Soil Science: Understanding Soil Formation, Properties, and Compaction, Study Guides, Projects, Research of Economics of Education

Download Soil Science: Understanding Soil Formation, Properties, and Compaction and more Study Guides, Projects, Research Economics of Education in PDF only on Docsity! SCHOOL OF AGRICULTURE, TECHNICAL STUDIES AND NATURAL SCIENCES PAGE 19 OPEN DISTANCE AND E-LEARNING (ODEL) P.O BOX 90-90128, MTITO ANDEI COURSE CODELADM 2104 COURSE TITLE BASIC SOIL SCIENCE 1 Table of Contents v COURSE DESCRIPTION..............................................................................................................................4  Importance of soil structure........................................................................................................5  Aerobic behaviour of soils...........................................................................................................5  Saturated soil water flow.............................................................................................................5  Hydraulic conductivity.................................................................................................................5  Ion Exchange and Cation Exchange Capacity...............................................................................5  Anion behaviour of soils..............................................................................................................6  Fertilizers and animal foodstuffs ACT..........................................................................................6 Figure 6: Adapted from LotusArise (2021), Soil Profile (Soil Horizons).......................................................12 Biogeography UPSC on 24th June 2023......................................................................................................12 3.6.1 Importance of soil structure...................................................................................................29 4.4.1 Aerobic behaviour of soils......................................................................................................36 4.6.1 Saturated soil water flow........................................................................................................40 4.6.2 Flow of water in unsaturated.................................................................................................40 4.6.3 Hydraulic conductivity...........................................................................................................41 5.8 Ion Exchange and Cation Exchange Capacity.........................................................................52 5.8.2 Cation exchange......................................................................................................................54 5.8.3 Anion behaviour in soils.........................................................................................................55 5.8.4 The nature and amount of clay..............................................................................................56 9.4 Factors Affecting Soil Organic Matter..............................................................................................80 10.4 Chapter 345 fertilizers and animal foodstuffs ACT..............................................................84 a) Discuss five factors that affect decomposition of organic matter in soils (4 marks).........................88 c) Using a suitable illustration, examine how rock outcrops influence adjacent soils and plant growth characteristics at fine scales in karst areas (6 marks)..............................................................................88 PURPOSE OF THE COURSE 2 . Course Outline Week one and Two  Definition, Composition ,Cons tituents, the Soil Profile and Important Irish Soils  Introduction  Soil as an ecosystem  The Concept of Basic Soil Science  The soil Profile.  Important Irish Soils Week Three  Soil Formation and Its Importance  Introduction  Soil Formation  Factors Influencing Soil Formation  Soils and Food Security  Functions of soils  Soil as a Medium of Plant Growth Week Four  The Science of Soil Physics and its importance  Soil Aeration  Introduction  Soil Texture and Textural Classification  Importance of soil texture  The Soil Textural Triangle  Properties of various soil textural classes  Soil Structure and Its Importance  Importance of soil structure  Factors Influencing Soil Structure  Soil Compaction  Causes and Effects of Soil Compaction  Effects of soil Compaction and Pore Space and Crop Production  Drainage and Infiltration of Water:  Aerobic and Anaerobic behaviour of Soils  Aerobic behaviour of soils  Anaerobic behaviour of soils Week Five  Cont.: Soil aeration  Water retention by Soils  Characteristics of retained Water  4Water Movement in Soils under Saturated and Unsaturated Conditions  Saturated soil water flow  Hydraulic conductivity Week six CAT 1 Week Six  Soil Chemistry  Introduction  Soil mineralogy and Soil Fertility:  Effects of soil Mineralogy on Soil pH and Cation Exchange Capacity  Fundamentals of layer Silicate Clay Structure  Classification of Layer Silicate Clays  Permanent and pH Depended Charges on Layer Silicate Clays  Charge Properties of Soil Organic Matter 5  Ion Exchange and Cation Exchange Capacity  Ion-Exchange/Cation exchange Week Seven  Soil Acidity  Introduction  The Nature of Soil Acidity  Buffer Capacity in Spoils  Factors That Influence Spoils Buffering Ability  Use of Lime to Control Soil Acidity  Benefits of Lime:  Aluminum toxicity  Anion behaviour of soils Week Eight and Nine  Fertilizer Sources  Organic Fertilizers  Introduction  Sources and Availability OF Nitrogen (N), Phosphorous (P) and Potassium (K).  Types of Fertilizer  Fertilizers with compounds vs. those without Week Ten  Soil Organic Matter, Manures and Waste Materials  Introduction  Soil Organisms  Organic Matter Cycling  Factors Affecting Soil Organic Matter Content  The Concept of Carbon-to-Nitrogen Ratios Week Eleven  Fertilizers and Animal Food Stuffs  Fertilizers and Animal Food Stuffs  Expected outcomes  Introduction  Legislation of sale of fertilizers and liming materials substances. Week Twelve  Fertilizers and animal foodstuffs ACT  Fertilizer use in Kenya and the Government guiding regulations 6 LECTURE ONE 1.0 Definition, Composition and Constituents, the Soil Profile and Important Irish Soils 1.1 Learning Outcomes By the end of this lecture, you will be able to: i. Demonstrate an understanding of the elementary aspects of soil formation. ii. Discuss basic soil physical, chemical, biological and morphological properties. iii. Explain the behavior of soils in managed and natural landscapes. 1.2 Introduction Different writers have given different definitions of the word soil or dirt as referred to some people. We may define soil as the unconsolidated mineral or substance that makes up the earth's surface and is modified by geography, creatures, climate, time, and parent material. C. E. Kellog (1960) stated that "the soil is a collection of natural bodies occupying a portion of the earth crust that supports the plant growth and has properties due to the integrated effect of climate, and vegetation acting upon parent material as conditioned by relief over a period of time," whereas Buckman and Brady define soil as a dynamic natural body on the surface of the earth where plants grow and is composed of mineral and organic materials, as well as living flora. Keep in mind that there are two ideas about soil: a) Soil is thought of as a natural body, a biochemically weathered and synthesized product of nature, b) Soil is thought of as a natural home for plants and other living things. Hence we can consider soil as three phased as shown below: 7 1.3 Soil as an ecosystem  When we discuss ecosystems, many of us neglect to bring up a few truly fascinating ecosystems. You would be astounded by the life and interactions that go on below the surface of soils. Indeed, if you could transform yourself into a little bug that burrows into the ground, you would be astonished by the components of this amazing ecosystem. Most of them are so little that it is difficult for the eyes to see.  A rudimentary food web in the soils may be seen by looking at the figure 2 below. The flow of energy is indicated by the arrow.  Plant roots are present in the soils. That is a biological element. For survival, the roots need nutrition, moisture, air, and temperature (abiotic factors). To prevent dangerous germs from adhering to the roots, fungus and bacteria are also necessary. There is also very small organic stuff present.  Arthropod shredders, such as weevils, millipedes, termites, and worms, which dig and churn the soils while consuming fungus and bacteria on dead plant material, supply nutrients and air. The chemicals required for the breakdown of other organic material are found in the feces (droppings) of these arthropods.  Mites, weevils, and bug eggs are also a source of food for birds and other tiny animals like moles. They all rely on moisture, air to survive.  Hence to maintain the ecosystem's longevity, it is crucial that each of its biotic and abiotic components is functioning properly as shown below: Figure 4: Soil as an ecosystem: Adapted from eSchooToday on 23rd June 2023 1.4 The Concept of Basic Soil Science The study of soil as a natural resource on Earth's surface includes soil formation, categorization, and mapping, as well as its physical, chemical, biological, and fertility aspects and how these properties relate to how soils are used and managed. The most crucial and fundamental natural 10 resource that sustains life on earth is soil. The weathering of rocks, the addition of organic matter, and ensuing profile development are what cause soil to form. Microorganisms that are in charge of the soil's many ecological and agricultural tasks use it as a natural abode. Being a natural environment for plants, soil offers support, nutrients, and water for plant growth and development. Soil has a crucial role in the ecosystem, acting as a natural and universal sink for a range of contaminants and preserving its quality. 1.5 The soil Profile The soil, which is the highest layer of the earth's crust, is mostly made up of rock and minerals that sustain life. A vertical cross-section of the soil made up of layers that run parallel to the surface is called a soil profile. These are referred to as "soil horizons."  