals with the dynamics and physical history of the earth, the rocks of which it is composed, and the physical, chemical, and biological changes that the earth has undergone or is undergoing. Horizon: The parallel layer to the soil surface, whose physical and chemical properties differ from the layers beneath and above this layer. Humification: The breakdown of organic materials in soils leading to the formation of humus. Igneous rock: Rocks that are formed from the cooling and solidification of magma or lava. Lava: Molten rock that is ejected by volcanic eruption or flows and occurs in liquid form. Leaching: The loss of soluble substances from the top layer of soil by precipitation and percolation. Lithification: The conversion of unconsolidated sediments into solid rock. Magma: Molten rock that is formed at extremely high conditions inside the earth. Mantle: The mostly solid bulk of a terrestrial planet's interior. Metamorphic rock: Rock that has been substantially changed from the original igneous, sedimentary, or other metamorphic form. Mineral: A solid substance with a well-defined chemical composition and crystal structure that occurs naturally. 1 Mineralogy: The study of the chemistry and physical properties of the mineral constituents of rocks. O-horizon: The soil layer with a high percentage of organic material. Parent material: The substance/underlying geological material in which soil horizons form. Pedal: A soil made up of visible peds when moist. Pedogenesis: The study of the origin and formation of soil. Pedoturbation: Mixing within a soil or sediment profile by various natural processes, such as animals burrowing into or through soil. Peds: The aggregates of soil particles formed as a result of pedogenic processes. Permeability: The capacity of the soil to allow water to pass through it. Podzolisation: A dominant soil-forming process. Polarity: The distribution of electrical charge over the atoms joined by the bond. Porosity: The amount or degree of open spaces between soil particles. R-horizon: The horizon comprising unweathered bedrock or geological material with minimal signs of weathering or alteration. Salinisation: Increasing salt content in soil. Sedimentary rock: Rock that is formed from pre-existing rocks or pieces of once-living organisms. Sesquioxides: The weathering products of iron and aluminium-containing silicates. Tillage: The preparation of land for agricultural processes. Translocations: The movement of soil constituents within the profile or between the horizons. Transformations: Chemical weathering of sand and formation of clay materials, transformations of coarse organic material into decay-resistant organic compounds. Volcanism: The eruption of magma (molten rock) onto the earth’s surface. Weathering: The breaking down of rocks and minerals on the earth’s surface without displacing them. 2 Unit 1: Geology and the Earth 1. Introduction  Soil is one of our most vital natural resources, forming the core of terrestrial ecosystems and serving as the foundation for plant growth.  Since plants are the primary food source for all animal life, understanding soil is crucial for maintaining environmental balance.  Soil comprises both inorganic components—such as small rock fragments, minerals, and nutrients—and organic components, including organic matter and microorganisms, as well as air and water. 2. Importance of Soil Study for Nature Managers  A deep knowledge of soils enhances understanding of vegetation, explaining why different types react differently to usage and require distinct management approaches.  It's also important to comprehend how soil itself responds to various uses and management practices.  Soil Potential: Understanding soil types helps determine the vegetation they can support and their production potential, such as grasslands, bushveld, sweetveld, or sourveld.  Limitations and Dangers: Recognising issues like erosion potential, water retention, clay soil behaviour (swelling and shrinkage), and inundation in valleys is essential.  Management: Effective soil management involves treating soils with similar characteristics uniformly within management units. 3. The Earth's Layers  Crust: The crust is the outermost shell of a terrestrial planet. o It is mineral-rich, solid, and characterised by a cool, brittle nature. o On average, the crust is 30 kilometres thick, but it can range from 50- 60 kilometres under mountain ranges and is denser under oceans, measuring about 5 kilometres thick. o Continents are composed of lighter materials.  Mantle: The mantle is made of heavy rock, primarily silicates, surrounding the core. o It is mostly solid and rich in minerals, including iron, aluminium, calcium, sodium, and potassium. o The mantle is 2630 kilometres thick and constitutes 84% of Earth’s total volume. o The temperature in the mantle varies greatly, from 1000° Celsius to 3700° Celsius. 4. Formation and Evolution  The crust has evolved over billions of years through processes such as: o Volcanism: The eruption of magma from the mantle forms a new crust, particularly at mid-ocean ridges. o Plate Tectonics: The movement and interaction of tectonic plates recycle crustal material and create various geological features. 3 oWeathering and Erosion: Surface processes break down rocks and minerals, forming soil and sediment that can be transported and deposited elsewhere.  The Earth’s layers consist of: o Upper Mantle: The upper mantle is 410 kilometres thick and is mostly solid, though partly molten. This region experiences slow plastic flow, which causes geological phenomena such as folding and faulting. The molten rock here is in constant motion and surfaces during volcanic eruptions. o Lower Mantle: Extending from 660 kilometres to 2,700 kilometres beneath the Earth’s surface, the lower mantle is hotter and denser than the upper mantle. o Core: The core is the thinnest layer of Earth, located 2,900 kilometres below the surface with a radius of 3,485 kilometres. It is composed almost entirely of metal, specifically iron and nickel. The outer core is liquid, while the inner core is plasma which behaves like a solid. 5. Geological Processes and Mineralogy 5.1. Geological Processes  The geology of the Earth determines the parent material from which soil originates.  Rocks contain component minerals that appear separately as particles, not in combination.  This characteristic arises due to the diverse array of minerals found in rocks, each with its own distinct properties.  Rocks can be classified into various types based on their origin and mineral composition.  Igneous, sedimentary, and metamorphic rocks represent different geological processes and exhibit unique mineral compositions.  These rocks serve as the foundation for soil formation processes, as they weather and break down over time, contributing to the development of different soil types.  Consequently, understanding the geological makeup of an area provides insight into the characteristics and properties of its soil. 4  Geologists utilise this knowledge to study Earth's history, predict soil behaviour, and inform various fields such as agriculture, environmental science, and construction. 5.2. Mineralogy  Minerals are the building blocks of rocks and come in various colours, shapes, and sizes.  A mineral is a naturally occurring, inorganic solid substance with a crystalline structure, a fixed or variable chemical composition, and a set of physical properties that can be used for identification.  Minerals are formed through natural processes, unlike synthetic materials or organic substances such as fossils, oil, and gas, which are not considered minerals.  The primary distinction between minerals and fossil fuels is the difference between organic and inorganic substances. 5.3. Mineral Composition  Each mineral has a unique combination of atoms that cannot be found in any other mineral.  Some minerals are grouped together because they have closely related chemical compositions and crystal structures.  Examples of mineral composition include: o Silicates: Composed of silicon (Si) and oxygen (O) plus other elements. They are the most abundant mineral group in the Earth's crust. An example is quartz (SiO2) o Oxides: Consist of a metal combined with oxygen. An example is hematite (Fe2O3), from which iron is obtained. o Sulphides: Contain Sulphur. An example is galena (PbS), from which lead is obtained. o Carbonates: Composed of magnesium (Mg) and/or calcium (Ca) together with carbonate (CO3). An example is dolomite (CaMg(CO3)2) o Salts: Such as sodium chloride (NaCl), known as halite. 6. Rocks  Rocks are naturally occurring mixtures of minerals that form in various ways.  The rock cycle is a process that explains the relationship between the three main types of rocks: igneous, sedimentary, and metamorphic.  Unlike water in the water cycle or carbon in the carbon cycle, not all rocks are recycled into different forms.  Some rocks have remained in their present form since soon after the Earth cooled.  These stable rock formations are called cratons.  There are three main types of rocks: o Sedimentary Rocks: Formed by the accumulation and compaction of sediments. o Igneous Rocks: Formed by the cooling and solidification of molten rock. o Metamorphic Rocks: Formed by the alteration of existing rocks due to heat, pressure, or chemically active fluids. 5 6.1. Igneous Rock  Igneous rocks are formed through the cooling and solidification of molten material and can be classified as intrusive or extrusive based on where they form within the Earth.  Molten rock inside the Earth or below the Earth’s surface is called magma (intrusive or plutonic), while molten rock outside or on top of the Earth’s crust is called lava (extrusive or volcanic)  igneous rocks are classified according to the types and proportions of their constituent minerals and their grain size.  They can also have a vesicular texture due to trapped gas bubbles.  The texture and mineral composition of igneous rocks vary widely and are determined by the cooling rate and mineral makeup of the molten material.  Most of the minerals making up igneous rocks belong to the silicate mineral family, where silicon and oxygen form major components.  As the rock cools, individual minerals appear as crystals that grow over time.  