Introductory Soils Chapter 1 PDF

Introductory Soils Chapter 1 Concepts of soil and approaches to study soil The word soil is derived from a Latin word ‘Solum’, which means floor Soils are the natural bodies in which plants grow  They underlie the foundation of houses and factories and determine whether t...

Introductory Soils Chapter 1 Concepts of soil and approaches to study soil The word soil is derived from a Latin word ‘Solum’, which means floor Soils are the natural bodies in which plants grow  They underlie the foundation of houses and factories and determine whether these foundations are adequate.  They are the beds for roads and highways and influence the length of life of these arteries  Soil is the mixture of minerals, organic matter, gases, liquids and a myriad of micro- and macro- organisms that can support plant life. Soils provide readily available nutrients to plants and animals by converting dead organic matter into various nutrient forms Soils supply plants with mineral nutrients held in place by the clay and humus content of the soil  A person's concept of soil depends on his/her viewpoint and experience. Engineer - loose or broken rock material at the earth's surface This material can be used for building purposes. Geologist - highly weathered rock. Horticulteralist - material that needs sterilizing and modification before using for plant growth in greenhouses. Farmer and Agronomist - material at earth's surface with biological, chemical, and physical properties that enable it to support plant growth. There are two general types of soil: Mineral – forms from rock and sediment – most soils form from minerals and are called mineral soils Organic – forms from peat, muck, and plant remains – may form in swamps or very wet areas  Mineral Soils 4 major components of mineral soils  – Functions of Soil in our Ecosystem  Soils perform five key functions in the global ecosystem. Soil serves as a: medium for plant growth, regulator of water supplies, recycler of raw materials, habitat for soil organisms, and landscaping and engineering medium Soil Function: Medium for plant growth As an anchor for plant roots and as a water holding tank for needed moisture, soil provides a hospitable place for a plant to take root Some of the soil properties affecting plant growth include: soil texture (coarse of fine), aggregate size, porosity, aeration (permeability), and water holding capacity. An important function of soil is to store and supply nutrients to plants The ability to perform this function is referred to as soil fertility The clay and organic matter (OM) content of a soil directly influence its fertility Greater clay and OM content will generally lead to greater soil fertility Soil Function: Regulator of Water Supplies  As rain or snow falls upon the land, the soil is there to absorb and store the moisture for later use  This creates a pool of available water for plants and soil organisms to live on between precipitation or irrigation events  When soils are very wet, near saturation, water moves downward through the soil profile unless it is drawn back towards the surface by evaporation and plant transpiration. The amount of water a soil can retain against the pull of gravity is called its water holding capacity (WHC) This property is close related to the number of very small mircro-pores present in a soil due to the effects of capillarity The rate of water movement into the soil (infiltration) is influenced by its texture, physical condition (soil structure), and the amount of vegetative cover on the soil surface.  Coarse (sandy) soils allow rapid infiltration, but have less water storage ability, due to their generally large pore sizes  Fine textured soils have an abundance of micropores, allow them to retain a lot of water, but also causing a slow rate of water infiltration  Organic matter tends to increase the ability of all soils to retain water, and also increases infiltration rates of fine textured soils. Soil Function: Recycler of raw materials  As a recycler of raw materials, soil performs one of its greatest functions in the global ecosystem  Decomposition of dead plants, animals, and organisms by soil flora and fauna (e.g., bacteria, fungi, and insects) transforms their remains into simpler mineral forms, which are then utilized by other living plants, animals, and microorganisms in their creation of new living tissues and soil humus  Many factors influence the rate of decomposition of organic materials in soil  Major determinants of the rate of decomposition include the soil physical environment, and the chemical make-up of the decomposing materials  The activity levels of decomposing organisms are greatly impacted by the amount of water and oxygen present, and by the soil temperature  The