There are five different kinds of strata (or horizons) in soils. However, the majority of soils only display the A, B, and C horizons, however typical soils have five master horizons which include O, A, E, B, C and R horizons as shown below. Accessed from Shallesh Tiwari, 2020 on 24th June 2023 Accessed from Creator: Picasa (2019) on 24th June 2023 Figure 5: The Soil profile  Horizon "O”: This topsoil layer is mostly made up of organic components, thus the name "organic horizon." It has about 20 and 30 percent organic materials. Due to the presence of organic material, this soil horizon frequently has a dark brown or black 11 appearance. Such horizons are typical in forested environments and are discernible in pristine soil.  Horizon “A” Both biological matter and other degraded materials make up the "A" Horizon, the top mineral horizon. It is also known as the humus layer because it has heavily humified organic materials. so the color that is darker than others. Microorganisms and seed germination both take occur in this stratum.  Horizon “E”: Eluviation refers to the removal of substances in suspension or solution by the percolating water from higher layer to lower layer, and the "E" Horizon is the horizon of highest eluviation and is made up of nutrients that have been leached from the O and A horizons.  Horizon “B”: The "B" Horizon, which is the subterranean horizon, is less humus, soluble minerals, and organic matter than the top soil. It is the Horizon of Maximum Illuviation of Fe, Al oxide, and silicate clays. Illuviation is the deposition or accumulation of soil elements in the lower strata.  Horizon “C”: Unconsolidated material can be found below the solum on the "C" Horizon. Ca, Mg, carbonates, and cementation all accumulate. The least weathering has occurred here.  Horizon: “R”: Below the "C" horizon, in the cemented layer, lies the parent material, or bed rock. Various types of rocks may be found here. It is important to note that all of the weathered material in the profile is part of the regolith. The solum and the saprolite are the two parts of the regolith. The most worn area of the profile's top horizons are included in the solum. The layer right above the dense, consolidated bedrock but below the regolith is the saprolite, which is the least weathered. 12 2.1 Learning outcomes By the end of this lecture, you should be able to: i. To explain the factors that influence soil formation ii. Understand the functions of soils iii. Explain the role of soil health in mitigation of food insecurity 2.2 Introduction One of our most important natural resources is the soil as it is a fundamental pillar for the living organism’s food chain. The soil provide plants stability and nutrients. Furthermore, numerous microorganisms, including earthworms, rodents, reptiles, amphibians, squirrels, the Naked Mole rat, scorpions,, millipedes, some spiders species, centipedes, some bird species as well as a number of other earth’s underground animals, call it home. 2.3 Soil Formation  When air agents break down the earth's rocks physically and chemically, soil creation occurs and physical disintegration and chemical deterioration are the two basic weathering processes. These simultaneous physical and chemical actions on the parent material are crucial for the creation of soil. Remember that soil is the thin layer of material that covers the surface of the earth and is created by the weathering of rocks. According to R.B. Harrison and B.D. Strahm (2008), soil is crucial for ecological function because it serves as the foundation for the growth of terrestrial plants and provides them with nutrients, water, support, and protection from the elements.  In addition, soil serves as a carbon pool that may either operate as a source or sink for atmospheric carbon dioxide, a home for soil organisms, and a filtering mechanism for both surface and ground water.  Soils are formed from rocks (the parent material) by weathering and erosive forces of nature. Parent material is broken down by a variety of factors, including water, wind, gravity, temperature change, chemical reactions, living things, and pressure variations.  R. Amundson, (2014), the boundary conditions for the soil system greatly influence the process of soil formation. The physical composition of the landscape, which determines the characteristics of the 'geomorphic surface': the atmosphere/land border, is one of the crucial factors. Landscapes can be either stable, erosional, or depositional from a physical 15 standpoint. While wind and other forces can cause physical erosion or deposition, slope- driven transport predominates in many soil-mantled ecosystems.  As documented by R.B. Harrison, B.D. Strahm (2008), there are several perspectives from which to examine soil and soil development, including the study of soil science as a separate subject. The base for the development of terrestrial plants, including the provision of nutrients, water, temperature regulation, and support, is soil, which plays a crucial role in ecological function. Additionally, soil serves as a home for soil organisms, a source or sink of atmospheric carbon dioxide, a filtering system for surface and ground water, and a pool of carbon. Not a renewable resource like crops or forests, soil has frequently been compared to nonrenewable resources like coal or oil. However, soil is frequently a slowly replenishable resource, and deteriorated soil may occasionally be repaired.  Bernhard Lucke (2023) reported that in arid and semi-arid areas, in dry and semi-arid habitats, physical processes control soil formation and the soils are rarely do deep and severe soil formations occur because chemical weathering is negligible. The soil types that have little or no soil growth are what define these soils. In addition, significant amounts of parent materials are contributed by dust and Aeolian sediments, which can result in the formation of deep soil coverings as in desert fringe loess.  Lucke further adds that since the onset of the earliest civilizations, soils on desert fringes have been utilized for agriculture, and they may be quite productive when exploited in accordance with appropriate land use practices.  Mason and Zanner (2005) assert that the climatic conditions that favor grassland growth as well as the particular features of grassland ecosystems have a significant impact on soil formation in grasslands. The pace of mineral weathering is often constrained in most grasslands by frequent soil moisture deficiencies, which frequently cause secondary carbonate mineral buildup in the lower soil layers. 2.4 Factors Influencing Soil Formation There are five factors that influence soil formation, these are include: a) Climate 16  Rainfall and temperature are the two most crucial elements in this system, which also includes temperature, humidity, and wind. These have an impact on biological, physical, and chemical processes.  Climatic is the main determinant of soil formation, and soils exhibit the particular traits of the climatic zones in which they develop. Climate is also influenced by the feedback of carbon stored in soil strata that is released back into the atmosphere.  The pH, base saturation of the exchange complex, organic matter content, and the type and composition of clay are soil characteristics that are impacted by climatic circumstances. The rate of weathering and organic breakdown is influenced by temperature. These processes can take longer in climates that are colder and drier, but they move more quickly in climates that are warm and humid.  By lowering or increasing the amount of water present, evaporation, transpiration, and humidity alter the impact of precipitation. In locations with little rainfall, salts build up on the soil's top; in areas with heavy rainfall, salts are leached off into the lower layers, making the soil acidic. b) Parent material  The parent material is transported, deposited, and precipitated before being chemically and physically weathered and changing into soil. Mineral components are what solum is made of Quartz (SiO2), Calcite (CaCO3), Feldspar (KAlSi3O8), and Mica (K(Mg,Fe)3AlSi3O10(OH)2) are the common soil parent mineral minerals. The characteristics of the soil that is created will be influenced by the sorts of parent materials and the circumstances in which they decompose. Parental materials are categorized according on how they were deposited. c) Organisms a) The creation of soil is actively influenced by the flora (plants, microbes, grasses, and forests) and fauna (termites, earthworms, and rodents). As they create holes and burrows, soil organisms mix the soil, facilitating the movement of gases and moisture. Bioturbation is the name given to this process. 17 It thus becomes important to protect the soil from contamination. Soil contamination is occurs when a substance's concentration is higher than it would be in a natural environment. This can depend on the substance in question for example nutrient, pesticide, organic material, acidic or alkaline molecule, or trace element. In order to maintain the required productivity, integrated nutrient management (INM) which refers to the maintenance of soil fertility and plant nutrient delivery at an optimal level becomes critical. In order to avoid negative environmental effects from nutrient outflows, this is accomplished by maximizing the advantages from all potential sources of organic, inorganic, biological, and recyclable waste components in an integrated way. NEXT CLASS 2.6 Functions of soils 2.6.1 Soil as a Medium of Plant Growth The development and growth of plants are supported by soil. Its characteristics have an impact on plant root support, gas exchange, water relations, and fertility.  Solid, liquid, and gaseous phases make up the three main components of soil, which is a heterogeneous substance. The delivery of nutrients to plant roots is specifically influenced by all three stages. The major nutrient storage phase is the solid phase. This phase's inorganic particles include cationic nutrients like K, Na, Ca, Mg, Fe, Mn, Zn, and Cu, Mo, Bo, among others.  The organic materials from the primary N and, to a lesser degree, P and S reserves are critical to plants growth. Note that water drainage, pH levels, and organic matter are three key factors which affect plant growth, among other factors, it is important to note that nutrient transport in the soil is facilitated by the liquid phase of the soil, known as the soil solution. In summary, you can easily see that soil is helpful as a substrate for plant growth because it:  Supports roots and keeps them erect for growth.  Give plants the vital minerals and nutrients they need. 20  Provide air to allow for gas exchange between the atmosphere and the roots.  Prevent erosion and other harmful biological, chemical, and physical activities on plants.  