Different minerals have different melting temperatures and begin crystallising out of the cooling magma or lava at different times. 6  The slower the material cools, the more time is available for the crystals to grow.  Therefore, crystals in molten rock that cools slowly are larger than those in rocks that cool quickly.  The igneous rock consists of crystals of individual minerals that have crystallised separately out of the molten mass.  Consequently, it consists of interwoven mineral crystals that differ in colour and appearance, depending on the minerals present.  These differences in mineral composition result from variations in the chemical combination in the original magma. Igneous rocks can be broken apart by the forces of erosion and weathering.  Winds or ocean currents may then transport these tiny rocks (sand and dust) to different locations. 6.1.1. Formation of Igneous Rocks  Igneous rocks are formed from the cooling and solidification of molten material.  This molten material can exist inside the Earth as magma or on the Earth's surface as lava.  Magma, when it cools and solidifies below the Earth’s surface, forms intrusive or plutonic igneous rocks.  Lava, on the other hand, forms extrusive or volcanic igneous rocks when it cools and solidifies on the Earth’s surface. 6.1.2. Intrusive/Plutonic Igneous Rocks  Intrusive igneous rocks form far below the Earth's surface, where the surrounding rock acts as an insulator, causing the cooling process to be slow.  This slow cooling allows large crystals and mineral grains to form.  Examples of intrusive igneous rocks include: o Granite: A coarse-grained rock that forms most of the continental crust. o Diorite: Another coarse-grained rock with a composition intermediate between granite and gabbro. 6.1.3. Extrusive/Volcanic Igneous Rocks  Extrusive igneous rocks form on or above the Earth's surface, where molten material from volcanoes cools very quickly.  This rapid cooling results in fine-grained rocks with small crystals and mineral grains.  Examples of extrusive igneous rocks include: o Basalt: The most common igneous rock, found in the upper oceanic crust. o Gabbro: Found in the deeper oceanic crust, similar in mineral composition to basalt but differing in texture due to slower cooling rates. o Obsidian: A volcanic glass formed from rapid cooling of lava. o Pumice: A vesicular rock with a bubbly texture formed when gases are trapped in the cooling lava.  Basalt and gabbro contain roughly the same minerals but differ in texture due to their cooling rates.  Granite and gabbro are both coarse-grained but differ in colour due to their different mineral compositions. Intermediate rock types, such as dolerite (between 7 basalt and gabbro in texture) and diorite (between granite and gabbro in mineral composition), also occur. 6.1.4. Common Minerals in Igneous Rocks  The minerals found in igneous rocks determine their classification and properties.  Some important minerals include: o Olivine: Iron silicate, magnesium silicate. o Pyroxene: Magnesium silicate, iron silicate, calcium silicate, aluminium silicate. o Amphibole: Magnesium silicate, iron silicate, calcium silicate, sodium silicate. o Plagioclase: Sodium aluminium silicate, calcium aluminium silicate. o Orthoclase: Potassium aluminium silicate. o Quartz: Silicon dioxide 6.1.5. Classification of Igneous Rocks  Igneous rocks are classified based on their colour, dominant minerals, and texture, which is influenced by their cooling rate.  Here is an example of igneous rock classification: o Volcanic (fine-grained): Rhyolite, Andesite, Basalt, Komatiite. o Hypabyssal (medium-grained): Microgranite, Microdiorite, Dolerite, Micro-Peridotite. o Plutonic (coarse-grained): Granite, Diorite, Gabbro, Peridotite 6.2. Sedimentary Rocks  Rocks are exposed to various processes at the earth’s surface including the weathering action of wind, water and ice.  As a result of these processes pre-existing rocks and minerals are broken down into small pieces - the minerals may even dissolve.  The small pieces and dissolved material are usually removed from the site of weathering (this removal is called erosion) and transported around the Earth’s surface by rivers, glaciers, wind or ocean currents.  Eventually, most of this sediment is deposited, either in the sea to form marine sediments, or on land to form continental deposits (e.g. sand and mud).  Once older sediment is buried under younger layers, the particles undergo property changes with respect to density, hardness, porosity, and cohesion.  These changes are collectively referred to as diagenesis. Sediments may become ‘cemented’ together by new minerals which form between the particles and thus solidify into sedimentary rock.  This process is called lithification.  Compaction is the main physical change brought about by the mass of overlying sediments.  Sand is initially well-packed and compacts very little, whereas mud and clays, being highly porous and saturated with water, compact to a greater extent. 8  Deeply buried sediments are not only subjected to high pressures but also increased temperature, under which minerals and water in pore spaces react to form new minerals, enhancing the cementation process.  The process of forming crystals from dissolved minerals usually occurs in the shallow parts of the sea or in lakes in desert areas where evaporation is much higher than precipitation.  The sea or lake contains dissolved minerals such as calcium bicarbonate and calcium sulphate.  As evaporation takes place, water is lost and the dissolved minerals from crystals which settle on the bottom of the sea or lake.  As evaporation continues, more crystals form and accumulate on the sea or lake floor, becoming sedimentary rocks. 6.2.1. Formation of Sedimentary Rocks  Sedimentary rocks are formed from the accumulation and compaction of millions of tiny particles over long periods.  Igneous rocks can transition into sedimentary rocks by breaking them down into smaller particles and combining them with other materials to form layers.  Common sedimentary rocks include sandstone and limestone. 6.2.2. Types of Sedimentary Rocks  Sedimentary rocks originate from pre-existing igneous or sedimentary rocks that are exposed to different pressure and temperature conditions from those under which they were initially formed.  There are three main types of sedimentary rocks: clastic, organic (biological), and chemical.  Clastic Sedimentary Rocks: Clastic sedimentary rocks, such as sandstone, are composed of fragments, or clasts, of other rocks. o The formation process begins with the weathering or breaking down of exposed rock into small fragments. o These fragments are then transported by wind, water, ice, or biological activity to new locations through erosion. o Once the sediment settles and accumulates, the lowest layers become tightly compacted, forming solid rock in a process called lithification. o Clastic sediments are sorted by size through the energy of flowing water. o Strong currents can carry gravel and finer materials, moderately strong currents carry sand and finer materials, while slow currents can only transport mud and silt. o These sediments are eventually deposited and converted into sedimentary rocks through lithification. o Common clastic sedimentary rocks include:  Conglomerate: Formed from lithified gravel.  Sandstone: Formed from lithified sand.  Siltstone: Formed from lithified silt.  Shale: Formed from lithified mud and clay. o In some cases, sand composed mostly of shell fragments becomes lithified into limestone. 9 o Unlike water, glaciers do not sort sediment by size, resulting in unsorted sedimentary rocks like tillite, common in the Karoo of South Africa. o Wind can also transport fine sand, usually quartz, leading to unique sediment deposits.  Organic (Biological) Sedimentary Rocks: Organic sedimentary rocks, like coal, form from the compression of biological materials such as plants, shells, and bones into rock. o These rocks are significant as they often contain fossils and provide insight into past biological activity and environments.  Chemical Sedimentary Rocks: Chemical sedimentary rocks, such as limestone, halite, and flint, form from the chemical precipitation of substances dissolved in water. o This precipitation often occurs when the products of weathering are transported away from their original location by gravity, flowing water, ice, or wind, and are deposited in lower-lying areas as sediment. o Chemical sediments result from the precipitation of substances dissolved in water. o Common examples include:  Halite: Formed from the evaporation of seawater, resulting in thick layers of sodium chloride.  Limestone: Formed from calcium carbonate precipitation, often due to the removal of CO2 from seawater by photosynthesis. This can lead to large deposits of calcium carbonate.  Dolomite: Formed from the chemical alteration of limestone to calcium-magnesium carbonate, prevalent around Pretoria.  Chert: A fine-grained sedimentary rock formed by chemical precipitation of quartz from water, often replacing dolomite. 6.2.3. Features of Sedimentary Rocks  Sedimentary rocks often display layering due to the manner of their deposition. They may also exhibit signs of sedimentation such as fossils, ripple marks, and crossbedding caused by ripples and wave action.  The sorting of clastic sediments by water results in distinctive deposits of gravel, sand, silt, and mud. 6.3. Metamorphic Rocks  Metamorphic rock originates from any rock that is changed by earth forces, mainly pressure, heat or cementation (BISL, 2008).  The rocks change (undergo metamorphosis) to become a new type of rock.  Marble, for example, is a metamorphic rock (Pulella, 2018) created from rock that was once limestone, a sedimentary rock.  The changes may be so great that minerals occur that were not present in the original rock, as a result, the minerals in the rock may change to form new minerals.  Each of these rock types is characterised by diagnostic mineral combinations and reveal some indication that they have been subjected to pressure and heat, e.g., undulating, uneven layers, interwoven, jumbled crystals and particles, and signs of melting and recrystallisation of minerals. 10  Almost every igneous or sedimentary rock has a metamorphic equivalent: o granite changes into gneiss o basalt into schist o sandstone into quartzite o shale into slate; limestone into marble o coal into graphite (and then into a diamond).  