chemical makeup of a material, especially the amount of the element nitrogen present in it, has a major impact on the ‘digestibility’ of any material by soil organisms  More nitrogen in the material will usually result in a faster rate of decomposition  Through the processes of decomposition and humus formation, soils have the capacity to store great quantities of atmospheric carbon and essential plant nutrients  This biologically active carbon can remain in soil organic matter for decades or even centuries  This temporary storage of carbon in the organic matter of soils and biomass is termed carbon sequestration  Soil organic carbon has been identified as one of the major factors in maintaining the balance of the global carbon cycle Soil Function: Habitat for soil organisms  Soil is teeming with living organisms of varied size  Ranging from large, easily visible plant roots and animals, to very small mites and insects, to microscopically small microorganisms (e.g. bacteria and fungi.)  Microorganisms are the primary decomposers of the soil, and perform much of the work of transforming and recycling old, dead materials into the raw materials needed for growth of new plants and organisms  Most living things on Earth require a few basic elements: air, food, water, and a place to live  The decomposers in soil have need of a suitable physical environment or ‘habitat’ to do their work  Water is necessary for the activities of all soil organisms, but they can exist in a dormant state for long periods when water is absent  Most living organisms are ’aerobic’ (requiring oxygen), including plant roots and microorganisms, however some have evolved to thrive when oxygen is absent (anaerobes)  Greater soil porosity and a wide range of pore sizes (diameter) in the soil allows these organisms to ’breathe’ easier  Soil textural type has a great influence on the available habitat for soil organisms.  Finer soils have a greater number of small ’micro- pores’ that provide habitat for microorganisms like bacteria and fungi  In addition to the need for suitable habitat, all soil organisms require some type of organic material to use as an energy and carbon source, that is to say they require food.  An abundant supply of fresh organic materials will ensure a robust population of soil organisms. Landscaping and engineering medium  Soils are the base material for roads, homes, buildings  The properties of concern in engineering and construction applications include:  bearing strength, compressibility, consistency, shear strength, and shrink-swell potential.  These engineering variables are influenced by the most basic soil physical properties such as texture, structure, clay mineral type, and water content Soil science has two main branches of study:  Edaphology and Pedology (from Greek: pedon, "soil"; and logos, "study")  Pedology is focused on the formation, morphology, and classification of soils in their natural environment  and does not focus primarily on the soil’s immediate practical use  Whereas Edaphology is concerned with the influence of soils on organisms  Edaphology is concerned with the influence of soils on living things, particularly plants  Edaphologists consider the various properties of soils in relation to plant production.  The term is also applied to the study of how soil influences man's use of land for plant growth as well as man's overall use of the land. Chapter 2. Soil formation  Soil formation is the conversion of rock into soil  The rock may be limestone, sand, shale, peat, etc  Instead of using the term rock, soil scientists prefer to use the expression parent material, which means the underlying geological material  Soil formation, or pedogenesis, is the combined effect of physical, chemical, biological and anthropogenic processes working on soil parent material 2.1. Soil forming rocks and minerals The mineral material from which a soil forms is called parent material. Rock, whether its origin is igneous, sedimentary, or metamorphic, is the source of all soil mineral materials and the origin of all plant nutrients with the exceptions of nitrogen, hydrogen and carbon  Typical soil mineral materials are: Quartz: SiO2 Calcite: CaCO3 Feldspar: KAlSi3O8 Mica (biotite): K(Mg,Fe)3AlSi3O10(OH)2 Classification of parent material Parent materials are classified according to how they came to be deposited Residual materials are mineral materials that have weathered in place from primary bedrock. Transported materials are those that have been deposited by water, wind, ice or gravity. Cumulose material is organic matter that has grown and accumulates in place. Transported materials Aeolian processes (movement by wind) are capable of moving silt and fine sand many hundreds of miles, Water-transported materials are classed