Keep moisture (water) in place and provide proper aeration Now, let us look at the key essential tasks carried out by soils in the global environment. i. Plant growth medium, Soil offers a suitable environment for a plant to establish root since it acts as an anchor for plant roots and a water reservoir for vital moisture. The soil texture (course of fine), aggregate size, porosity, aeration (permeability), and water holding capacity are some of the soil characteristics that have an impact on plant growth. Physical characteristics of the soil and plant development. This maize plant's exposed roots provide indications of preferred development to the right, away from the region where dirt has been compacted in the wheel track area on the left. Photo Credit to Dr. John Doran Figure 8: Exposed maize plant roots The ability of soil to hold and provide nutrients to plants is a crucial function. Soil fertility is the capacity to carry out this job. A soil's organic matter (OM) and clay content have a direct impact on its fertility. Soil fertility will typically increase as clay and organic matter concentration increase due to enhancement of Cation Exchange Capacity (CEC) to be discussed in later chapters. ii. Soil as a regulator of water supplies As rain or snow falls on the ground, the soil is there to absorb and store the moisture for later use, thus acting as a regulator of water supply (Hydrologic Buffer). As a result, a reservoir of water becomes accessible for plants and soil organisms to survive in between periods of precipitation or irrigation. For plant development and water supply, soils absorb, store, modify, filter, and release water.. The finer the soil particles, the greater the soil's power to trap contaminants and stop them from seeping into ground water. The 21 capacity of soil to hold water is known as its water retention capacity, and it is correlated with the soil's particle size distribution. iii. Recycler of raw materials; from the ruins of the old, new life arises. The leftovers of once-live tissues are used by plants, fungi, and insects as a source of energy and to produce new living tissues. One of the most important tasks that soil performs in the global ecology is the recycling of basic materials. Dead plants, animals, and other species are broken down by soil flora and fauna (such as bacteria, fungus, and insects) into simpler mineral forms, which are subsequently used by other living organisms to produce new living tissues and soil humus iv. Soil organisms' habitat: In the soil, which is populated by organisms of all sizes, more than half of the "life" on the planet may be discovered. There are all sizes in between, from massive, plainly visible plant roots and animals to tiny mites and insects to microscopic-sized microorganisms (such as bacteria and fungus). Microorganisms, which are the primary decomposers of soil, perform the majority of the effort needed in converting and recycling old, dead materials into the basic elements necessary for the production of new plants and animals. Photo: Courtesy of USDA-NRCS A burrowing earthworm leaves behind'middens' of feces on the soil's surface, where bacteria and other soil creatures can further decompose it. On their way to forming humus, several organisms continuously ingest and digest organic compounds in soil. Image from the USDA-NRCS Figure 9: A burrowing earthworm v. Soil carbon: Soil is the largest repository of terrestrial organic carbon, accounting for more than twice as much carbon as plants. vi. Landscaping and engineering medium = (Building materials) = Soils are the foundation material for roads, homes, buildings, and other structures set upon them, but the physical characteristics of different soil types are greatly variable. 22  It is observed that soils are quite distinct and diversified, depending on location, and thus contain varying ratios of building blocks. Air space exists between these building blocks. The various relative textural particles which include sand, silt, and clay create the soil's texture. It is noted that despite their tiny size, clay particles contain a lot of surface area.  Aggregates of soil mineral particles, with or without organic components, are called soil peds. They normally withstand several wetting and drying cycles, but over time they break down and reform, particularly in topsoil where decomposable organic compounds bind particles together. Shape, size, and strength are used to categorize various types of soil aggregates. 3.2 Soil Texture and Textural Classification  Sand, silt, and clay particles make up soil. The percentage of different-sized mineral particles in the soil also affects the texture of the soil. One can simply ask, what exactly is soil texture, and why is it significant?  The fineness or coarseness of a particular soil sample and the distribution of the three types of soil particles are referred to as soil texture. (Silt, clay, and sand) 3.3 Importance of soil texture Soil texture is important because it has influence on:  The soil's capability to store water for use by plants and animals is known as its water holding capacity.  It also has an impact on permeability, which refers to how easily air and water may enter or pass through soil.  Affects soil workability, which refers to how quickly and easily soil can be worked following rain.  Some root crops, such as carrots and onions, will have a tough time growing in a fine- textured soil.  Be aware that fine-textured black cotton soils may significantly impair the development of onions' bulbs in clay. 25 The relative size sand, silt and clay soil particles can be illustrated as shown below Figure10: Spoil particles - Adapted from other sources Credit, The University of Waikato. Class activity: Separation of soil particles  You are required to demonstrate the various sizes present in a given soil sample. You will need a glass jar, soil sample and water. Fill two thirds of the glass jar with water. Add your soil sample and shake thoroughly and let the contents settle. You will notice that various soil particles present settle according to their sizes. 26 Figure 11: Size of soil particles - Credit: Ameliamurtha 3.4 The Soil Textural Triangle As was previously mentioned, the texture of the soil provides information on the relative numbers (amount) of the various particles in the soil that are different sizes. The soil textural triangle is a tool used by soil scientists to identify these soil particles and compares the relative particle compositions. A typical soil textural triangle is as shown below: 27 3.6.1 Importance of soil structure  It increases tilth and permeability.  Prevents crusts from forming that weaken crop stands by resisting the pounding action of raindrops.  In addition to permeability, soil structure also affects other soil characteristics including porosity, aeration, heat transmission, and water flow.  The structural changes that a farmer makes to a field through plowing, cultivating, draining, liming, and manuring are all significant.  The amount of water and air in a soil sample is controlled by the structure of the soil.  For respiration, plant roots and seed germination need enough air and oxygen. Soil structure has an impact on this.  Microbial activity also needs access to water and air in the soil to function normally. Class activity Activity1: Read widely in this section make an observation of the different types of soil structure found in your school compound. Make sketches and see if they compare with the general soil structure types given below. Figure 12: Common types of soil structure 3.6.2 Factors Influencing Soil Structure 30 a) External load  Compaction  Poor drainage  Unnecessary tillage  Treatment of wet soil  Heavy rain b) Natural processes  Biological activity  Amount and species distribution of worms  Microbes  Liming and use of manure  Cultivation techniques c) Soil properties  Soil texture  Soil organic matter, amount and quality  Clay content  Soil p H  Ion concentration in soil solution  Amount and quality of cations  Cation exchange capacity (CEC) 3.7 Soil Compaction According to Alakukku, L. (2012) from University of Helsinki (Finland), soil compaction is the increase in bulk density or reduction in porosity of soil caused by externally or internally imposed loads. It is also referred to as soil structure deterioration or degrading. The phrase soil compaction is a term used to describe the mechanical densification of soil. The diagram below shows non–compacted soil on the left and a compacted soil in the right. 31 Figure 13: Soil compaction - created by Neil Hansen, University of Minnesota, 2003 3.7.2 Causes and Effects of Soil Compaction  Subsoil compaction has an impact on a variety of ecosystem services and soil processes.  Generally speaking, soil compaction is becoming a bigger issue as farm tractors and other field equipment get bigger and heavier.  The bulk of field operations, which are frequently carried out when the soils are wet and more prone to compaction, can be linked to soil compaction.  Tillage tools and heavy machinery might harm the soil's structure.  The ability of a soil to store and convey water, nutrients, and air essential for plant root activity is determined by the soil's structure.  Soil compaction occurs when soil particles are pressed together, reducing pore space between them. 3.7.2 Effects of soil Compaction and Pore Space and Crop Production Heavily compacted soils contain few large pores and have a reduced rate of both water infiltration and drainage from the compacted layer. This occurs because large pores are the most effective in moving water through the soil when it is saturated. Indeed, in a compacted soil, roots must exert greater force to penetrate the compacted layer. Reduced soil aeration affects root metabolism and thus plant growth. 32 4.0 Soil Aeration 5.0 Increasing the air exchange in the soil allows for improved oxygen penetration and the elimination of extra carbon dioxide. This process is known as soil aeration. To promote gas mobility and soil health overall, it entails forming channels or pores inside the soil. Healthy plants require aeration because it encourages root growth, nutrient uptake, and microbial activity. It can be accomplished using a variety of techniques, including appropriate soil management practices, tilling, mechanical cultivation, and the use of specialist equipment like spike or core aerators. Farmers and gardeners may increase crop output, avoid soil compaction, and promote greater water drainage by enhancing soil aeration. 