12 Soil as a Resource

B. Rupini

Structure

 

15.0 Objectives

 

15.1 Introduction

 

15.2 Definitions of soils

 

15.3 Composition of soil

 

15.4 Soil profile

 

15.5 Properties of soil

 

15.6 Soil as a resource

 

15.6.1 Resource for minerals

 

15.6.2 Resource for water

 

15.6.3 Resource for fossil fuels

 

15.6.4 Resource for raw materials

 

15.6.5 Resource for organic matter

 

15.6.6 Resource for biological habitat

 

15.6.7 Resource for archeological materials

 

15.7 SAQ

 

15.8 Conclusion

 

15.9 Key Words

 

15.10 References and Suggested Further Readings

 

 

Dear learners this unit mainly focuses on why soil itself is a resource and a source for many other resources to the living community by explaining important concepts of soil. Accordingly the objectives have been framed.

 

15.0 OBJECTIVES

 

 

  • After reading this unit you will be able to
  • define the soil;
  • understand the composition and profile of the soil; describe the properties of soil; and
  • discuss various resources of soil.

 

 

15.1 Introduction

 

 

 

Soils are limited nonrenewable natural resources. Soils provide a base for food production, water and many ecosystem functions. Healthy soil is important for healthy life on earth. The word SOIL was originated from the Latin Word “SOLUM” which means ‘FLOOR’. The global scientific community recognized the significance of soil and observed ‘International year of soils’ in 2015. The predicted global population by 2050 raised an alarm to protect the soil and its resources from degradation. The soil is a component of pedosphere existing in the interface between atmosphere and pedosphere. The concept of soil formation is explained by ‘pedology’ and the influence of soil on plants and human beings is described by ‘edaphology’.

 

 

 

Learn more

 

Pedology (Derived from Greek word pedon which means soil or earth) is a branch of soil science that deals with soil genesis, classification and mapping where as Edaphology (Derived from Greek word edaphos that means soil or ground.

 

15.2 Definitions

 

We have learnt in lower classes that the soil is a visible surface of the earth which is loose in structure and formed from rocks and minerals by weathering process. Some of the definitions given by scientists are as follows

According to Whitney (1892) – Soil is a basket of nutrient that supplies all the nutrients for plant growth.

 

According to Hilgard (1892) – Soil is more or less a loose and friable material in which plants, by means of their roots, find a foothold for nourishment as well as for other conditions of growth”.

 

 

According to Dokuchaiev (1900) Russian scientist, Father of soil science – Soil is a natural body composed of mineral and organic constituents, having a definite genesis and a distinct nature of its own.

 

 

According to Joffe (1936) –“Soil is a natural body of mineral and organic constituents differentiated into horizons – usually unconsolidated – of variable depth which differs among themselves as well as from the underlying parent material in morphology, physical makeup, chemical properties and composition and biological characteristics”.

 

 

According to Jenny (1941) – Soil is a naturally occurring body that has been formed due to combined influence of climate and living organisms acting on parent material as conditioned by relief over a period of time.

According to Soil Science Society of America (1970) – Soil is the unconsolidated mineral matter on the surface of the earth that transformed by eugenic and environmental factors of parent material, climate (including moisture and temperature effects), macro and microorganisms and topography, all affecting over a period of time and producing a product, that is “SOIL” that differs from the material  from which it is derived in many, physical, chemical, biological and morphological properties and characteristics.

According to Dr. W.E.H. Blum – Soils not only serve for agriculture and forestry, but also for filtering, buffering and transformation activities between the atmosphere and the ground water, protecting the food chain and drinking water against pollution and biodiversity. As soil provides

 

nutrients, water, air and anchorage and supports life on Earth, it can be called as Soul of Infinite Life (SOIL).

 

 

By observing definitions of different scientific community we can understand that the soils are four dimensional open systems with time and space which connect to the atmosphere, biosphere, hydrosphere and lithosphere that are formed from mineral and organic substances on the earth’s crust by the effect of various environmental factors administered for long time.