as either alluvial, lacustrine, or marine. Ice moves parent material and makes deposits in the form of terminal and lateral moraines in the case of stationary glaciers. Parent material moved by gravity is obvious at the base of steep slopes as talus cones and is called colluvial material Weathering of parent material and soil formation Weathering is the process whereby solid rocks of the earth’s crust are broken down to form the parent material of the soil The weathering of parent material takes the form of physical weathering (disintegrating), chemical weathering (decomposition) and Biological weathering  physical weathering (disintegrating In physical weathering the only change is the change of rock into a range of finer particle sizes of the rock without any accompanying chemical change This disintegration has a two fold purpose: It promotes a fine state of division which is necessary in soil. It facilitates the action of chemical weathering agents by increasing the free surface area of the particles. Agencies of physical weathering: Heat and Cold Freezing and Thawing Erosion by Streams Wind chemical weathering (decomposition) Chemical weathering of minerals differs from physical weathering, in that minerals in the original rocks are decomposed and new substances originate Chemical weathering involves the change in the composition of rocks, often leading to a 'break down' in its form This is done through a combination of water and various chemicals to create an acid which directly breaks down the material. A number of distinct types of chemical weathering may be distinguished as below. Simple solution The solution of salts in water results from the action of bipolar water on ionic salt compounds producing a solution of ions and water. Carbonation In carbonation, the reaction of carbon dioxide in solution with water forms carbonic acid. Carbonic acid will transform calcite into more soluble calcium bicarbonate. CO2 + H2O H2CO3 Carbon dioxide Water Carbonic acid H2CO3 + CaCO3 Ca(HCO3)2 Carbonic acid Calcium carbonate Calcium bicarbonate Oxidation of a mineral compound is the inclusion of oxygen in a mineral, causing it to increase its oxidation number and swell due to the relatively large size of oxygen, leaving it stressed and more easily attacked by water (hydrolysis) or carbonic acid (carbonation). Biological weathering  Strictly speaking there is no biological weathering  Essentially it is physical and chemical weathering by biological agencies  The presence of vegetation accelerates weathering processes by producing carbon dioxide in respiration and by providing materials for humus and humic acid.  Plant roots helps in widening the cracks and crevices (splits) in the rocks through which water can percolate freely The opening of subsoils to weathering is also done by worms which make burrows up to a depth of five feet, thus introducing both air and water in lower layers Soil forming factors 1 Climate  The principal climatic variables influencing soil formation are effective precipitation (i.e., precipitation minus evapotranspiration) and temperature, both of which affect the rates of chemical, physical, and biological processes.  The temperature and moisture both influence the organic matter content of soil through their effects on the balance between plant growth and microbial decomposition  Climate is the dominant factor in soil formation, and soils show the distinctive characteristics of the climate zones in which they form  The direct influences of climate include:  A shallow accumulation of lime in low rainfall areas as calcite  Formation of acid soils in humid areas  Erosion of soils on steep hillsides  Deposition of eroded materials downstream 2. Topography  The topography, or relief, is characterized by the inclination (slope), elevation, and orientation of the terrain  Topography determines the rate of precipitation or runoff and rate of formation or erosion of the surface soil profile  The location of a soil on a landscape can affect how the climatic processes impact the soil  Slope and aspect affect the moisture and temperature of soil  Soils at the bottom of a hill will get more water than soils on the slopes  Therefore, soils on steep terrain tend to have rather shallow, poorly developed profiles in comparison to soils on nearby, more level sites  Depressions allow the accumulation of water, minerals and organic matter and in the extreme, the resulting soils will be saline marshes or peat bogs  Intermediate topography affords the best conditions for the formation of an agriculturally productive soil. 3. Organisms (Biological factors) All plants, animals, micro-organisms, and humans affect soil formation. Animals, soil mesofauna and micro-organisms mix soils as they form burrows and pores, allowing moisture and gases to move about In the same way, plant roots open channels in soils Plants with deep taproots can penetrate many metres through the different soil layers to bring up nutrients from deeper in the profile. Plants with fibrous roots that spread out near the soil surface have roots that are easily decomposed, adding organic matter  Humans impact soil formation by removing vegetation cover with erosion as the result  Their tillage also mixes the different soil layers, restarting the soil formation process as less weathered material is mixed with the more developed upper layers  Earthworms, ants and termites mix the soil as they burrow, significantly affecting soil formation.  