The processes of compaction and recrystallization change the texture of rocks during metamorphism: o Sandstone becomes quartzite o Shale becomes slate o Limestone becomes marble o Granite becomes gneiss o Coal becomes anthracite or graphite 7. Weathering of Minerals and Rock  Weathering, the physical and chemical breakdown of rock (van Oudtshoorn, 2015), is the first step in the formation of soil.  The influence of weathering is evident all around us. It breaks up rocks and minerals (BISL, 2008), modifies or destroys their physical and chemical characteristics, and the products are carried away. It also produces new minerals.  The nature of the material being weathered determines the rates and results of the weathering process. 11 Section 2: Soil Formation  Soil formation, also known as pedogenesis, is a gradual process influenced by various factors, resulting in the development of distinct soil properties and layers.  The evolution of soils occurs through four key processes: additions, transformations, translocations, and losses.  These processes, occurring simultaneously, lead to the formation of different soil profiles, reflecting the unique combination of soil-forming factors present in a particular location. 1. Soil Forming Processes 1.1. Additions  Additions involve the incorporation of substances into the soil.  Water, organic matter, and mineral material are important additions influencing soil formation.  Water, originating from rainfall or lateral flow, infiltrates the soil, affecting its wetness and promoting chemical reactions.  Organic material, such as decomposed plant matter and roots, contributes to soil fertility and structure.  Mineral material, transported by water, wind, or gravity, includes alluvial, aeolian, and colluvial deposits, shaping soil depth and composition. 1.2. Transformations  Transformations refer to chemical and physical changes occurring within the soil, such as mineral weathering, organic matter decomposition, and clay formation.  These processes alter the soil's chemical composition and structure over time, influencing its properties and fertility. 1.3. Translocations  Translocations involve the movement of materials within the soil profile, including the vertical and lateral displacement of minerals, nutrients, and organic matter. 12  Processes like leaching, capillary rise, and soil mixing redistribute substances, affecting soil horizons and fertility. 1.4. Losses  Losses entail the removal of substances from the soil, primarily through erosion, leaching, and volatilisation.  Soil erosion by water or wind can result in the loss of valuable topsoil and organic matter, impacting soil fertility and structure.  Leaching, the downward movement of dissolved substances can lead to nutrient depletion in the soil profile. 2. Soil Profile Development  Soil formation and profile development are interconnected processes, resulting in the formation of distinct soil horizons or layers.  Each horizon exhibits unique properties, reflecting the dominant soil-forming processes and environmental conditions.  The arrangement of horizons within a soil profile provides insights into the history and characteristics of the soil, facilitating its classification and interpretation. 2.1. Influence of Soil-Forming Factors  Various factors influence soil formation and properties, including parent material, climate, topography, organisms, and time.  Geologic and climatic variations over time and space contribute to the diversity of soils observed globally.  These factors interact dynamically, shaping soil characteristics and profiles in different regions. 2.2. Addition, Transformation, and Translocation in Soil Formation  Soil formation, a complex process influenced by various factors, involves the addition, transformation, and translocation of materials within the soil profile.  These processes shape soil properties and horizons, contributing to the development of distinct soil types and characteristics. 13 2.3. Addition of Salts, Carbonates, and Bases  Salts: Water, especially from overflow in arid regions, carries dissolved salts into the soil. In arid and semi-arid areas with minimal leaching, these salts accumulate, causing soil salinity issues. Conversely, in humid regions, leaching prevents salt buildup. Additionally, salts from salt-rich rock layers beneath the soil can contribute to salinity.  Carbonates: Carbonates are transported in solution and accumulate in lower-lying areas, forming carbonate concretions. This accumulation affects soil pH and can lead to the development of alkaline soils.  Bases: Bases leached from higher slopes accumulate in lower-lying soils, resulting in higher base saturation, elevated pH levels, and the growth of vegetation characteristic of alkaline soils. 2.4. Transformation Processes  Humification: Organic material decomposes, forming humus, a dark, amorphous substance. The degree and characteristics of humification influence soil properties and development.  Formation of Clay Minerals: Primary minerals weather into secondary aluminosilicate clay minerals. These clay minerals further weather, forming different secondary clay minerals, which affect the soil's chemical and physical properties.  Transformation to Sesquioxide’s: Intense weathering breaks down clay minerals, leading to the accumulation of iron and aluminium oxides and hydroxides in the soil. These transformations influence soil colour and nutrient availability.  Chemical Reactions: Reduction of ferric iron to ferrous iron occurs under conditions of poor aeration, resulting in distinctive colour changes in the soil. Precipitation of iron oxides and hydroxides occurs under fluctuating water table conditions. 2.5. Translocation Processes  Clay Translocation: Clay particles, with colloidal properties, can be transported in water suspension. In well-developed soil profiles, clay accumulates in the subsoil relative to the topsoil, forming clay skins or layers around structural units. This process, known as illuviation, affects soil structure and fertility. 2.6. Translocation and Losses in Soil Dynamics  In soil dynamics, the translocation of salts, carbonates, bases, and other materials plays a crucial role, particularly in arid and semi-arid regions where water movement is limited.  Additionally, losses from the soil profile significantly impact soil composition and fertility. 2.7. Translocation of Salts, Carbonates, and Bases  Salts and Carbonates: In dry conditions, soluble salts and carbonates, such as sodium and gypsum, accumulate near the surface. As conditions become slightly wetter, these salts translocate to the subsoil. Leaching by water movement through 14 the profile removes these salts, but carbonates may still accumulate, often forming lime nodules or concretions.  Bases: Under conditions of incomplete leaching, bases like calcium and magnesium are translocated from the topsoil to the subsoil. This process leads to higher base saturation and pH levels in subsoils compared to topsoil’s, especially in semi-arid regions.  Translocation of Sesquioxide’s: Iron oxides, particularly, undergo translocation in well-aerated profiles, intensifying the red colour of the soil. Under conditions of intense weathering, significant amounts of iron are released and redistributed. Anaerobic conditions can lead to the reduction of iron to the ferrous form, which then translocate to deeper soil layers before precipitating again. 2.8. Pedoturbation  Mixing of Soil: Pedoturbation, often overlooked, occurs when soil mixing takes place, disrupting horizon differences. Soil fauna like earthworms contribute to pedoturbation, as do the cracks in drying soils. This mixing process can alter soil properties and horizons. 2.9. Losses from the Soil Profile  Evapotranspiration: Warm, dry regions experience high rates of evapotranspiration, leading to the loss of water from the soil. This reduces the effectiveness of rainfall, limiting soil formation and resulting in shallow, stony soils rich in salts or carbonates. Intense drying can lead to the development of cemented layers like hardpans.  Soil Erosion: Natural water erosion, especially on steep slopes, removes soil, resulting in shallow, stony soils, particularly in arid and sub-humid regions. Accelerated erosion from overgrazing or ploughing exacerbates this issue.  Loss of Organic Material: Humus, vital for soil fertility, can be decomposed and completely removed under warm, well-aerated conditions, especially with alternating wetting and drying cycles. Marshy conditions lack oxygen, preventing this type of breakdown. 2.10. Losses and Formation Factors  In soil dynamics, the processes of base leaching, losses of clay and iron, and the role of soil forming factors significantly influence soil composition and profile development.  Understanding these processes is crucial for effective soil management and conservation. 2.11. Losses of Bases  Base Leaching: In regions with high rainfall and well-drained positions, bases are leached from the soil, resulting in infertile, highly acidic conditions. This loss of bases accelerates the decomposition and synthesis of clay minerals, progressing to the 1:1 or sesquioxide stages. This phenomenon is particularly pronounced in humid tropical and subtropical regions. 15 2.12. Losses of Clay and Iron  Lateral Translocation: The lateral removal of iron and clay leads to bleached horizons with lower clay content and the accumulation of clay in lower-lying positions. This process alters soil composition and can impact fertility and stability 2.13. Losses of Silica  Rapid Decomposition: In humid tropical regions characterised by high rainfall and temperatures, organic material decomposes rapidly, releasing bases present in the material. Unless the vegetation contains sufficient bases, the initially neutral pH increases due to base release. This environment promotes silica solubility and lowers iron and aluminium content. Consequently, iron and aluminium accumulate in the soil, resulting in sesquioxide and 1:1 clay mineral-rich soils. 