15.3 Composition of Soil

Soil is a major component of the environment and essential for existence of life on earth. Soil is formed by the process weathering of rocks and by erosion. The soil forming factors and processes which influence the formation of soil is depicted in fig.15.1

 

Fig-15.1 Soil formation

 

The soil is made up of four major components like minerals (45%), water (25%), air (25%) and organic matter (5%).These will be discussed in detail in the proceeding sections. These components influence the properties of soils. To understand the properties of soil first we will go through the soil profile.

 

15.4 Soil profile

 

 

The soil has several layers beneath the earth’s crust. We will observe several layers in the vertical segment of the soil from the ground surface to parent material arranged in a sequential manner known as a soil profile. The main divisions in natural soil profile are known as horizons named as O, A, B, C

 

  • & These natural layers or soil horizons are arranged parallel to the earth’s surface with non-uniform boundaries, different thickness and composition. These horizons provide information to pedologists to classify the soil in a particular location. The brief description of these horizons is here under.

 

 

Horizon O: – The O horizon represents soil layer which contains organic matter combined with minerals and clay. It is subdivided into two types.

O1– It comprises of fallen leaves, twigs, lichens, and other decomposed plant and animal material which can be easily recognized.

 

O2– It comprises of putrefied organic material that cannot be recognized.

This horizon is observed in forest and wetland soils.

Horizon A: It is the top most layer of the soil and contains major portion of the organic matter with significant biological activity and fertile in nature. Because of this reason it is known as the biomantle in which concentrations of all organisms of soil is more and bound to plant roots. This soil layer is suitable for cultivation. This layer is susceptible to leaching as the minerals and other soil constituents  dissolve in rain water and percolates down wards. Horizon A is subdivided into A1, A2 & A3 on the basis of their components.

 

A1– It is dark in color due to the presence of more organic matter. It is rich in humus which is a combination of decomposed organic matter, clay and a small portion of mineral content.

 

 

A2 – It is a lower part of the A1 horizon which is exhausted with reference to mineral oxides, clay and iron by leaching activity. Hence it is also known as eluvial horizon. It is rich in quartz and large size minerals.

A3– It is an intermediate layer that connects A and B horizon with major characteristics of A1 & A2.

 

 

Horizon B: The leached material like clay matter, oxides of iron, and aluminum from horizon A transported either by chemical or mechanical process and accumulated in this zone known as illuvial horizon and the soils accumulation is known as Alfisols. The presence of sesquioxides imparts dark red color to this layer. It is subdivided into B1, B2 & B3.

 

B1– It is an intermediate layer between A & B.

B2– In this zone accumulation of leached material is at maximum in addition to the formation of sesquioxides in the zone itself. Since the accumulation of different components is high, the color of the zone is darker than other zones.

 

B3– It is an intermediate layer that connects B and C with more similar properties of B2.

The horizon A and B are together known as SOLUM.

 

 

Horizon C: This is the horizon of parent material from which the formation of soil starts. Along with the parent material the other soils from A, B horizons containing carbonates or sulphates, calcium and magnesium translocated by natural process. Those soils have mixed properties of B & C and known as BC horizon.

 

Horizon R: It is the hardest layer in all horizons composed of bed rock by which all above layers are formed.

 

15.5 Properties of soil:

 

The diverse nature of soil composition and its profile is not uniform on the earth’s crust. To understand its uses, affects on the environmental habitat, its resources and its health it is necessary to learn the properties of soil. Testing of soil suitability for particular use these properties provides ample information to the pedologists. These properties are governed by parent material, topography and soil formation factors.

The properties are grouped under two categories. They are

 

  1. Physical properties
  2. Chemical Properties

 

Physical Properties: Physical properties of the soil mainly deal with the color, soil texture and structure. These properties depend on mineral composition, amount and size of the particles. Dear learner we will discuss about some of the important soil physical properties. They are 1.Color 2.Texture 3.Structure,4.Density,5.Porosity

 

  1. Color: It is an important and basic property to understand the classification of the soil. The change in the color of adjacent soils reveals about whether the soil formed from its parent material assigned as lithochromic or imparts the color in development process known as pedochromic color. Factors that influence the soil colour are presented in table 15.1.