4.1 Learning outcomes This lecture will educate you on different types of respiration, water retention in soils, characteristics of retained water in soils, water movement in soils, hydraulic conductivity, 4.2 Introduction  This lecture deals with “dynamic relationship” of the amount of air in the soil. Generally, animals that make holes in the soil allow for easier access to air, water, and nutrients enter the in what is simply known as soil aeration.  The dynamic equilibrium is dominated by two key soil components, air and water.  Your plants' roots may not receive enough nutrients if the soil is compacted, which might cause them to become weak or even die.  The process of increasing air exchange within the soil is known as soil aeration. It entails boosting the soil's oxygen and carbon dioxide circulation, which is essential for plants to grow and flourish.  Compacted or inadequately aerated soil can restrict root development, lower nutrient availability, and obstruct water penetration, hence soil aeration is essential. We can have several methods through which soil is aerated, these include: 35 1. Mechanical Aeration: In this technique, compacted soil is physically broken up using equipment like aerators or tillers. The soil can be spiked or corked to make holes, or tilled to make the soil more pliable. 2. Aeration naturally: Soil aeration is a result of organic processes including freeze-thaw cycles, earthworm activity, and plant root development. These procedures aid in forming air passageways inside the soil. Compost or well-rotted manure are two examples of organic matter that may be added to the soil to improve its structure and porosity. As a sponge, organic matter allows air to easily circulate through the soil. The benefits of soil aerations include: -  Improved soil aeration makes it possible for roots to receive oxygen, which is necessary for their ability to breathe and for their general wellbeing.  Improves soil permeability  Improves Nutrient absorption  The ability of healthy roots to absorb water and nutrients from the soil is improved.  Enhances good drainage, an important prerequisite towards minimization of risks associated with fungal diseases  Creates a favorable environment for healthy root growth and development Availability of nutrients:  Proper soil aeration encourages the breakdown of organic matter and the release of nutrients, increasing their accessibility to plants.  Additionally, it helps stop nutrient leaking. 4.3 Drainage and Infiltration of Water:  A well-aerated soil structure permits infiltration of water into the soil.  Facilitates effective drainage and is simple to use. 36  This lessens the risk of waterlogging and increases the amount of water available to plants. Enhanced soil structure:  By generating pore spaces, soil aeration helps to reduce compaction and enhances soil structure and as a result, the flow of roots, water, and air is improved, helping to maintain a healthy soil environment. We can conclude that: -  Keeping healthy soil for plant development requires the crucial practice of soil aeration and enhances soil structure overall, root growth, nutrient availability, water circulation, and soil structure.  Gardeners, farmers, and land managers may maximize plant growth and production by using the right soil aeration techniques. 4.4 Aerobic and Anaerobic behaviour of Soils Aerobic and anaerobic behaviour of soils refers to the presence or absence of oxygen in soil environments. Here are short notes on aerobic and anaerobic behaviour of soils:  The structure of soil particles in aerobic soils permits air to flow freely via the pores (open spaces between soil particles). Anaerobic soils, on the other hand, have a restricted air movement within their soil pores because of a high moisture or water table level. 4.4.1 Aerobic behaviour of soils Because aerobic soils have a sufficient amount of oxygen, they support the development and activity of aerobic organisms including bacteria, fungus, and earthworms. Oxygen is essential for many soil activities, such as the breakdown of organic matter, nutrient cycling, and the formation of soil structure. Under aerobic conditions, friable, well-aerated soils that have high drainage and are favorable to plant development can occur. Aerobic soils often contain a more balanced microbial population, which aids in preventing the growth of hazardous diseases. The presence 37 water to pass through and lowers water retention. Compacted soils, which have limited porosity, have less area for water to occupy, which can result in poor water retention.  Compaction: Due to compression, compacted soils have less pore space, which limits water penetration and retention. Compaction can happen both naturally and as a result of human activity, such as using large machinery or walking a lot. The ability of compacted soils to retain water is frequently inadequate.  Organic Matter Content: Soils with more organic matter tend to retain water better. The soil becomes more porous when organic stuff, such as decayed plant and animal debris, is present. The capacity of these aggregates to keep water improves the soil's capacity to hold moisture. Low levels of organic matter in soils may result in less water retention.  Drainage: For water retention, proper drainage is crucial. Poorly draining soils, such as clayey soils with a high water-holding capacity, are susceptible to saturation and flooding. Inadequate drainage makes it impossible to properly store excess water, which reduces water retention.  Slope and Erosion: Steep slopes can cause water to rush off and away from the soil, which prevents water from being retained. The top layer of soil, which is often richer in organic matter and has superior water-holding capacity, can also be removed through erosion, which lowers the soil's ability to retain water. When evaluating soils' capacity to keep moisture, it's crucial to take into account certain water retention properties that soils lack. Combining these elements can affect plant development, groundwater recharge, and the total water retention capacity of various soil types. 4.6 Water Movement in Soils under Saturated and Unsaturated Conditions  Due to the variety in the states and directions in which water flows as well as the variety in the forces that cause it to move, movement of water inside the soil is a very complicated process. Saturated flow, unsaturated flow, and water vapour flow are the three main forms of water movement within the soil that are generally recognized (Fig. 15). Under the action of gravity, liquid water travels through soil pores that are filled with water (saturated state). In an unsaturated state, water surrounds soil particles in thin films, which surface tension causes 40 to move. Along the vapour pressure gradient, water in the vapour form diffuses through pores in air. Water always moves along energy gradients, or from higher potential to lower potential. Figure 15: Different types of water movement in soils (Source:http://www.terragis.bees.unsw.edu.au/terraGIS_soil/images/water_fig_9.jpg: accessed on June 25th 2023) Under the action of gravity, liquid water travels through soil pores that are filled with water (saturated state). This can lead to soil erosion. Remember sheet erosion, rill and gulley erosion) 4.6.1 Saturated soil water flow All of the soil's macro- and micro-pores are filled with water when it is saturated, and any water flow under this circumstance is referred to as a saturated flow. The hydraulic gradient, which is the hydraulic force pushing the water through the soil, and hydraulic conductivity, which is the ease with which the soil pores admit water movement, are the two parameters that determine the saturation flow of water. 4.6.2 Flow of water in unsaturated  The soil's macrospores are emptied and primarily filled with air as gravity drainage proceeds, while the micropores, or capillary pores, are mostly filled with water and some air.  Unsaturated flow is the word used to describe the water movement that takes place in this situation.  The water potential in an unsaturated flow state is equal to the sum of the metric potential and gravitational potential. 41  Metric potential is only relevant when water is moving horizontally. Water moves downward due to a combination of capillary action and gravitational potential. Metric potential and gravitational potential are in opposition as water rises in capillaries.  You should be able to advice farmers when to plough so as to minimize soil compaction. As discussed in the earlier section on page 30, ploughing during the wet period increases soil compaction and should be avoided. Famers should plough and prepare their farms during the dry season. 4.6.3 Hydraulic conductivity This is the ratio of Darcy's flow rate to the hydraulic gradient and is used to determine hydraulic conductivity. Hydraulic conductivity, in theory, is a measurement of the ease with which water may permeate soil or rock: high values signify permeable material, while low values signify less permeable material. In other words, it is how quickly pore fluid escapes from the compressed pore space is determined by hydraulic conductivity (Peter Aird, 2019) and it is one of the most unpredictable yet crucial elements in the estimate of contaminant transport time is hydraulic conductivity. In Darcy's law (Shackelford, 2013), it is described as how liquid flows through porous material, the coefficient of proportionality is referred to as hydraulic conductivity. It is in record that aquifer pumping tests are the most trustworthy way to measure hydraulic conductivity (Jun Lu, 2015). As reported by Akker and Soane (2005), soil is sheared and kneaded during the compaction process by wheels (tractors, combine harvesters, vehicles etc.) which is particularly sensitive to saturated hydraulic conductivity. 42 industrial processes, agricultural methods, and poor waste disposal. In order to restore the health of the soil, soil remediation procedures try to minimize or remove these toxins.  Crop productivity, environmental care, and sustainable land management all depend on an understanding of soil chemistry. It helps researchers, farmers, and politicians to make well-informed choices on pollution prevention, soil conservation, and nutrient management, which improves soil health and ecosystem sustainability. 