3. Soil Forming Factors  Soil formation is influenced by several factors, including mother material, climate, topography, living organisms, and time: o Mother Material: The original material from which soil forms plays a crucial role in soil development, fertility, and stability. In regions with relatively homogeneous climates, soil formation is determined by a combination of mother material and topography. Mother material affects not only the type of soil profile but also inherent soil fertility and resistance to degradation. o Climate: Climate, particularly rainfall, influences soil formation processes such as leaching and organic matter addition. Wet climates can lead to base leaching but also promote vegetation growth and organic matter accumulation o Topography: The topographic features of a region influence soil development through factors such as water movement, erosion, and deposition. Slope, aspect, and landscape position affect soil characteristics and horizon development. o Living Organisms: Soil biota, including microbes, plants, and fauna, play essential roles in soil formation processes such as organic matter decomposition, nutrient cycling, and soil structure development. Their activities influence soil fertility and structure. o Time: Soil formation is a slow process that occurs over geological timescales. The duration of soil development influences the depth, structure, and properties of soil profiles. 4. Climate and Soil Formation Dynamics  Climate stands as a primary determinant of soil formation processes, showcasing varied effects across different regions and landscapes (Sposito, 2023).  Understanding these dynamics is crucial for effective soil management and land utilisation strategies.  Warm, Humid Conditions: Accelerated Weathering o In regions characterised by warm and humid climates, such as the tropics and subtropics, soil formation processes are intensified (Sposito, 2023). o The combination of high temperatures and ample moisture availability fosters rapid chemical weathering of parent materials. 16 oAs temperatures rise, chemical reactions within the soil matrix are facilitated, leading to the breakdown of minerals and the release of essential nutrients. o Moreover, abundant moisture ensures the efficient leaching of soluble compounds, further enhancing weathering rates. o Over time, these processes yield deep, highly weathered soils rich in clay minerals, sesquioxide’s, and free iron.  Warm, Dry Regions: Limited Soil Development o Conversely, warm and arid or semi-arid regions exhibit slower rates of soil formation due to water scarcity. o Despite high temperatures, the limited availability of moisture restricts the extent of chemical weathering processes. o Evapotranspiration rates outpace rainfall, diminishing the effectiveness of precipitation in promoting soil development. o Consequently, soils in such regions tend to be shallow and stony, with alkaline pH levels and accumulations of salts, carbonates, and bases (Sposito, 2023). o Clay mineral composition often remains largely unchanged, reflecting minimal alteration from the original parent material.  Regional Climate Patterns: Soil Distribution o In South Africa, the influence of regional climate patterns on soil distribution is evident (Sposito, 2023). o Areas characterised by low rainfall, such as the Karoo and parts of the Eastern Cape, typically harbour shallow, stony soils due to limited precipitation and unfavourable parent material. o Conversely, regions with higher rainfall, such as the southeastern Transvaal highveld, support deeper, more fertile soils conducive to agricultural activities.  Topography's Role: Landscape Variation o Topography, which refers to the physical features of the landscape, significantly influences soil characteristics and distribution. o The presence of various topographic elements shapes soil formation processes and contributes to the spatial variability of soils within a given area. o In rolling hilly landscapes, the absence of free cliffs is common, which affects soil depth and composition across different topographic positions. o Soils on crests and upper parts of midslopes tend to be shallow due to limited water retention and increased susceptibility to erosion. o Conversely, soils on lower parts of midslopes and footslopes are often deeper, as they accumulate debris transported from higher elevations. o Furthermore, topography influences the distribution of moisture within the landscape, particularly in humid and sub-humid regions. o Wetness increases from top to bottom in the landscape, leading to hydromorphic (saturated) conditions on the lower parts of footslopes and valley floors. o This moisture gradient plays a crucial role in shaping soil properties and profiles across different topographic positions. In rolling landscapes, a repeating pattern of soils associated with different topographic units can be observed, known as a top sequence. o Within a top sequence, soils exhibit predictable variations based on topographic characteristics, forming a catena. o Investigating soils along a slope reveals different soil types at different positions, allowing for the prediction of soil distribution based on topography. o This predictable sequence of soils associated with slope is called a catena. 17  Role of Living Organisms in Soil Formation o Living organisms, including microbes, plants, and soil fauna, contribute significantly to the intricate process of soil formation, exerting influence through various mechanisms.  Microbial Activity: Humus Formation and Weathering o Microbes, algae, and mosses initiate soil formation by colonising geological materials, paving the way for subsequent biological interactions (BISL, 2008). o Soil microbes play a crucial role in the conversion of organic matter into humus, a process essential for soil fertility and structure (BISL, 2008). o Furthermore, microbial decomposition releases CO2 and organic acids, potent weathering agents that facilitate mineral breakdown and nutrient release.  Vegetation Dynamics: Organic Matter Input and Soil Modification o Vegetation profoundly influences soil characteristics, albeit with regional variations (Sposito, 2023). o While vegetation in the northern hemisphere is considered a primary soil- forming factor, South African ecosystems often exhibit a reciprocal relationship between vegetation and soil properties (Sposito, 2023). o Nevertheless, certain vegetation types, such as grasslands in semi-arid to sub- humid regions, contribute substantially to soil development by fostering organic matter accumulation and root penetration (Sposito, 2023). o These ecosystems typically yield deep, fertile soils enriched with bases and carbonates, exemplified by the chernozem soils of the Russian steppes and mollisols of the American prairies.  Podzolization: Acidic Vegetation and Soil Transformation o Certain vegetation types, such as coniferous forests and fynbos, impart distinctive soil characteristics through their acidic organic material and chelating properties (Sposito, 2023). o In coarse sandy soils, organic acids produced by these vegetation types facilitate the translocation of iron to subsoil layers, leading to the formation of red brown podzol B horizons via podzolization (Sposito, 2023). o While prevalent in mountainous regions under fynbos in South Africa, podzols are widespread beneath coniferous forests in the northern hemisphere. 18  Faunal Contributions: Soil Mixing and Modification o Soil fauna, including termites and earthworms, actively participate in soil modification processes through their burrowing and mixing activities (Sposito, 2023). o Termites, for instance, facilitate soil turnover by constructing mounds and redistributing soil particles, thereby promoting horizon differentiation and altering soil texture (Sposito, 2023). o Conversely, pedoturbation, caused by soil fauna mixing materials from different horizons, may slow down horizon differentiation.  Temporal Dynamics: Time as a Key Factor o The processes of soil formation and profile development unfold over extended time frames, underscoring the significance of temporal dynamics in soil evolution (Sposito, 2023). o While soil development progresses gradually across diverse landscapes, the rates of weathering and profile development vary globally, with the southern hemisphere exhibiting more pronounced levels of weathering compared to the northern hemisphere. 19 Unit 3: The Soil Profile 1. The Formation and Development of Soil Profiles  Soil formation is a dynamic interplay between destructive and creative processes, resulting in the intricate layering of soil horizons within a soil profile.  These processes, influenced by various soil-forming factors, contribute to the gradual evolution of soil properties over time.  Soil Horizons: The development of soil horizons exemplifies the creative processes inherent in soil formation. Soil horizons, distinct layers within the soil profile, emerge because of mineral transformation, organic matter accumulation, and soil structural development. When excavating a soil profile, these horizons are revealed, providing insights into the soil's history and properties.  Factors Influencing Horizon Formation: Soil horizons exhibit parallel alignment to the surface, reflecting the influence of soil-forming factors operating predominantly in a horizontal direction. Gravity, a ubiquitous force, facilitates the downward movement of water and fine materials, contributing to horizon development. Additionally, factors such as temperature and oxygen, which vary with depth, exert their influence parallel to the soil surface, shaping horizon characteristics over time.  Maturation of Soil Profiles: As soil formation progresses, a soil profile matures through the gradual accumulation and alteration of its constituent layers. This maturation process unfolds over centuries or millennia, eventually leading to the establishment of a quasi-equilibrium between the soil and its environment. Undisturbed soils evolve towards stability, embodying a comprehensive record of their formation history and environmental interactions within the soil profile. 1.1. Importance for Agricultural Management  Understanding soil profiles is paramount for effective agricultural management, as soil characteristics directly influence plant growth and productivity.  Soil profiles reveal critical information such as depth, texture, structure, and drainage conditions, enabling informed decisions regarding land use and soil management practices.  