Ex: Lithochromic color: Red soil evolved from red sand stone Pedochromic color: Red soil developed from granite stone.

  1. Texture:

 

Soil texture is a combination of sand, silt and clay particles in relative proportions that can be determined by quantitative analysis. It is a basic property of a soil that cannot be altered in a given sample area and collectively known as soil separates which can be defined by its size. There are several organizations that have established the size limits of these particles and classify the soil types in to different categories. For example :The American system developed by USDA, the English system or British system (BSI),The International system (ISSS) and the European system(table 15.2). When the soils are wet clay soils sense sticky, sand soil sense gritty and the silt sense smooth. The nature of soil separates presented in table 15.3.

 

 

Soil texture is an important tool to guide for soil management methods and provides the value of the land. It also provides the information about percolation and water holding capacity.

 

  1. Soil Structure: The structure of the soil provides information about the condition of the soil and various activities like movement of quantity of air and water, bacterial activity that occur in a particular soil.

 

Soil structure is defined as the systematic arrangement of primary particles of soil like sand, clay & silt and secondary particles like group of primary particles combined to form granules or aggregate particles is known as soil structure.

 

The primary soil particles are solids and the secondary particles are porous, spongy and are not solids. Majority of soils are combination of individual single particles and aggregate particles. Primary soil particles are structure less in nature where as secondary particles are granulated structure. The aggregates or secondary soil particles are formed by the joining of individual primary particles with colloids of hydroxides of aluminium, oxides of iron, clay and decomposed organic matter by dehydration process under pressure.

Significance: Soil structure plays important role in water infiltration, aeration, bearing moisture, fertility and also to resist the erosion.

 

  1. Density: The density of the soil is defined with reference to the solid particles is the weight per unit volume of the solid portion of soil is called particle density or true density. The bulk density of the soil is defined with reference to dry soil is the weight of dry soil per total unit volume. This total unit volume is the combination of solids and pores that contains air and water.

 

The particle density of the soil may vary on the basis of the presence of the constituents. High density is observed in soils containing heavy minerals and decrease in soil density observed in soils with more organic matter. The bulk density decreases with increase in the fine texture of minerals in the soil.

 

The bulk density provides the information about the porosity of the soil and useful to understand and estimate the physical behavior of the soil.

 

 

  1. Porosity: Porosity is the space between the primary particles and between or within the aggregates which will be filled with air and water. The larger pores are termed as macropores or non-capillary pores (> .05mm) and smaller pores are known as micropores or capillary pores (< .05mm). The soil air is inversely proportional to the soil water since water displaces air in the soil. The size of the pore space is depends on soil texture, soil structure and vegetation. Air movement and water percolation is high in the presence of macropores and low in case of micropores.

 

Chemical Properties:

 

 

pH: pH is the number of hydrogen ion concentration or the number of hydroxyl ion concentrations which is a measure of acidity and alkalinity of a particular compound. When the hydrogen ion concentration is more the soil is acidic. If the hydroxyl ion concentration is more the soil is basic or alkaline. A logarithmic pH scale ranges from 0-14 is used to measure it. The pH >7 indicates alkaline and the pH <7 indicates acidic soils.

In natural soils the mineral composition of the parent matter, weathering reactions and the temperature influence the ph values. In warm and humid climatic conditions weathering reactions are high and acidification of soil predominates where as dry climate favors neutral or alkalinity of the soil.

 

Significance: The pH the soil provides the information about availability of certain plant nutrients, water quantity and the suitability of the soil for different plants.