Earthworms ingest soil particles and organic residues, enhancing the availability of plant nutrients in the material that passes through and out their bodies  They aerate and stir the soil and increase the stability of soil aggregates, thereby assuring ready infiltration of water  As they build mounds, some organisms might transport soil materials from one horizon to another.  In general, the mixing activities of animals, sometimes called pedoturbation, 4. Time  Time is a factor in the interactions of all the above  Soil is always changing  It takes about 800 to 1000 years for a 2.5 cm (1 inch) thick layer of fertile soil to be formed in nature  Over a period of between hundreds and thousands of years, the soil will develop a profile that depends on the intensities of biota and climate. 5 Parent material:  The primary material from which the soil is formed  Soil parent material could be bedrock, organic material, an old soil surface, or a deposit from water, wind, glaciers, volcanoes, or material moving down a slope  Few soils weather directly from the underlying rock  These "residual" soils have the same general chemistry as the original rocks  Chapter 3 and 4 Important Physical and Chemical properties of Mineral Soils  Physically, soils are composed of mineral and organic particles of varying size.  The particles are arranged in a matrix that results in about 50 percent pore space, which is occupied by water and air.  This produces a three-phase system of solids, liquids, and gases  Essentially, all uses of soils are greatly affected by certain physical properties.  The physical properties considered in this chapter include: texture, structure, porosity, density, color, and temperature.  SOIL TEXTURE  The relative proportions of (sand, silt, and clay)  The mineral particles, originally from solid rock, assumed their present form because of physical and chemical processes called weathering  At some stage in the weathering process, mineral particles became a favorable medium for plant growth, that is, they were able to provide storage of water, air, and mineral nutrients, as well as space in which roots could grow  Organic matter then accumulated near the soil surface due to the decomposition of plant residues  cont,…  Generally, organic matter further improved the properties of the soil as an environment for plant growth  Soil texture relates primarily to particles smaller than 2 millimeters (.080 inches) in diameter  Types of soil texture 1. Sand soil: − The soil that dominated by sand(2-0.02) − Posses good drainage & aeration − It is easy to handle during tillage − less surface area − poor water retention and nutrient content 2. Silt soils: The intermediate size, feels smooth when dry and but not sticky when moist. − Because the smaller particle size promotes smaller pore spaces between particles, − silty soils have a slower water intake rate but a higher water holding capacity than sandy soils.  Cont,…. − These are difficult for storage because they often lack aggregation  This results in high density and a pore size too small for suitable water percolation and aeration. Nevertheless, silt is an essential component of the medium textured  Cont,… 3. Clay soils: − The soil that dominated by clay (< 0.002mm) − High absorption & retention capacity for moisture and nutrient − It is difficult to handle during tillage practice − They have sticky and plastic nature − Fine pores and poor drainage  cont,… While soils high in clay are difficult to manage because of their great strength and sticky nature An intermediate amount of clay in a soil improves its capacity to hold water and plant nutrient ions (either in solution or by adsorption) The swelling and shrinking of clay also helps form favorable structure in medium textured soils  cont,… Another useful and often used grouping of soil textures includes the following three categories: −Coarse-textured soils - Sands, loamy sands, and some sandy loams. −Medium-textured soils - Loams, sandy loams, silt loams, and some sandy clay loams and clay loams. −Fine-textured soils - Clays, sandy