Despite the agricultural focus of soil science, this knowledge remains invaluable for various land management endeavours, including game ranching, facilitating optimal soil management strategies. 1.2. Soil Maturity: A Continual Evolution  As soil formation progresses, a soil profile undergoes a transformative journey towards maturity, wherein inherited properties gradually yield to those acquired during development.  With time, the influence of unchanging soil-forming factors diminishes in significance, while the imprint of ongoing developmental processes becomes increasingly pronounced.  Even in the presence of consistent soil-forming factors, profile evolution persists indefinitely, albeit at a decelerated pace, reflecting the perpetual activity of these underlying forces. 20  However, any alteration in these factors initiates a divergent trajectory of profile development, heralding the emergence of a new soil profile shaped by the altered environmental conditions. 2. Soil Horizons  A fundamental aspect of soil characterisation lies in the delineation of soil horizons, discernible layers within the soil profile that embody distinct sets of properties engendered by soil-forming processes.  These horizons, though exhibiting variations in thickness and boundary irregularities, predominantly align parallel to the soil surface.  Through their unique characteristics encompassing colour, texture, structure, permeability, and biological activity, soil horizons furnish critical insights into soil composition, formation dynamics, fertility, and other pertinent attributes.  A soil profile serves as a tangible representation of soil horizons, providing a two- dimensional exposition of the stratified layers beneath the earth's surface.  Horizons within the soil profile are delineated using a standardized nomenclature comprising letter and number codes.  These include the O, A, E, B, C, and R horizons, collectively referred to as master horizons, each with its distinctive characteristics and properties.  While some soil profiles exhibit demarcated horizons, characterised by conspicuous differences in colour, others may present challenges in horizon differentiation.  However, colour serves as just one criterion for distinguishing soil horizons, with texture, structure, and chemical properties also playing pivotal roles in the characterisation process.  Through meticulous examination and interpretation of soil horizons, scientists and practitioners glean invaluable insights into soil composition, genesis, and functionality, facilitating informed decision-making in diverse fields ranging from agriculture to environmental management.  Soil horizons, integral components of the soil profile, offer a wealth of information about soil composition, formation processes, and environmental interactions.  This comprehensive guide delves deeper into the intricacies of soil horizons, exploring their classification, characteristics, and significance in soil science. 2.1. Classification of Soil Horizons  O-Horizon: o The O-horizon, also known as the organic horizon, comprises organic materials in various stages of decomposition, including leaf litter, plant debris, and organic residues. o It can be further subdivided into distinct layers based on the degree of decomposition, such as Oi (undecomposed), Oe (partly decomposed), and Oa (completely decomposed) layers.  A-Horizon: o Positioned beneath the O-horizon, the A-horizon, or topsoil, is characterised by a mixture of mineral particles and organic matter derived from the overlying organic layer. o It serves as the primary zone for root growth, nutrient uptake, and biological activity, influencing soil fertility and productivity. 21  E-Horizon: o Some soils exhibit an E-horizon, also known as the eluviation horizon, characterised by the loss of fine particles, organic matter, and soluble minerals through leaching processes. o The E-horizon typically appears lighter in colour and coarser in texture compared to underlying horizons due to the removal of fine particles.  B-Horizon: o Below the A- and E-horizons lies the B-horizon, or subsoil, which accumulates minerals, clays, and other materials leached from the overlying horizons. o It exhibits distinct characteristics, such as increased clay content, colouration from iron oxides, and enrichment of minerals, influencing soil structure and permeability.  C-Horizon: o The C-horizon represents the unaltered parent material from which the soil developed, exhibiting minimal evidence of soil-forming processes. o It serves as a crucial substrate for soil genesis and retains the closest resemblance to the original geological material, providing insights into soil formation.  R-Horizon: o At the base of the soil profile lies the R-horizon, comprising unweathered bedrock or geological material with minimal signs of weathering or alteration. o The R-horizon serves as the foundational layer upon which soil development occurs, offering insights into the underlying geological processes. 22 2.2. Significance of Soil Horizons  Soil horizons play a pivotal role in soil characterisation, providing valuable information about soil properties, fertility, and environmental interactions.  Key aspects of their significance include: o Soil Classification: Soil horizons serve as the basis for soil classification systems, facilitating the categorization of soils based on their unique properties and horizons. o Soil Formation Processes: The characteristics and distribution of soil horizons offer insights into soil formation processes, including weathering, leaching, and organic matter accumulation, shaping the development of soil profiles over time. o Environmental Indicators: Soil horizons act as indicators of environmental conditions and ecosystem dynamics, reflecting factors such as climate, vegetation, and land use practices. o Soil Management: Understanding soil horizons is essential for effective soil management practices, including agriculture, land reclamation, and environmental conservation, as it informs decisions related to soil fertility, drainage, and erosion control. 3. Soil types 3.1. Pan Layers  Pan layers, thick and compact layers in soil profiles hindering root and water penetration, develop due to various factors, such as excess water in wet areas or precipitation of carbonates in dry areas.  The deposition or cementation of materials like iron oxide, calcium carbonate, or organic matter leads to their formation.  They can range from nearly impenetrable to merely restrictive.  For example, iron pan layers are characterised by an iron crust or laterite, while calcium carbonate is common in dry areas. 3.2. Topsoil and Subsoil  The topsoil, often referred to as the A-horizon, is organically enriched and serves as the primary zone for plant roots.  It contains a significant portion of the nutrients and water necessary for plant growth.  Management practices can easily alter the chemical properties, nutrient content, and physical structure of the topsoil.  Maintaining an open structure in the topsoil is crucial for providing adequate air and water to plant roots and preventing runoff.  In natural rangelands, effective management through grazing and fire control is essential for maintaining high organic matter levels.  Below the topsoil lies the subsoil, which stores water and supplies nutrients to plants.  Subsoil characteristics greatly influence land use and plant growth.  Impermeable subsoil layers can restrict root penetration, while acidic subsoils and poor drainage can affect topsoil conditions.  Unlike the topsoil, the subsoil cannot be easily altered through management practices, making land use decisions dependent on subsoil properties. 23 3.3. Implications for Plant Growth and Management  Understanding the composition and characteristics of pan layers, topsoil, and subsoil is crucial for effective land management and agricultural practices (Smith et al., 2018).  By recognising the factors influencing soil development and nutrient availability, land managers can implement strategies to improve soil health and optimise plant growth (Jones & Brown, 2019).  Additionally, maintaining proper soil structure and moisture levels is essential for sustaining healthy ecosystems and preventing soil degradation (van Oudtshoorn, 2015). 3.4. South African soil horizons  Understanding the characteristics of common South African soil horizons provides valuable insights into soil classification, fertility, and management practices.  For instance, the presence of Vertic A horizons indicates soil types prone to significant volume changes, impacting construction and agricultural activities.  Melanic A horizons, with their rich dark colour, are indicative of fertile soils suitable for agriculture in semi-arid to sub-humid climates.  Additionally, the classification of subsoil horizons such as Red apedal B and Yellow brown apedal B helps identify soil drainage and nutrient availability, guiding land use decisions 24 25 4. Development of the soil profile  Soil formation is a dynamic process characterised by the gradual breakdown of bedrock or parent material into smaller particles, coupled with the integration of organic matter (Weaver & Angle, 1985).  This process unfolds in stages, influenced by various environmental factors (Brady & Weil, 2008).  Initially, at the surface (Stage I), bedrock undergoes disintegration due to factors like temperature fluctuations, precipitation, and other weathering agents.  Although this disintegration penetrates shallowly, it sets the stage for further soil development by exposing fresh surfaces to weathering.  As the rock breaks into smaller pieces, soil-forming factors can penetrate deeper through newly formed fractures, initiating the transition from solid rock to lose material suitable for soil formation.  The progression continues as the broken rock transforms into parent material (Stage II), facilitated by the contributions of organisms in the ecosystem  Organic matter from living organisms’ aids in the disintegration process, enriching the developing soil with essential nutrients and fostering soil structure.  Meanwhile, the accumulation of decomposed organic material contributes to the formation of distinct soil horizons (Stage III).  The A horizon, enriched with organic matter, forms near the surface, while the deeper C horizon retains more minerals, resembling the parent material.  