5.3 Soil Mineralogy Soil mineralogy is the study of soil minerals, including their composition, behavior, and physical and chemical characteristics. The minerals that are present in the soil have a significant impact on its fertility, structure, and other properties. Soil minerals are naturally occurring inorganic compounds that make up a significant portion of the soil matrix. They develop as a result of the weathering and decomposition of the parent rocks and minerals throughout time. Common soil minerals include quartz, feldspar, mica, clay minerals, and various metal oxides. The compositions and physical traits of the minerals contained in soil vary widely, according to soil experts. They can be separated into primary minerals—inherited from the parent material—and secondary minerals—produced by the weathering processes of the soil. Primary minerals are non-clay minerals with low surface area (silica minerals) and little reactivity, according to Berkowitz et al. (2008). These minerals mostly affect liquid and vapour movement physically. Clay minerals like kaolinite, montmorillonite, and illite are classified as secondary minerals, whereas quartz, feldspar, and mica are primary minerals. According to Low (1961), secondary minerals are clay minerals with a significant surface area and high reactivity that affect the chemical movement of liquids and vapors. According to the information that is currently available, quartz, one of the most common minerals and a silica mineral, makes up to 95% of the sand fraction. The amount of silica mineral depends on the original material and the degree of weathering. Quartz is naturally rounded or angular as a consequence of physical wear because of the close packing of the crystal structure and the high activation energy required to modify the Si-O-Si bond. 45 5.3.1 Soil mineralogy and Soil Fertility: Because they give plants crucial nutrients, soil minerals are necessary for soil fertility. Potassium, phosphorus, calcium, magnesium, and trace elements are among the elements found in many minerals and are essential for plant development. The general fertility of the soil is impacted by the presence of certain minerals, which alter soil pH, cation exchange capacity, and nutrient availability. Minerals in the soil have a big impact on the texture and structure of the soil. The shape and distribution of mineral particles in the soil determine its texture, which has an impact on drainage, aeration, and water-holding capacity. The vast surface area and high water-holding capacity of clay minerals in particular can lead to soil compaction and hinder root development. 5.3.2 Effects of soil Mineralogy on Soil pH and Cation Exchange Capacity The minerals in the soil affect the cation exchange capacity (CEC) and soil pH. Calcium carbonate, which is present in certain minerals like calcite and dolomite, helps balance acidity and increase the pH of soil. CEC, which impacts the availability of nutrients to plants, is the capacity of soil minerals to hold and exchange cations (positively charged ions). Analysis of Soil Mineralogy: X-ray diffraction (XRD), scanning electron microscopy (SEM), and chemical analysis are a few of the methods used to study soil mineralogy. These techniques offer information on the composition, crystalline structure, and abundance of soil minerals while also assisting in their identification and characterization. For maintaining soil fertility, increasing crop yield, and implementing sustainable agricultural methods, it is essential to have a solid understanding of soil mineralogy. Scientists and agronomists may decide wisely on soil management, fertilizer supplementation, and soil amendment techniques to maximize plant development and safeguard the environment by researching the mineral composition of soils. 46 5.4 Fundamentals of Layer Silicate Clay Structure The essential structural components of silicate clays are the tetrahedral and octahedral sheets. These sheets are joined into several layers within the crystals by oxygen atoms that are shared. Ionic bonding, the attraction of positively and negatively charged atoms—holds planes of oxygen atoms in silicate clays—the bulk of which are made up of silicon and aluminum atoms— together. A layer is made up of silicon and aluminum ions sandwiched between three or four planes of oxygen atoms. Based on the colloid mineral's silicate and crystalline properties, colloids are categorized into three classes. They are silicate clays, crystalline layer silicate clays, and silicate clays with noncrystalline layers. Layered silicates, like montmorillonite (MMT), have an octahedral film of either magnesium or aluminum hydroxide sandwiched between two fused silica tetrahedral sheets. As documented by Kumari and Mohan. (2021), the primary raw materials for making clay are clay minerals that are created in the presence of water, such as kaolinite, smectite, chlorite, and micas. There are several clays used to create various structures, all of which have different mining sources. They are classified as hydrous phyllosilicates and may be found either in interlayer space or on the surface of planets. They include silica, alumina, and water along with varying amounts of inorganic ions, such as Mg2+, Na+, and Ca2+. 5.4.1 Classification of Layer Silicate Clays Silicate clays are divided into three types according to the quantity and configuration of tetrahedral (silica) and octahedral (alumina-magnesia) sheets present in the crystal units or layers. 1.1 clay minerals 2:1 type clay minerals 2: 1: 1 (or) 2: 2 types clay minerals 47 Figure 17: Adapted from Pohl WL. DOI: 10.1002/9781444394870 5.6 Permanent and pH Depended Charges on Layer Silicate Clays The layers of layer silicate clays like smectite, illite, and kaolinite are layered one on top of the other. Tetrahedrally coordinated sheets of silica (SiO4) and octahedrally coordinated sheets of aluminum (AlO6) or magnesium (MgO6) ions make up these layers. Weak van der Waals forces and electrostatic interactions keep the layers connected.  Layer silicate clays may have persistent charges as a result of ion substitutions occurring inside the crystal lattice. For instance, part of the aluminum ions in the octahedral layer of smectite clays can be replaced by magnesium ions, resulting in a permanent negative charge. Similar to this, a positive permanent charge is produced when silicon replaces aluminum in the tetrahedral layer of illite and kaolinite clays.  Charges that are depending on pH may also be seen in layer silicate clays in addition to permanent charges. The dissociation of hydroxyl (OH) groups on the surface of the clay particles is what causes the pH-dependent charges. The clay particles' surface becomes protonated at low pH levels (acidic conditions), producing a positive charge. In contrast, the surface becomes deprotonated at high pH levels (alkaline conditions), producing a negative charge.  Layer silicate clays' pH-dependent charges are critical to a number of soil and environmental processes. The cation exchange capacity (CEC), nutrient retention/release, water movement, and adsorption/desorption of pollutants are all influenced by these charges. The exact clay 50 mineral, its chemical make-up, and the pH of the surrounding solution all influence the size and kind of the pH-dependent charges.  It is noted that studying the interactions of layer silicate clays with other substances in various environmental and commercial applications requires an understanding of the permanent and pH-dependent charges on these materials. Activity 1.  Study the figure below and explain how clays get their charge Figure 18 Activity 2 Examine the importance of ion exchange under the following:  Retention and liberation of plant nutrients  Control of soil structure  Control processes of soil formation  Reclamation of acid and alkali soils  Influence the effect of fertilizer and fertilizer practices 5.7 Charge Properties of Soil Organic Matter For its interactions with other soil components and its overall function in soil fertility and nutrient cycling, soil organic matter (SOM) has a number of charge features that are significant. Key points about the charge characteristics of soil organic matter are as follows: 51  Negative Charge: Indeed, the presence of functional groups containing oxygen, such as carboxyl (-COOH) and phenolic (-OH) groups, gives soil organic matter a net negative charge. These negatively charged sites in the soil draw and hold onto positively charged ions known as cations.  The capacity of organic matter in soil to hold and exchange cations is known as cation exchange capacity (CEC). With its negative charge, SOM functions as a cation exchange complex, holding onto and preventing the loss of important nutrients including calcium (Ca2+), magnesium (Mg2+), potassium (K+), and ammonium (NH4+) from the soil.  The availability of nutrients is influenced by the negative charge of SOM, which releases cations into the soil solution when plants require them. The SOM releases an equivalent number of cations into the soil as cations are extracted by plants, ensuring a constant supply of nutrients for plants.  Organic soil material functions as a buffer against pH fluctuations in the soil. The soil pH may be stabilized by it by absorbing and releasing hydrogen ions (H+), avoiding sharp swings that could harm plant development.  Soil Organic Matter has some ability to retain anions, which are negatively charged ions, even though it mostly attracts cations. The availability and transport of anions like nitrate (NO3-) and phosphate (PO43-) in the soil can still be impacted by this retention, despite it often being less effective than cation retention.  Charge Density: The amount of negative charge present in a unit weight or volume of soil organic matter is referred to as the charge density of SOM. Various factors, including soil type, the makeup of the organic matter, and management techniques, can affect charge density. The CEC and nutrient retention capabilities of SOM are improved by higher charge density.  The management of soil fertility, nutrient cycling, and the preservation of a healthy soil ecosystem depend heavily on an understanding of the charge characteristics of soil organic matter. These characteristics control the availability of nutrients, pH regulation, and overall productivity of natural and agricultural systems. 52 5.8.2 Cation exchange In order to understand Cation Exchange Capacity with ease, it is important to remember your form chemistry on water hydrolysis.  The amount of total negative charges in the soil that bind to cations of plant nutrients like calcium (Ca2+), magnesium (Mg2+), and potassium (K+) is known as the cation exchange capacity, or CEC. Thus, a soil's capacity to provide nutritional cations to the soil solution for plant uptake is described by the CEC.  The entire capacity of a soil to contain exchangeable cations is known as cation exchange capacity (CEC) and it is challenging to considerably change CEC since it is an innate soil property.  