 

 

Ion exchange capacity: Soil colloid particles like clay, sesquioxides and organic matter protect and preserve the essential plant nutrients from leaching. These nutrients will be taken up plant roots as and when required. The soil collides filled with the nutrients are referred as colloidal bank. These nutrients present in the form of charged ions. The soil colloids at temperate regions have negative charged nutrients which adsorbs cations. In tropical regions soil colloids adsorb anions. These exchange of ions between phases of solid-liquid, solid-solid or liquid-liquid is called as ion exchange reaction. If the cations are exchanged it is called cation exchange reaction where as anion is exchanged that is known as anion exchange reaction. The capability of holding exchanged cations (positive charged ions) is known as cation-exchange capacity (CEC) where as anion (negative charged ions) holding ability is referred as anion-exchange capacity (AEC).CEC increases with increase in pH, Whereas AEC increases with decrease in pH.

 

 

Units to measure CEC is C mol (P+)/kg

 

AEC is C mol (e-)/kg which denote centimoles of charge per kilogram.

Examples of cations:Ca2+,Mg2+,K+,NH4+,H+ and Na+.

Anions: SO42-, Cl-, PO43- and NO3-

The relative order of cation exchange is H > Ca > Mg > NH4 > K > Na

Anion exchange is OH>H2PO4>SO4> NO3>Cl

Significance: i.This is the only process for plants to take up the nutrients from the soil.

 

  1. It provides the information about the soil fertility
  • It provides the information about availability of nutrients and need of the fertilizers.

15.6 Soil as a resource: Soil is a non-renewable resource for land use, agricultural productivity and ecological services. In this chapter we are going to discuss about some important soil resources.

 

 

15.6.1 Resource for mineral: So far we have learnt that the major part of the soil composition is by minerals. Now we will discuss about the mineral resource in detail. Minerals are crystalline natural inorganic constituents of rocks with uniform chemical properties. The minerals are the product of the solidified liquefied magma. The fundamental unit in the mineral is silica (SiO2), since the high abundance of oxygen and silica on earth. The classification of minerals can be studied on the basis of their composition and crystal structure. They are

  1. Silicate minerals
  2. Non-silicate minerals

 

Silicate minerals: Silicate minerals are rock-forming minerals and consist of silicon and oxygen atoms bonded by tetrahedral geometry. The basic atomic structure of silicate mineral is [SiO4]4- and the silicon atom is placed at the centre of the tetrahedron and the four corners are occupied by oxygen atoms. The remaining negative charges on the tetrahedron are balanced by suitable cations like sodium (Na+), potassium (K+), magnesium (Mg2+), iron (Fe2+) aluminium (Al3+) and water molecules or with other oxygen atoms of another silicate unit to get the stability. The silicate minerals are subdivided into six groups on the basis of their structure and bonding of oxygen and silicon (table 15.4).

 

 

Ortho/Neosilicate: These minerals are formed of either Mg or Fe cations. These are also described as Ferro magnesium minerals. Their color varies from dark brown, black and green due to the presence of Iron and Magnesium. The examples of Ferro magnesium minerals are Olivine, Pyroxene and Amphibole. These are categorized under different groups because of their structural differences. Olivine has a single tetrahedral structure in Ortho/Neo silicate structural group. Pyroxene and Amphibole are grouped under chain structural group. Pyroxene has single chained structure in which two oxygen atoms shared in individual tetrahedron. Amphibole has double chain structure in which more than two oxygen atoms contribute to their structure. Augite is an important mineral found in soils and basic rocks as well under Pyroxene group which are basic in nature. The Amphibole minerals are found in acidic rocks.

 

Sorosilicate: These silicate minerals have double silicate tetrahedron structure that is linked by single oxygen atom. Epidote has the complex structure consists of chains of aluminium oxide tetrahedrons connected by single or double silicate tetrahedrons and generally exist in hour-glass shape. They are rare minerals with 2:7 silicon-oxygen ratio with chemical formula Ca2 (Al, Fe) Al2O (SiO4) (Si2O7) (OH).

 

Cyclosilicate: These mineral tetrahedrons are connected in ring structures with 1:3 silicon-oxygen ratios. The important cyclo -minerals are beryl and tourmaline with chemical formulas (Be3Al2Si6O18) & NaFe2+3Al6 Si6O18 (BO3)3(OH) 4

 

Phyllosilicate: Silicate clays are important class under phyllosilicates which have ( phyllon-leaf like structure) leaf like or sheet or layer like structure. Examples are Clay silicates.