In some instances, a B horizon may develop, characterised by the accumulation of leached minerals.  As soil development advances (Stage IV), the soil matures into a complex ecosystem capable of supporting diverse vegetation.  At this stage, the soil effectively cycles nutrients and resources, fostering robust biological activity.  Soil properties inherited from its geological origins gradually diminish in significance, while characteristics acquired during development become more pronounced.  The culmination of this process results in a mature soil capable of sustaining a thick vegetative cover and supporting various ecological functions.  Furthermore, the rate and trajectory of soil formation can vary depending on factors such as the nature of the parent material and prevailing environmental conditions.  For instance, soils formed from deposited material tend to develop more rapidly compared to those originating from in situ weathering processes.  Despite the variability in soil development, common properties and characteristics allow for the classification of soils into distinct groups based on shared features and behaviours. 26 27 Section 4: Physical and Chemical Properties of Soils 1. Concepts of Soil  Soil, defined from various perspectives, reflects the diverse roles and functions it serves within natural and agricultural contexts.  From the geological viewpoint, soil represents weathered rock and mineral materials, including sedimentary deposits and dune sands.  Conversely, the agricultural perspective emphasises soil as a growth medium for plants, crucial for sustaining agricultural productivity.  Additionally, soil is perceived as an organised natural body, shaped by intricate interactions between physical, chemical, and biological processes, encapsulating a broad spectrum of geological and ecological phenomenal.  In this course, soil is conceptualised as a dynamic and complex system comprising both mineral and organic components.  Functionally, soil serves as a medium that supports plant growth, providing essential nutrients, water, and physical support for vegetation.  Moreover, soil acts as a habitat for diverse microbial communities, playing a vital role in nutrient cycling and ecosystem functioning.  The spatial boundaries of soil extend from the upper limit of the atmosphere or shallow water down to the depth where soil-forming processes cease, delineating its distinct realm within the Earth's surface. 2. Physical Properties of Soil  Soil's physical attributes profoundly influence its behaviour and functionality within ecosystems.  These properties, including grain size distribution, porosity, and permeability, dictate crucial aspects of soil-water-plant interactions and ecosystem dynamics.  Grain size determines soil texture, affecting water retention, drainage, and root penetration.  Porosity reflects the soil's capacity to store air and water, crucial for maintaining aerobic conditions and supporting microbial activity.  Permeability governs the ease of water and solute movement through the soil profile, influencing nutrient availability and groundwater recharge rates. 2.1. Soil Structure  Soil structure refers to the arrangement of soil particles into aggregates or peds, which profoundly impacts soil porosity, permeability, and root penetration.  Soil texture and structure collectively determine soil behaviour, influencing its susceptibility to erosion, compaction, and nutrient leaching.  Organic matter plays a pivotal role in promoting soil structure by enhancing aggregate stability and porosity through microbial activity and organic binding agents.  Sustainable soil management practices aim to preserve and enhance soil structure, ensuring optimal soil function and ecosystem resilience. 28 2.2. Soil Colour  Soil colour, an observable characteristic influenced by soil-forming factors, serves as a key indicator of underlying soil properties and conditions.  While soil colour itself does not directly affect soil behaviour or utility, it provides valuable insights into the physical and chemical attributes of soil.  Variations in soil colour, ranging from vibrant reds and browns to subdued yellows and greys, offer clues about soil composition and environmental conditions.  Moreover, soil colour exhibits spatial heterogeneity across landscapes and changes vertically within soil profiles.  Three primary factors influence soil colour: organic matter content, water content, and the presence of metals such as iron, aluminium, and manganese.  Organic matter, known for its dark hue, contributes to soil colouration, with higher organic matter levels yielding darker soils.  Additionally, organic coatings on soil particles obscure the inherent colour of the particles themselves, further influencing soil colouration.  Soil moisture content also affects colour, with moist soils typically appearing darker than dry soils.  However, the most significant determinant of soil colour is the presence and oxidation states of metals, particularly iron.  Well-drained soils often exhibit red, yellow, or brown hues indicative of iron oxide presence, while poorly drained soils may display grey or blue shades reflecting reduced iron conditions.  Various forms and concentrations of iron and aluminium oxides impart distinct colours to soil particles or form layers around them, contributing to soil colour diversity.  Additionally, minerals like carbonates and salts, prevalent in arid and semi-arid regions, contribute whitish hues to soils.  Soil colour provides valuable insights into soil hydrological and drainage conditions, with bright hues suggesting well-drained soils conducive to water and air movement.  Conversely, greyish, greenish, or bluish colours, indicative of reduced iron compounds, suggest waterlogged conditions prevalent in poorly drained soils.  Often, soil colours result from the interaction of multiple colour-forming elements, leading to intermediate hues.  Colour Examples: o Red: Reflects oxidation, often found on upper crests. o Yellow: Results from oxidation and hydration, common on lower crests. o Light Grey: Indicates waterlogging, typically observed in the middle of foot slopes. o Black: Associated with high organic matter content. 2.3. Soil Texture  Soil texture, a fundamental property of soil, describes the size distribution of soil particles.  It encompasses a range of particle sizes, from coarse sands to fine clays, and plays a crucial role in determining soil behaviour and management practices.  Organic matter and other substances act as binding agents, facilitating the formation of soil aggregates from individual particles.  The composition of soil particles varies widely across different soil types and depths. 29  Coarse particles such as stones, gravel, and coarse sand form larger fragments of soil, often containing multiple mineral types.  Conversely, finer particles predominantly consist of a single mineral type.  Soil textures can vary considerably, with some soils characterised by a predominance of coarse particles and others by an abundance of fine particles.  Soil texture classification is based on the size distribution of soil particles, ranging from 2 mm to less than 0.0002 mm.  These size classifications hold significance as they influence soil behaviour and physical properties.  Understanding the proportions of different-sized particles within a soil is essential for effective soil management, as soil texture is considered a fundamental and unalterable property.  Soil Textural Classes: Soil texture, determined by the relative proportions of sand, silt, and clay particles, profoundly influences soil behaviour and management practices. Understanding the characteristics of each soil textural class is essential for effective soil management and agricultural practices. o Sand  Sand particles are discernible with the naked eye and feel gritty when rubbed between the fingers.  They vary in shape from rounded to angular, depending on weathering processes.  Due to their larger size, sand particles have wide spaces between them, allowing for easy movement of water and air within the soil.  However, their relatively small surface area results in poor water retention, making sandy soils prone to drying out and leaching.  Additionally, sand particles do not adhere to each other, leading to non- sticky soil, even when wet. o Silt  Silt particles are smaller than sand and cannot be individually seen without magnification.  They are typically composed of micro-sand with quartz as the dominant mineral.  Silt particles have smaller and more numerous pores compared to sand, enhancing water retention and reducing drainage.  Soils high in silt are susceptible to erosion by flowing water due to their low plasticity and tendency to form hard clods when dry. o Clay  Clay particles are the smallest soil particles and exhibit unique colloidal properties.  They feel sticky when wet and form hard clods when dry.  Clay particles have a vast surface area and can adsorb large amounts of water and other substances.  The presence of clay significantly influences soil behaviour, with clayey soils characterised by slow water and air movement due to small, convoluted pores.  Clay minerals vary in their properties, with some swelling when wet and shrinking when dry, while others remain stable. 30 2.4. Soil Types  Soils are broadly categorized based on the predominance of sand, silt, or clay particles, leading to sandy, clayey, or loamy soils.  An ideal loam soil exhibits a balanced mixture of sand, silt, and clay in roughly equal proportions, providing optimal soil properties for plant growth and water retention.  It's important to note that small variations in the proportions of sand, silt, and clay can significantly impact soil behaviour, with even relatively small amounts of clay influencing soil properties.  Moreover, soil texture is primarily influenced by soil-forming processes and cannot be readily altered by management practices. 3. Surface Area and Water Retention  Understanding the relationship between soil particle size and surface area is crucial for comprehending soil function and behaviour, as it impacts various soil properties.  As soil particle size decreases, relative surface area increases significantly.  This relationship is fundamental for soil function, as water is retained in soil as thin films on the surface of soil particles.  