It affects the soil's capacity to retain crucial nutrients and acts as a buffer against the acidity of the soil. Generally, greater clay percentage soils often have greater CECs.  The CEC of organic materials is quite high.  Sandy soils strongly rely on organic matter with a high CEC to retain nutrients in the topsoil. See the illustration below for closer details. Figure 19: Adapted from Agri learner on 25th June 2023 at (21.16 hours) 55 5.8.3 Anion behaviour in soils  Anions, which are ions that have a negative charge, are very important in the chemistry of soil. For a variety of agricultural, environmental, and soil management methods, it is crucial to comprehend how they behave in soils.  Adsorption and Desorption: Soil particles, especially clay minerals and organic materials, can be adsorbed with anions from the soil. Adsorption is influenced by variables such soil pH, ionic strength, and anion concentration. Additionally, anions are capable of being desorbed from soil particles, which makes them mobile in soil solutions.  Different leaching tendencies are seen by anions in soils. Nitrate (NO3-) and chloride (Cl-) are two highly mobile anions that are prone to leaking through the soil profile and possibly reaching groundwater sources. Water quality and nutrient management may be affected by this.  Anions in soils frequently engage in competition with one another for adsorption sites. Weaker anions, like sulfate (SO42-), can be driven off the surface of soil when stronger anions, such phosphate (PO43-), are present. For nutrient management and fertilizer application, it is essential to comprehend these competitive interactions.  Plant Uptake: Through their root systems, plants take ions from the soil. The availability of anions, which are crucial for plant nutrition, has an impact on crop output. Plant absorption and total nutritional status may be affected by soil characteristics that affect the availability of anion, such as pH and the amount of organic matter.  Conclusion: The complicated process of anion behavior in soils is regulated by a number of variables, including soil characteristics, pH, and competitive interactions. For nutrient management, environmental preservation, and sustainable agriculture practices to be successful, it is essential to comprehend these processes. Our understanding of anion behavior is continually being enhanced by ongoing study and strides in soil science, which results in better soil management tactics.  Effects of pH: Anion behavior is substantially influenced by soil pH. Low pH 56 makes anions like phosphate and sulfate more likely to be retained. On the other hand, anions like nitrate can be more easily leached in alkaline circumstances. Optimizing anion availability and reducing environmental effects need careful pH regulation.  In this case, anions rather than cations will be attracted to and retained by the soil. Anions are negatively charged as opposed to cations. In decreasing strength order, phosphate, sulfate, nitrate, and chlorine are the anions that soil particles hold and keep.  The release of fixed phosphate into the soil and subsequent increase in its availability to plants is mostly caused by the phenomena of anion exchange. The clay minerals' OH ions are mostly replaced during the anion exchange process. The role of anion exchange cannot be ignored as Proteins, amino acids, sugars/carbohydrates, and other acidic materials having a negative charge are frequently purified at higher pH values using anion exchange chromatography. The intensity of the material's negative charge determines how tightly the substance bonds with the resin.  For instance, anion exchangers have positive charges that may reversibly bind counterions (an ion having a charge opposite to that of the substance with which it is associated) that are negatively charged. Anions with stronger affinity, such as negatively charged amino acids on the surface of proteins, can substitute the anion exchanger's counterion, which is typically a weakly binding one like Cl. 5.8.4 The nature and amount of clay Compared to sand and silt, clay is a form of soil particle that is smaller in size. It is the result of the long-term weathering and breakdown of rocks, minerals, and organic materials. Depending on a number of variables, including climate, parent material, terrain, and time, the kind and quantity of clay in soils can differ dramatically. 57 LECTURE SIX 6.0 Soil Acidity Generally, the pH scale is used to measure soil acidity. The concentration of hydrogen ions in the soil solution is determined by the pH of the soil. The greater the acidity, the lower the pH of the soil. On a logarithmic scale of 1 to 14, with 7 representing neutrality, pH is measured. A pH of 4 has 10 times the acidity of a pH of 5, and 100 times the acidity of a pH of 6, respectively. 6.1 Learning outcomes By the end of this lecture, you should be able to: i. The nature of acidic soils ii. The concept of soil buffer and liming effects iii. Factors affecting soil buffer capacity iv. The influence of soil pH on nutrient availability v. The highs and valleys of phosphorous fixation under various pH levels 6.0 Introduction On a scale of 1 to 14, the acidity of soil (and all other substances, for that matter) is determined. Anything with a pH value under 7 is regarded as acidic. Everything mentioned above is regarded as being alkaline. An ideal pH range for garden plants is 6 to 7.5. Because phosphorus in the soil is soluble between pH values of 6 and 7.5, where it dissolves in water and is absorbed by plant roots, this range of pH is ideal for garden plants. One of the three macronutrients that all plants require, phosphorus is the middle number in the NPK ratios you see on bagged fertilizers and is essential for the plant to blossom and produce fruit. 6.1 The Nature of Soil Acidity  Available literature indicates that above 7 is alkaline, below 7 is acidic, and pH 7 is neutral. A pH of 6 is 10 times more acidic than a pH of 7, according to the logarithmic scale used to determine pH. Both calcium chloride (pHCa) and water (pHw) can be used to measure the pH of soil, and the results will differ according on the technique. 60  The level of acidity or alkalinity (pH level) in the soil is referred to as soil acidity. It is a crucial aspect of soil that has a big impact on how plants grow and develop. The quantity of hydrogen ions (H+) in the soil solution determines the kind of acidity in the soil. Higher hydrogen ion concentrations are associated with acidic soils, whereas lower concentrations are associated with alkaline or basic soils.  Acidic materials like organic matter decay, acidic parent materials, and the buildup of specific elements like aluminum and hydrogen are the main causes of soil acidity. Acidity in soil can be caused by several things, including biotic processes and anthropogenic activities. Let's investigate some essential elements of soil acidity  The pH scale has a range of 0 to 14, with 7 representing neutrality. Alkaline soil has a value over 7, whereas acidic soil has a value below 7. To grow most effectively, most crops demand a pH range between slightly acidic and neutral.  Effects on Plant Nutrient Availability: The availability of crucial plant nutrients is impacted by soil acidity. Phosphorus, potassium, calcium, and magnesium are among the nutrients that are less accessible to plants as pH falls. Nutrient deficits may result from this, which would be detrimental to plant development and productivity.  Aluminum Toxicity: Aluminum ions can be liberated from very acidic soils and turn poisonous to plants. Aluminum toxicity limits plant development by affecting root growth and nutrient and water absorption.  Acidity of the soil has an impact on the variety and activity of soil microorganisms. In soils with high levels of acidity, the populations of some microorganisms may decline because they are more susceptible to acidity. This may have an effect on nutrient cycling and organic matter decomposition, affecting soil fertility and the general health of the ecosystem.  Acidic soils frequently have poor structural qualities, which can result in problems including compaction and inadequate water infiltration and drainage. Acidity can weaken soil aggregates and make soil particles cluster together, which makes it harder for roots to penetrate and for nutrients to be absorbed.  Improvement: A procedure known as liming can be used to change the acidity of soil. Liming is the process of raising the pH of the soil and lowering its acidity by adding 61 minerals like limestone (calcium carbonate). Liming contributes to improving nutrient availability, reducing soil acidity, and improving overall soil health.  While certain plants, like rhododendrons and blueberries, perform well in acidic soils, most agricultural crops require a pH range between slightly acidic and neutral. For optimal plant development and sustainable agriculture, maintaining the ideal pH level through frequent soil testing and suitable additions is crucial. It is important that you are able to discuss soil acidity in various parts of Kenya. For example, how would you compare the soil reaction levels from North Eastern Kenya Counties from regions in central Rift Valley such as Kericho, Nandi, Bomet, Vihiga and Kakamega counties? 6.2 Buffer Capacity in Spoils  Nutrients would be completely washed away with the first significant rainstorm if the surface soil didn't have the potential to act as a buffer. Due of the high quality of the water, this is a crucial component of water management.  Generally speaking, a buffer's buffering capacity is the number of protons or hydroxide ions it can take in without significantly changing its p H. In this regard, the higher buffering capacity denotes the presence of a greater quantity of buffering components, which enables it to absorb more protons or hydroxide ions with no change in p H.  The ability of a soil to tolerate pH changes when external causes, such as the addition of acidic or alkaline chemicals, modify its chemical composition is referred to as buffer capacity in soils. It gauges the soil's capacity to sustain the ideal pH for plant development and the operation of soil microbes.  The mineral and organic makeup of a soil essentially determines its buffer capacity. Significant levels of minerals and organic matter that may absorb or release ions are present in soils with high buffer capacity, which helps to maintain a pH that is reasonably constant. However, these compounds are few in soils with low buffer capacity, making them more sensitive to pH fluctuations. 