clays, silty clays, and some sandy clay loams, silty clay loams, and clay loams. Soil texture triangle  Properties of soil particle size Sand Silt Clay Porosity mostly small pores small pores large pores predominat predominat e e Permeabilit rapid low to slow y moderate Water limited medium very large holding capacity Soil particle small medium very large surface  cont,… Soil structure refers to the arrangement of soil particles. Sand, silt, and clay seldom occur as separate units in the soil; rather, they combine into aggregates held together by small binding forces of clay and organic matter. The size and form of aggregation is known as the structure of the soil. Soil structure is one of the more important physical characteristics of soil  How does soil structure impact on nutrient availability? Clay Loam Sand clay (micelles) : silt:.02-.002 + sand: 2 mm-.02 less than.002 mm sand + clay mm in size. This soil is excellent for it Very large particles Very fine combines the high which have large particles. Very surface area and pockets between large surface area; water retention of them. This is great charged to hold clay with the air for aeration ions. spaces of sand.  Cont,… Can compact Can be greatly thus detrimental for the effecting root plant because growth by not there is very little allowing enough surface area. Thus pores for air and nutrients will not water. Think of bind to it and swampy soils water just runs through  cont,… Plant growth is strongly influenced by soil structure. Soil structure affects movement of water, air, and roots through the soil Soil structural aggregate may vary from a fraction of an inch to several inches in diameter; may be approximately spherical, elongated, or plate like; and may be held together strongly or weakly  Types of structure 1 granular: sphere shape arranged particles arranged round the point and bounded by curved or very irregular surfaces  Common in surface soils with high organic matter content  A granular structure provides an ideal environment for plant roots  And is particularly helpful for establishing plants from seeds or transplants  The larger pores between the granular aggregates are continuous, and roots may penetrate them with ease  Water drains readily through this soil, yet moisture is held back sufficiently in the aggregates to supply root needs.  Granular structure occurs in loam soils and in some clay soils near the surface.  One of the good things about clay is its promotion of granular structure (by swelling and shrinking) in medium textured soils.  A greater organic matter content also results in better granular structure of a soil  cont,… 2. Prismatic and Blocky :  Prismatic and blocky structures most often occur as the result of shrinking and cracking of clay loams and clay soil layers (called horizons) upon drying.  Prismatic or blocky aggregates may vary considerably in size but are always coarser than those of granular structure.  The aggregates swell when wet and fit together so tightly that water drains through them rather slowly  cont,… Plant roots may follow cracks downward but do not usually penetrate to the centers of prismatic or blocky aggregates. Thus, the roots may not have access to a significant portion of the water and nutrients in these soils. 3 Platy structure: the particle are arranged in horizontal plane  These most often occur when silty soil materials are deposited in thin layers by stream overflow  cont,… Intensive cultivation usually results in some breakdown of the natural soil structure. Forces holding soil particles together in aggregates may not be strong enough to resist the crushing effect of heavy equipment, or the shearing effect resulting from working the soil at too high a moisture content. Excessive traffic over the land results in a compact soil mass in which large pores have collapsed due to crushing of the granules  cont,… In the absence of large pores, water penetration becomes very slow. The small pores, still present, may fill slowly with water after irrigation, and drain even more slowly because water is held strongly by particle surfaces This has two serious effects − Water movement to lower depths is very slow; − Little or no airspace is left in the compacted soil  Soil structure decline results  Reduced crop yields,  Greater surface run-off and  The potential for soil erosion,  Which may reduce water quality.  Soil compaction may result in less infiltration and drainage, thereby reducing amounts of water available for plant growth  Other impacts include a reduced ability of soils and plants to withstand stress imposed by diseases, climate and machinery effects (arising from lower organic matter levels) and reduced resistance to wind and water erosion. Grade of structure - Describes stability of the aggregates. Terms for grade of structure are as follows: 0, Structure less, no observable aggregation or no definite and orderly arrangement of natural lines of weakness. Massive if coherent as in clays and single grained, if no coherent, as in sands. 