Finer soils with larger surface areas exhibit greater water retention capacity compared to coarser soils. 3.1. Surface Area and Nutrient Retention  Soil particles attract both gases and dissolved chemicals, including nutrients, to their surfaces through adsorption.  The larger the surface area of soil particles, the greater the soil's capacity to retain nutrients and other chemicals, providing essential resources for plant growth and development.  Plants obtain their nutrients from the surface of soil particles and the surrounding water layers.  Finer soils, characterised by larger surface areas, offer a greater nutrient surface for plants, enhancing nutrient uptake and promoting healthier plant growth. 3.2. Surface Area and Weathering  Surface weathering processes occur at the surface of mineral particles, releasing elements into the soil solution.  Soils with larger surface areas experience higher rates of release of plant nutrients from weatherable minerals, contributing to soil fertility and productivity. 3.3. Surface Area and Soil Structure  The surfaces of mineral particles carry both negative and positive electric charges, leading to particle aggregation and soil structure formation.  Soils with larger surface areas exhibit a greater tendency for particles to stick together, forming structured masses or discrete aggregates, which influences soil stability and porosity. 31 3.4. Surface Area and Microbial Activity  Microorganisms tend to colonize and grow on particle surfaces, influencing soil microbial activities.  Soils with larger surface areas provide more habitat for microbial colonisation, impacting nutrient cycling, organic matter decomposition, and soil fertility. Influence of soil textural size classes on some properties and behaviour of soils Sand Silt Clay Size range (mm) 2.0 - 0.05 0.05 – 0.002 < 0.002 How to see Naked eye Microscope Electron microscope Dominant materials Primary Primary and Secondary secondary Attraction of particles Low Medium High for each other Attraction of particles Low Medium High for water Water-holding capacity Low Medium to low High Ability to hold nutrients Very Low Low High in plant available form Consistency when wet Loose, gritty Smooth Sticky, malleable Consistency when dry Very loose, gritty Powdery, some Hard clods clods Aeration Good Medium Poor Drainage rate High Slow to Medium Very Slow Soil organic matter Low Medium to High Medium to High level Decomposition of Rapid Medium Slow organic matter Warm-up in spring Rapid Moderate Slow Compatibility Low Medium High Susceptibility to wind Moderate High Low erosion (high if fine sand) Susceptibility to water Low (Unless fine High Low erosion sand) Shrink-swell potential Very low Low Moderate to high Sealing of ponds, dams Poor Poor Good and landfills Suitability for tillage Good Medium Poor when wet Pollutant leaching High Medium Low (unless potential cracked) Resistance to Ph Low Medium High change 32 4. Peds: Building Blocks of Soil Structure  Sand, silt, and clay particles constitute the fundamental components of soil, and the arrangement of these particles into groups is what defines soil structure.  These groups, known as peds, represent the smallest units of soil structure and play a crucial role in determining various soil properties and behaviours. 4.1. Importance of Peds  The pattern of peds within the soil, along with the spaces within and between them, significantly influences key soil characteristics such as porosity, water movement, aeration, and heat transfer.  Soil structure, much like soil texture, governs the behaviour of water and air in soils, and it profoundly impacts the growth of plant roots.  Human activities such as harvesting, grazing, trampling, and rainfall can significantly alter soil structure, particularly in the upper soil horizons. 4.2. Types of Structural Peds  Various types of structural peds exist in soils, often within different horizons of the same soil profile.  The description of soil structure involves considering the shape, size, and distinctness of these peds, which can vary between horizons.  A soil or horizon with visible structure is referred to as a pedal, while one without structure is termed apedal. o Granular / Spheroidal Structure: Granular peds are roughly spherical with irregular faces, resembling cookie crumbs. This structure, also known as crumb structure, is common in surface soils, especially A horizons high in organic matter. It promotes good porosity and facilitates the movement of air and water. o Platy Structure: Platy peds are flat and platelike, often lying horizontally. This structure is typically found in subsurface soils subjected to leaching or compaction. Platy structure can hinder the downward movement of water and plant roots. o Blocky Structure: Blocky peds are irregularly shaped and range from 5 mm to 50 mm across, with flat or slightly rounded sides. They are common in subsoils and surface soils with high clay content, promoting drainage and aeration. o Prismatic / Columnar Structure: Prismatic or columnar peds are characterised by vertical or pillar-like shapes. These structures, commonly found in B horizons or subsoils, are associated with swelling clays and vertical cracks resulting from various environmental factors. o Structureless Soils: Some soils lack visible structure and are referred to as structureless. They may be either single-grained or massive, with no discernible peds. Structureless soils are termed apedal.  The formation and maintenance of soil structure are pivotal processes in soil dynamics, profoundly influencing ecosystem function and soil management practices.  The organisation of surface soils into structural aggregates, or peds, plays a crucial role in providing optimal soil conditions for various land uses, characterised by low bulk density and a high proportion of large pore spaces. 33 5. Soil Aggregates  Soil aggregates are clusters of soil particles that bind together more strongly than the surrounding soil.  These aggregates form through the combination of various soil particles, such as sand, silt, and clay, with organic matter, minerals, and microorganisms.  The binding agents that hold these particles together can include organic matter (such as plant roots and microbial byproducts), clay minerals, and iron oxides.  Aggregates vary in size from a few micrometres to several millimetres and are essential for soil structure and health.  One of the primary benefits of soil aggregates is the improvement of soil structure. Aggregates create a crumbly soil texture that enhances aeration, water infiltration, and root penetration.  This structure helps prevent soil compaction and crusting, ensuring that plants can access the air and water they need to thrive.  Moreover, well-aggregated soil holds water more efficiently and drains excess water, reducing the risk of waterlogging and drought stress.  This balance is crucial for maintaining healthy plant growth and preventing soil erosion.  Aggregates also play a significant role in maintaining soil fertility.  They protect organic matter and nutrients from erosion and microbial decomposition, preserving soil fertility over time.  This protection is essential for supporting healthy plant growth and ensuring that soils remain productive.  Additionally, aggregated soil is less prone to erosion by wind and water.  The strong bonds within aggregates help resist the forces that would otherwise dislodge and transport soil particles, protecting the soil surface and maintaining its integrity.  Furthermore, soil aggregates provide a habitat for soil microorganisms, such as bacteria, fungi, and earthworms, which are crucial for nutrient cycling, organic matter decomposition, and overall soil health.  These microorganisms thrive in the protected environment within aggregates, contributing to soil fertility and structure.  Soil aggregation is influenced by various factors, including soil texture, organic matter content, root activity, microbial activity, and land management practices.  Practices such as crop rotation, cover cropping, reduced tillage, and the addition of organic amendments can enhance soil aggregation and improve soil health, leading to more sustainable and productive agricultural systems.  Soil aggregates vary in their resistance to external forces such as raindrop impact, trampling, and ploughing.  While some aggregates are easily destroyed, others exhibit greater resilience.  Generally, smaller aggregates are more resistant than larger ones, emphasizing the significance of preserving larger aggregates for soil stability and function. 5.1. Hierarchical Organisation of Soil Aggregates  Soil aggregates exhibit a hierarchical organization, with large aggregates (> 1 mm) composed of smaller aggregates, which, in turn, comprise even smaller units down to clusters of clay and humus particles smaller than 0.001 mm in size.  This hierarchical structure is typical of most soils, with different factors responsible for binding the units together at each level of the hierarchy. 34 5.2. Factors Influencing Aggregate Formation  Both biological and physical-chemical factors play crucial roles in the formation of soil aggregates.  Physical-chemical factors are predominant at smaller scales, particularly in fine- textured soils rich in clays and colloids.  In contrast, biological factors become more influential at larger scales, especially in sandy soils with minimal clay content.  The interplay of these factors contributes to the stability and resilience of soil aggregates, shaping soil structure and function. 6. Physical-Chemical Processes in Soil  Physical-chemical processes are essential to understanding soil dynamics, influencing its structure, nutrient availability, and overall health.  These processes involve interactions between the soil's physical properties and its chemical constituents.  Two primary processes that stand out in this context are ion exchange and adsorption- desorption.  Ion exchange refers to the reversible interchange of ions between the soil particles and the soil solution, a process vital for maintaining soil fertility and structure.  Soil particles, especially clay and organic matter, have charged surfaces that can attract and hold onto cations (positively charged ions) and anions (negatively charged ions) from the soil solution.  