62 6.4.1 Benefits of Lime:  When lime is applied correctly, it may stimulate plant growth, improve soil structure, increase nutrient availability, and encourage beneficial microbial activity. It also aids in avoiding nutritional deficits brought on by excessive soil acidity.  Monitoring and Reapplication: It may take many months for the full benefits of lime application to appear. It is advised to regularly check the pH of the soil; if necessary, further lime treatments may become necessary in the future to keep the pH level desired.  Although lime is useful in reducing soil acidity, it is important to take into account the particular needs of the plants being produced. It is vital to balance pH modifications in accordance with the fact that certain plants prefer slightly acidic soils.  Always remember to get specialized advice from local soil specialists or agricultural extension organizations depending on your soil's characteristics and the needs of your particular crops.  NB: Detailed research carried out in Australia demonstrates that limed result in higher yields both in grains or biomass. See Msimbira and Smith, 2020. Effects of Liming in crop production. Government of Western Australia Benefitial of liming  Crop yield improvement … (Ca ions, Mg ions, /// high absorption of nutrients / biotic active)  Nutrient availability … (Ca ions, Mg ions,  Improved microbial activity  Improved legume fixation,  Calcium and magnesium addition. The diagram below summarizes the benefits of liming. 65 Figure 21: A diagram to show the benefits of liming acidic soils. Adapted from Mahmud, M. S., & Chong, K. P. (2022). Effects of Liming on Soil Properties and Its Roles in Increasing the Productivity and Profitability of the Oil Palm Industry in Malaysia. Agriculture, 12(3), 322. MDPI AG. Retrieved from http://dx.doi.org/10.3390/agriculture12030322 In Below is a figure showing the relative soil activity and biomass accumulation under different soil pH levels. You will notice that under neutral or near neutral p H levels, plant thrive, indicating highest levels of soil fertility. Hence it is important to test soils and lime as necessary. 66 Figure 22: An illustration to show plant nutrient availabilities and deficiencies over a wide soil p H levels. Adapted from Msimbira, & Smith, (2020). The Roles of Plant Growth Promoting Microbes in Enhancing Plant Tolerance to Acidity and Alkalinity Stresses. Frontiers in Sustainable Food Systems. 6.5 Aluminum toxicity Here, you have to understand that Aluminum is largely abundant on the earth’s surface where it has not effect on crops and plants under near neutral, neutral or alkaline soils. Aluminum becomes soluble when the pH of the soil decreases. A slight pH lowering can cause a significant rise in soluble aluminum which then hinders root development and thus greatly limiting availability to water and nutrients Al & pH graph with rule of thumb Al toxicity. Roots of barley grown in acidic subsurface soil are shortened by Aluminum toxicity. Fact Sheet Soil Acidity: Source: Soilquality.org.au. Accessed on 9thJuly, 2023 at 18.04 hrs. (EAT 16.04) 67  Between a p H of 6-7, nutrient availability is optimum but at 5.5, nutrient availability (NPK) becomes problematic and gets worse as the soil becomes more acidic. Similarly, soil becomes more alkaline than 7.0, nutrient availability (some minor elements) will also begin to suffer.  Note that at pH of 6 -7, microorganism activity starts to peak and become more active and this can be critical for nitrogen fixation.  As you are now aware, Phosphorous (P) plays a critical role in a plants growth and development. However, depending on the soil p H level, this phosphorous may not n=be available to plants as required due to various chemical reactions. The diagram below shows what can be referred to as the Hills and Valleys of Phosphorous fixation across soils of varying pH. The correlation of this diagram and the one immediate above is clear. 70 Adapted from: Barrow, N.J. The effects of pH on phosphate uptake from the soil. Plant Soil 410, 401– 410 (2017). https://doi.org/10.1007/s11104-016-3008-9 Class activity:  Discuss the behaviour of pesticides in soils. Think along these lines: - Microbial degradation, leaching, surface run- off, deterioration of soil property, percolation of pesticide in water table, & effects of soil micro and Macro fauna including non-target species . 71 Lecture seven 7.0 Fertilizer Sources Any substance of natural or synthetic origin that is given to the soil to offer one or more plant nutrients is referred to as fertilizer and according Bill Gates, two out of every five people on Earth today owe their lives to the higher crop outputs that fertilizer has made possible.” 7.1 Learning outcomes By the end of this lecture, you should be able to understand: i. The sources of nitrogen, phosphorous and potassium fertilizers. ii. Categories of fertilizers iii. Impacts of fertilizer application on crop growth and production 7.2 Introduction Fertilization is essential for plants to develop robustly and provide their highest possible yield. For optimal development and output, plants need both macronutrients and micronutrients. The origins of the main key macronutrients are examined in this chapter. 7.3 Sources and Availability OF Nitrogen (N), Phosphorous (P) and Potassium (K). You realize that the availability of these fertilizers is based on their global production and usage trends up until 2021. Generally, we can have Inorganic fertilizers are divided into 5 different categories which include: 1. Nitrogenous Fertilizers 2. Phosphate Fertilizers 3. Potassium Fertilizers 4. Compound Fertilizers 5. Complete Fertilizers (NPK) 72 2) Discuss potential practices that can be adopted to replace the use of inorganic fertilizers 3) Examine the role of agroforestry in soil fertility management in Kenya 4) With respect to animal and human health, explore the challenges faced by farmers during crop production respect to microplastic pollution. 75 LECTURE EIGHT 8.0 ORGANIC FERTILIZERS Organic fertilizer comprises animal manure, sewage sludge, compost, rendering waste, guano, brewery waste, digestate, and other bio-wastes. It also includes organic amendments given to soils other than the direct deposition of excreta by grazing animals. They are natural substances devoid of synthetic chemicals, organic fertilizers come from plants or animals. They support sustainable agriculture by nourishing plants and enhancing the health of the soil. 8.1 Learning outcomes i. Soil fertility improvement ii. Nutrient recycling iii. Long term soil fertility management iv. Environmental sustainability 8.2 Introduction Manures are animal and plant feces that are utilized as plant nutrition sources. As they break down, they release nutrients. Agriculture has long practiced the skill of gathering and using wastes from animal, human, and vegetable sources to increase crop yield. Manures are organic compounds made from animal, human, and plant waste that have complex organic forms of plant nutrients. Fertilizers are organic or synthetic compounds that contain plant nutrients. In addition to enhancing the physical characteristics of the soil, manures with low nutrient content per unit quantity have longer lasting effects than fertilizers with high nutrient content. Manure's principal sources include:  Cattle shed wastes from biogas plants, include dung, urine, and slurry  Night dirt, human urine, garbage, sewage, sludge, and sullage are examples of human habitation wastes.  Chicken Jitter, sheep and goat excrement  Bone meal, flesh meal, blood meal, horn and hoof meal, and other slaughterhouse wastes Fish squander  Byproducts of agro industries, such as oil cakes, bagasse and press mud, and waste from the processing of fruits and vegetables 76  Crop wastes, including sugarcane debris, stubs, and other materials  Weeds, tank silt, green manure crops, and green leaf manuring material. Water hyacinth s activity Read, discuss and make notes on the following  Green manure.  Farmyard manure.  Compost manure.  Types of inorganic manures  Eutrophication  Legislation for sale of fertilizers and liming materials in Kenya Question 1) Discuss why inappropriate use and application of fertilizers occurs in Kenya LECTURE NINE 9.0 Soil Organic Matter, Manures and Waste Materials 9.1 Introduction Soil organic matter is the crucial component of soil consisting of decomposed plant and animal materials that improves soil fertility, structure, and nutrient-holding capacity. You will remember that soil organic matter is the portion of soil that is made up of organic materials, such as plant roots and other soil organisms, as well as animal, plant, and microbial remains in various stages of decomposition. From your previous studies, you will remember that organic matter can be divided into three categories which include: Active organic matter  Nutrients that are simple for bacteria to consume and utilise for metabolism are present in active organic materials. These materials have often just been in the soil for less than five years. A valuable source of active organic matter is fresh agricultural 77 9.4 Organic Matter Cycling  A supply of active organic matter is provided by the addition of organic materials to the soil, such as residues (leaves and roots) or organic amendments (manures and composts). Soil organisms need this for metabolism and growth after that.  The amount of residue produced by this process diminishes because it stimulates the growth of soil organisms and allows CO2 to be respired into the atmosphere.  Plants and microorganisms utilize the nutrients that are released during decomposition and biomass turnover.  Organic matter can be stabilized by adhering to soil particles and by occlusion (entrapment in aggregates).  