1, Weak and poorly formed indistinctive peds,  2, Moderate and well-formed distinctive peds, moderately durable and evident, but not distinct in undisturbed soil.  3, Strong and durable peds that are quite evident in undisturbed soil, adhere weakly to one another, and become separated when the soil is disturbed. Maintaining Soil Structure  Till soil only at the proper moisture contents Never till when the soil is too wet. This will cause the soil to become cloddy. Aggregates are easily destroyed.  Add the proper amounts of lime and fertilizer. Proper plant growth will lead to the development of good soil structure.  Grow grasses and legumes. These plants may help form unstable aggregates and their organic matter will help stablize the aggregate. Growth of legumes will also give the soil more microorganisms which give certain beneficial fungi which will stabilize peds.  Maintain or increase organic matter contents of Ap horizon plant cover crops in fall and winter plant more grasses turn under crop residue add manure  SOIL COLOR Soil color is obvious and easily determine is one of the most useful characteristics in classification and identification Although color has no direct influence on the functioning or productivity of the soil, a great deal may be inferred about a soil from its color. A few broad generalizations may be made about soils of different colors  CONT,… Significance ( role ) of soil color 1. To determine the drainage properties 2. To observe the fertility of the soil 3. It shows the rate of weathering SOIL DEPTH Soil depth is important to the management of plant growth. The deeper the soil, the greater the total water and nutrient storage capacity available to plants. If a sub-soil layer has a noticeable increase in clay, water may accumulate above this layer, and roots may be injured because of poor aeration  This condition is often called water logging Density : is the mass of an object per unit volume  The density of soil can be expressed in two ways. (1) The density of solid (particle density), particles of the soil and 2 The density of the whole (Bulk density) soil that is inclusive of pore space 1 particle density (ρs ): is measure the density of soil particle ρs = ms/vs ρs = particle density ms = mass of solid soil vs = volume of solid soil  For a mineral soils the particle density will average about 2.65g/cm3  Mineral matter like Fe and heavy metals increases particle densities  Bulk density (ρb) :is the density of soil in its natural state including the pore spaces ρb = ms/ vT Ms= mass of dry soil VT = volume of total soil ( solid, water, air)  Bulk density values have various use − It determine storage capacity of water per soil volume − It determine the root penetration − To determine the air movement in the soil  cont,…  Generally soils with low bulk density have better physical condition than those with higher bulk densities.  Texture and structure of a soil, its total pore space and organic matter content are all related to bulk densities.  Soil density can be modified with aeration.  cont,… Porosity : Between the soil particles there are empty spaces which are occupied by air and water and are termed as pore spaces. Pore spaces between the aggregates of soil particles are macro pores and those between the individual particles of the aggregates are micro pores  cont,… Typically, sandy soils never become water logged and allow water to percolate down ward more rapidly than clay soils. Moisture content in sandy soils is relatively low when compared to clay soils. Clay soils contain a higher percentage micro pores when compared to sandy soils Clay soils are more susceptible to water logging which can adversely effect root respiration and microbial activity  Soil temperature and plant growth  Soil micro-organisms show maximum growth and activity at optimum soil temperature range.  All crops practically slow down their growth below the temperature of about 90C and above the temperature of about 500 C.  The biological processes for nutrient transformations and nutrient availability are controlled by soil temperature and soil moisture  Soil temperature has a profound influence on seed germination, root and shoot growth, and nutrient uptake and crop growth.  