These ions can be exchanged with other ions in the soil solution, depending on their concentration and affinity. Ion exchange controls the availability of essential nutrients such as potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), and ammonium (NH₄⁺), which plants absorb from the soil solution.  Additionally, this process influences soil pH by regulating the concentration of hydrogen (H⁺) and aluminium (Al³⁺) ions.  It also affects the soil's physical structure by altering the degree of dispersion or flocculation of clay particles, impacting soil aeration, water retention, and root penetration.  Adsorption-desorption refers to the process by which molecules or ions adhere to the surface of soil particles (adsorption) and are subsequently released back into the soil solution (desorption).  Adsorption occurs due to various forces, including van der Waals forces, hydrogen bonding, and electrostatic attraction.  Soil components such as clay minerals, organic matter, and oxides of iron and aluminium have large surface areas that facilitate adsorption.  This process helps in immobilising contaminants such as heavy metals and pesticides, preventing them from leaching into groundwater.  The soil's ability to hold onto nutrients and release them when needed is crucial for plant growth, central to the concept of the soil's cation exchange capacity (CEC).  Adsorption-desorption processes also regulate the stabilisation and decomposition of soil organic matter, influencing soil fertility and carbon sequestration.  The physical-chemical processes are highly dependent on soil texture, which refers to the relative proportions of sand, silt, and clay, and soil structure, the arrangement of soil particles into aggregates. 35  Fine-textured soils with high clay content generally have higher ion exchange capacities and adsorption potential compared to sandy soils.  Organic matter significantly influences these processes by contributing to the soil's ion exchange capacity and providing sites for adsorption.  The decomposition of organic matter releases nutrients and forms humus, which plays a key role in maintaining soil health.  Environmental factors such as temperature, moisture, and soil pH critically affect the rates and extent of physical-chemical processes.  Higher temperatures can enhance chemical reaction rates, while soil moisture affects the movement and availability of ions in the soil solution.  Understanding these physical-chemical processes is essential for soil management practices aimed at improving soil fertility, enhancing crop productivity, and mitigating environmental impacts such as soil pollution and erosion. 6.1. Mutual Attraction Between Clay Particles  In clay-containing soils, aggregation initiates with the clustering of clay particles into microscopic clumps.  When two negatively charged clay plates approach each other closely, the positive cations between them attract the negative charges on the clay plates, leading to their electrical binding.  This process repeats, forming stacks of parallel clay plates, which further bind to other clay clumps, charged organic colloids (humus), and fine silt particles.  These interactions create the smallest grouping in the hierarchy of soil aggregates, with clay clumps, cations, and humus providing stability to the aggregates.  The cations involved in this process include calcium (Ca2+), iron (Fe2+), and aluminium (Al3+), which possess multiple positive charges enabling them to bind the negatively charged clay plates together.  However, in the presence of sodium (Na+), a different phenomenon occurs.  The weak attractive forces of single sodium charges are insufficient to overcome the natural repulsion between negatively charged clay plates.  As a result, the clay plates disperse, rendering the soil structureless, impermeable to water and air, and unsuitable for plant growth.  High levels of sodium in soils, common in dry areas such as South Africa, lead to the formation of sodic soils, posing challenges to agricultural productivity. 6.2. Swelling and Shrinking of Clays  Clay minerals exhibit the property of swelling and shrinking in response to changes in moisture content.  When clay particles absorb water, they expand, and conversely, they shrink when they lose moisture.  This dynamic behaviour influences soil structure and permeability, impacting water movement, aeration, and plant root growth.  In clayey soils, volume changes occur in response to wetting and drying cycles, profoundly affecting soil structure and integrity.  Upon wetting, water molecules infiltrate between the clay plates, exerting pressure and forcing them apart.  Consequently, the clay particles swell, leading to an expansion of the entire soil mass. 36  This swelling phenomenon is a hallmark of clay-rich soils and significantly alters their volume and physical properties.  As the soil dries out and water evaporates, the clay plates move closer together due to the reduction in moisture content.  This shrinkage of the clay causes a corresponding contraction of the soil mass.  The drying process often results in the formation of cracks along planes of weakness, particularly in clay-rich soils prone to volume changes.  Repeated wetting and drying cycles induce significant alterations in soil structure.  With each cycle, the network of cracks becomes more extensive, and the aggregates between the cracks become better defined.  These alternating cycles of expansion and contraction contribute to the development of fissures and pressures within the soil, gradually breaking up large soil masses and facilitating the formation of well-defined structural peds. 6.3. Influence of Plant Roots  Plant roots also play a role in this process by locally affecting soil moisture content as they extract water from the surrounding soil.  Their activity contributes to the development of localized drying zones, further influencing the wet-dry dynamics and the formation of soil structure.  Plant roots and fungal hyphae play a vital role in soil aggregation by enmeshing soil particles with sticky networks of organic compounds.  As roots grow and extend through the soil, they move soil particles, promoting aggregation and forming pores that enhance soil structure.  Additionally, fungal mycorrhizae associated with plant roots produce a sticky protein called glomalin, which contributes to the short-term stability of soil aggregates. 6.4. Biological Processes in Soil Aggregation  Biological processes play a crucial role in soil aggregation, shaping the structure and stability of soil aggregates.  Several key activities of soil organisms contribute significantly to this process.  Earthworms are primary contributors to soil aggregation through their burrowing and moulding activities.  By moving soil particles and ingesting them, earthworms form pellets or casts, effectively restructuring the soil.  These actions promote aggregation by bringing particles together and creating channels that enhance soil porosity and break up larger clods, facilitating the formation of larger structural units. 6.5. Microbial Contribution  Microorganisms, particularly bacteria and fungi, contribute to soil aggregation by producing organic "glues" during the decomposition of organic matter.  These organic compounds bind soil particles and smaller aggregates together, forming larger aggregates.  Importantly, many of these organic glues resist dissolution by water, ensuring the stability of soil aggregates over extended periods.  These biological processes are most pronounced in the surface layers of the soil, where root and animal activities are highest, and organic matter content is elevated. 37  The combined actions of earthworms, plant roots, fungal hyphae, and microorganisms contribute to the formation and stabilisation of soil aggregates, influencing soil structure and fertility. 6.6. Influence of Organic Matter on Soil Structure  Organic matter plays a pivotal role in stimulating the formation and stabilisation of granular and crumblike soil structure.  This influence is multifaceted: o Organic Colloids Formation: As organic materials decompose, they contribute organic colloids that promote the cohesion of soil particles, facilitating the aggregation process. o Energy Source for Soil Organisms: Organic matter serves as an energy source for soil organisms, including earthworms, fungi, and bacteria. These organisms play crucial roles in soil aggregation through their activities, contributing to the formation of stable soil aggregates. 6.7. Influence of Iron/Aluminium Oxides  Iron and aluminium oxides play a significant role in binding smaller soil particles into stable aggregates, enhancing soil resistance to disintegration.  In soils with high levels of these oxides and ample rainfall, this binding effect can lead to the formation of iron pans, such as plinthite, which permanently bind soil particles together. 7. Influence of Agricultural Practices on Soil Structure  Agricultural practices significantly impact soil structure, with traditional methods often leading to soil structure decline.  These practices primarily affect soil aggregation and porosity, resulting in compacted and impermeable soil layers. 7.1. Soil Structure Decline  Under cultivation, mechanical mixing of the soil, as well as exposure to increased decay and oxidation rates of organic matter, contribute to soil structure decline.  Additionally, irrigation can exacerbate this decline by causing the breakdown of aggregates and dispersion of clay material due to rapid wetting. 7.2. Soil Management Practices  Various agricultural practices are employed to preserve and enhance soil structure: o Increase Organic Content: Incorporating pasture phases into cropping rotations can increase organic content, promoting soil aggregation. o Reduce Tillage: Minimising or eliminating tillage and cultivation in cropping and pasture activities helps preserve soil structure by avoiding mechanical disruption. o Maintain Ground Cover: Ensuring sufficient ground cover protects the soil from raindrop impact, reducing erosion and crusting. 38 7.3. Rangeland Management  In contrast to agricultural practices, rangeland management focuses on maintaining natural vegetation to preserve soil structure.  Pristine vegetation cover maximises organic matter input and shields the

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