While stable organic matter has longer turnover durations (decades), fresh leftovers will disintegrate over short time periods (days, months, or years). As the English people say, pictures speak louder than words. The figure below summarizes the different periods taken from active organic matter to Stable organic matter. Figure22: Credit: University of Minnesota Extension: Soil organic matter in cropping systems. Accessed on 25th June, 2023 (4.18 EAT) 80 According to the earlier chapters of this unit, soil organic matter significantly affects a number of physical, chemical, and biological aspects of soil, including its structure, water retention, available water capacity, thermal conductivity, erodibility, infiltration, formation of soil aggregates, color, soil compaction, soil aeration, pH, buffering capacity, CEC, base saturation, zeta potential, exchangeable cations, microbial population, and carbohydrate content. In fact, this will become much more crucial as we attempt to feed the estimated 9 billion people who will inhabit the earth by the year 2050. This indicates that in order to assure human existence, food production must multiply by six. The composition and pool of organic matter in the soil must consequently be kept as healthy as possible. The soil physical properties, physical processes and gasses emissions are given in the figure below. Figure 19: Soil Organic Matter and Its Impact on Soil Propwerties and Nutrient Status. Credit: Bashir, O. etal., (2021) Soil Organic Matter and Its Impact on Soil Properties and Nutrient Status. In: Dar, G.H., Bhat, R.A., Mehmood, M.A., Hakeem, K.R. (eds) Microbiota and Biofertilizers, Vol 2. Springer, Cham. 81 9.4 Factors Affecting Soil Organic Matter  Temperature - high temperatures favor decomposition of plant residue. Less organic matter in warm climates  Precipitation - O.M. is less where low soil moisture limits plant growth  Poorly drained soils - organic matter accumulates when oxygen supplies are too low for microbial activity. This is because plant residues are not decomposed so they accumulate. This can be observed in – swamps and marshes. However, once these soils are drained the OM decomposes causing subsidence.  Texture - Finer textures accumulate more organic matter.  Cropping  Harvesting plant material returns less to soil than the natural condition  Tillage increases rate of decomposition  Good fertility and crop rotation help maintain organic matter.  Activity:  Citing examples, give a detailed account how Cover crops contribute to soil health 8.6 The Concept of Carbon-to-Nitrogen Ratios After looking in the above, it would not be wise not to mention the relationship between Carbon to Nitrogen ratios and the rate of decomposition.  The C:N ratio (carbon to nitrogen) affects the rate of decomposition. The ratio also influences whether nitrogen will be immobilized (bound up) by the decomposer community as it breaks down the material or if nitrogen will be mineralized (released) when the material is degraded.  Why do various residue kinds decompose at different rates? The distinctions in residue chemistry and the dietary needs of decomposers provide the solution.. For soil microorganisms, a residue with a C: N ratio of roughly 25:1 offers the ideal ratio of energy and nutrients. High microbial biomass and activity cannot be supported by 82 LECTURE TWELVE 10.0 Fertilizers and Animal Food Stuffs 10.1 Expected outcomes By the end of this lecture and your further reading, you should be able to understand consumer safety, the safety, quality, and compliance of fertilizers and animal feedstuffs depend heavily on their legal features. In regard to fertilizers and animal feedstuffs, legal considerations are crucial for the following main reasons: Consumer safety, environmental protection, quality control, animal welfare, International Trade. In deed and as a rule of the thumb, in order to safeguard consumer health, protect the environment, ensure product quality, advance animal welfare, and ease international commerce, the legal issues of fertilizers and animal feedstuffs are crucial. Respecting these legislative criteria promotes sustainable agriculture, enhances public confidence in the sector, and improves the general security and welfare of both people and animals. 10.2 Introduction As you are already aware, using fertilizers and animal feed responsibly and sustainably is essential to reducing adverse environmental effects and ensuring the long-term viability of agricultural and animal production systems. In Kenya, The Fertilizer and Animal Foodstuffs Act of 2013 is largely responsible for regulating fertilizer laws for increased efficiency and enhancement of food security. The manufacture, importation, distribution, and usage of fertilizers inside the nation are all regulated by this legislation. The act aims to protect environmental and human health while promoting agricultural output by guaranteeing the quality, safety, and effectiveness of fertilizers. Manufacturers, importers, and distributors of fertilizer are required to register with the appropriate authorities and adhere to established criteria and rules. Along with quality control measures, labeling and packaging rules, and the creation of a Fertilizer Control and Certification Institute to supervise the rules' implementation and enforcement are also included in the legislation. These legislative actions are essential for supporting sustainable agriculture and appropriate fertilizer usage. 85 10.3 Legislation of sale of fertilizers and liming materials The Kenya fertilizer and Feed Act 345 regulates various aspects of the manufacturing, importation and placing on the market in Kenya of feed for animals and fertilizing substances. 10.4 Chapter 345 fertilizers and animal foodstuffs ACT Date of assent: 24th August, 1962. [Date of commencement: 4th August, 1967.]. You are asked to read the following documents among others to familiarize yourself with the legislation for sale, of fertilizers and liming materials in Kenya. 1) An Act of Parliament to regulate the importation, manufacture and sale of agricultural fertilizers and animal foodstuffs and substances of animal origin intended for the manufacture of such fertilizers and foodstuffs, and to provide for matters incidental to and connected with the foregoing: Fertilizers and Animal Foodstuffs Act (Cap 345) 2) Please see “Bio-fertilizer Regulation in Kenya: Legal Frameworks, Institutional and Capacity Limitations “written by: Tarus, D., Kabole M, Masso, C., Watiti. J, F. Nang’ayo Further reading Class/individual activity:  Discuss why inappropriate use of fertilizers occurs in Kenya  Special further reading topics i. Msimbira, L.A., & Smith, D.L. (2020). The Roles of Plant Growth Promoting Microbes in Enhancing Plant Tolerance to Acidity and Alkalinity Stresses. Frontiers in Sustainable Food Systems. 11. Specialized questions Below are selected questions to hasten your thinking and reasoning outside the box: 1) Discuss why pesticide mixtures could be a major problem in our aquatic ecosystems 2) Examine the importance of soil pH with respect to plant nutrient availability. 3) Explain four major causes of soil acidity. 4) Why should we focus on weed management options in agricultural systems? 5) Give a detailed account on factors that threaten biodiversity on the planet 86 6) Expound the factors that influence Fertilizer Use Efficiency (FUE) and Nutrient Use Efficiency (NUE) 7) Examine the impact of farm straw management and ammonia volatilization. 12. General revision questions 1) What is soil, and how is it formed? 2) What are the three primary soil particles, and what are their relative sizes? 3) How does soil texture affect soil fertility and water-holding capacity? 4) What are the main components of soil organic matter, and what role do they play in soil health? 5) How does soil pH influence nutrient availability to plants? 6) What is soil erosion, and what are some common causes and prevention strategies? 7) What is soil erosion, and what are some typical ways to stop it? 7) What role do soil microorganisms play in nitrogen cycling and soil fertility? 8) What are some typical techniques for assessing soil in order to determine its nutrient content and quality? 9) How do plant growth and root development are impacted by soil compaction? 10) What are the primary elements affecting the development of the soil profile and soil formation? 11) What are the major factors that influence soil formation and development? 12) What is the significance of soil structure, and how does it impact plant growth? 13) What are some common soil testing methods used to assess soil fertility and nutrient levels? 14) How do soil microorganisms contribute to nutrient cycling and plant health? 87 c) Using a suitable illustration, examine how rock outcrops influence adjacent soils and plant growth characteristics at fine scales in karst areas (6 marks) d) Discuss three ways how soils contribute to climate change mitigation (4 Marks) QUESTION FIVE (20 MARKS) a) Explain the term Biomagnification as used in soil science (2 Marks) b) Using a suitable diagram, expound what is referred to as “the hills and valleys” of phosphorous fixation across soils of varying pH. (8 Marks) c) Many chemicals are applied to the soil with the aim of increasing plant and crop production. Examine the general fate of these chemicals once applied. (10 Marks) . UNIVERSITY EXAMINATION FOR THE AWARD OF THE DEGREE OF BACHELOR OF SCIENCE IN EDUCATION (SCIENCE) 2020/2021 ACADEMIC YEAR DECEMBER SEMESTER 2021 UNIT CODE: LADM 2104 UNIT TITLE: BASIC SOIL SCIENCE DATE: DECEMBER 2021 TIME: 2 HOURS INSTRUCTIONS 5. Answer question one and any other two questions 6. Do not write anything on this question paper 7. Do not write in the page margins of the answer booklet 8. Begin each question answer on a new page 90 QUESTION ONE - COMPULSORY (30 MARKS) a) Using a suitable diagram, give four key components of an ideal soil sample (4 marks) b) Explain the phrase Isomorphous Substitution in a Clay Mineral (2 marks_ c) Give two reasons why Isomorphous Substitution is considered important in crop production and in environmental protection. (4 marks) d) Explain four factors that influence soil permeability (4 marks) e) Explain the phrase “soil compaction” (2 marks) f) Give two adverse effects of soil compaction (4 marks) g) Discuss five factors that influence soil formation (10 marks) QUESTION TWO (20 MARKS) a) Discuss two threats to soil biodiversity (4 marks) b) Give four functions performed by soils in arid and semi-arid areas (8 marls) c) Expound four factors that influence cation exchange capacity (8 marks) QUESTION THREE (20 MARKS) a) Discuss four roles of soil organic matter in soil fertility and its impact on plant growth (4 marks) b) Citing examples, give four causes of soil degradation (8 marks) c) Expound four major causes of soil acidity in Kenya (8 marks) QUESTION FOUR (20 MARKS) a) Examine the significance of soil pH with respect to plant nutrient availability (10 marks) b) Elaborate what is referred to as the Hills and Valleys of Phosphorous fixation across soils of varying p H (10 marks) 91 QUESTION FIVE (20 MARKS) a) Give a detailed account on the significance of soil sampling and testing in agricultural farming in Kenya (10 marks) b) Citing examples, explain the concept of soil erosion (4 marks) c) Discuss three causes and potential consequences each of soil erosion in faming systems in East Africa (6 marks) 92