cont,…  The various factors that control the soil temperature are soil moisture, soil colour, slope of the land and vegetative cover  Aeration can be used to control soil temperature, regulate soil moisture, improve drainage, stimulate microbial activity and improve overall soil profile Important soil chemical properties Soils are chemically different from the rocks and minerals from which they are formed in that soils contain less of the water soluble weathering products, calcium, magnesium, sodium, and potassium, and more of the relatively insoluble elements such as iron and aluminum. Old, highly weathered soils normally have high concentrations of aluminum and iron oxides The organic fraction of a soil, although usually representing much less than 10% of the soil mass by weight, has a great influence on soil chemical properties. Soil organic matter is composed chiefly of carbon, hydrogen, oxygen, nitrogen and smaller quantities of sulfur and other elements The organic fraction serves as a reservoir for the plant essential nutrients, nitrogen, phosphorus, and sulfur, increases soil water holding and cation exchange capacities, and enhances soil aggregation and structure The most chemically active fraction of soils consists of colloidal clays and organic matter. Colloidal particles are so small (< 0.001 mm) that they remain suspended in water and exhibit a very large surface area per unit weight  Cation exchange capacity Cation exchange is the ability of soil clays and organic matter to adsorb and exchange cations with those in soil solution Silicate clays and organic matter typically possess net negative charge because of cation substitutions in the crystalline structures of clay and the loss of hydrogen cations from functional groups of organic matter Positively-charged cations are attracted to these negatively-charged particles, just as opposite poles of magnets attract one another The quantity of cation exchange is measured per unit of soil weight and is termed cation exchange capacity Organic colloids exhibit much greater cation exchange capacity than silicate clays. Various clays also exhibit different exchange capacities. Thus, cation exchange capacity of soils is dependent upon both organic matter content and type of silicate clays. Cation exchange capacity is an important phenomenon for two reasons: 1. exchangeable cations such as calcium, magnesium, and potassium are readily available for plant uptake and 2. cations adsorbed to exchange sites are more resistant to leaching, or downward movement in soils with water. Calcium (Ca++) is normally the predominant exchangeable cation in soils In highly weathered soils, such as oxisols, aluminum (Al+3) may become the dominant exchangeable cation. The cations of calcium, magnesium, potassium, and sodium produce an alkaline reaction in water and are termed bases or basic cations. Aluminum and hydrogen ions produce acidity in water and are called acidic cations. The percentage of the cation exchange capacity occupied by basic cations is called percent base saturation. The greater the percent base saturation, the higher the soil pH. Soil Reaction (Soil pH) Refers to the degree of soil acidity or alkalinity. Soil reaction is important because it affects nutrient availability, microbial activity and plant growth. There can be three types of soil reactions 1.Acid 2.Alkaline 3.Neutral  Soil Acidity  The absence of bases or excess of H+  Importance: – Nutrient availability – Ion toxicity – Inhibit microbial activity – Influence on fertilizer efficiency – Influence on plant growth – Influence in the environment Is common in regions where precipitation is high enough to leach appreciable amounts of exchangeable bases from the surface layers of the soil Cont,…  So that the exchange complex is dominated by Hydrogen ions (H+) and Aluminum (AI+3)  At the solution phase of acidic soil both ions contributing to the concentration of hydrogen ion (H+) in the soil solution.  The absorption hydrogen is direct effect in soil solution because if the hydrogen ion concentration increase pH decrease and the reveres is true  Aluminum ions do so indirectly through hydrolysis reaction  Factors causing soils to become acidic Loss of exchangeable bases from the soil CEC – Leaching – Removal from plant uptake Production of organic acids from organic matter decay Use of fertilizers, particularly ammonium sources: (NH4)SO4, NH4NO3, Anhydrous ammonia, Urea Soil erosion: Loss of bases from surface runoff Parent material: Presence of acidic materials that weather giving rise to acid soils Weathering Cont,…  Acid soils therefore, occur widely in humid regions and affect the growth of plants  Soil acidity causes the following problems to plants; 1.Toxicity of certain substance to plant example SO4- + H+ → H2SO4 + H2O NO-3 + H+ → NH4 + H2O 2.Reduce the availability of plant nutrient example: NO3- + H+ → NH4 + H2O Al+3 + H2PO4 + H2O → H+ +Al(OH)3H2PO4 Cont,… 3.Reduce the activity of microorganisms Example:  Fungi resists to all pH but bacteria is difficult to survive at low (pH