 

These clay silicates are formed by either alteration or weathering process of various mineral rocks. These are two types of cay silicates-silica tetrahedron and aluminium or magnesium octahedron that are arranged in horizontal manner. Both the silicate sheets are held together by sharing oxygen atomswithin the crystal structure. Depends on the arrangement of tetra and octahedral sheets they are classified in to 1:1type, 2:1 type and 2:1:1 or 2:2 type clay minerals.

 

Clay minerals are extensively used in manufacture of ceramics and in construction.(Table 15.5).

3-dimensional silicate minerals: Feldspar and quartz belong to tectosilicates that have three dimensional tetrahedral structures.

 

 

Feldspars: The gaps of the tetrahedrons are filled by K+,Na+,Ca2+ or Al3+.Potassium feldspar also known as orthoclase, sodium feldspar known as albite and calcium feldspar anorthite are the important types which are rarely found in rocks. There are series of silicates observed in between albite and anorthite known as plagioclases. Feldspars are used in ceramic making.

 

Quartz: It is the best known mineral of the silicate. It occupies 12% of the lithosphere by composition. The general formula and the simplest of all silicate minerals are SiO2.It is very hard and weathering resistant than their layer counterparts. The modified quartz is cristobalite, opal, tridimite.The pure quartz is used in the glass manufacture and in construction.

 

 

Non-silicate minerals: Non silicate minerals are mostly recognized by their respective anions. Examples of carbonate minerals are calcite, that is calcium carbonate (CaCO3), dolomite, that is calcium magnesium carbonate (Ca Mg (CO3)2).The important and widely used sulphate mineral is gypsum CaSO4.2H2O.It is also known as plaster of paris used in plasters making. Minerals also available as sulfides like Pyrite (FeS2) is known mineral which is called fool’s gold due to its metallic golden color. Other examples are PbS and ZnS that are important sources of lead and zinc. Hematite (Fe2O3) & Magnetite (Fe3O4) are common iron ores. Halite, sylvite and Fluorite are the examples of halide mineral

 

15.6.2 Resource for water: One third of earth’s surface is covered with water in various bodies. Out of it 30% is ground water which includes soil moisture and wetlands. Now we will discuss different types of soil water in detail. The free water which found in large soil pores drained rapidly by gravitational force and is not available to plants and reduces the oxygen supply is known as gravitational water. The water in capillary soil pores which held by cohesion and adhesion forces against gravitational pull that is available for plants is known as capillary water. Some of the soil water found in the form of vapor on the surface of soil colloid particles is known as hygroscopic water.

 

Significance: Soil water is responsible for plant growth and acts as solvent medium for transportation of plant nutrients. It regulates the soil temperature and assists the biochemical activities of the soil.

 

 

15.6.3 Resource for fossil fuels:

 

Fossil fuels are energy sources like solid fuels (coal), natural gas and oil that are formed by the buried decayed organic material from plants and animals which were converted by physicochemical processes(anaerobic decomposition) over millions of years. Natural coal is considered as sedimentary rock which is formed in swamp soils. The anthracite is a harder form of the coal that is transformed at high temperature and pressure which is considered as metamorphic rock. It is composed mainly of  carbon and other elements like oxygen, hydrogen, nitrogen and sulfur. Natural gas is a gaseous hydrocarbon mixture mainly contains methane and trace amounts of CO2, N2, H2S and He. The crude oil is a mixture of liquid hydrocarbons and sulfur compounds present in micropores of sedimentary rock. Bituminous and oil shales are other fossil fuels under the soil.

 

15.6.4 Resource for raw material: Soil is a main source for raw materials that are used in modern appliance manufacturing. For construction of brick walled houses requires clay and plinthite as a raw material for making bricks. Clay and loamy soils used for plastering, sand and gravel used to concrete by mixing with cement. Tin, asbestos concrete are used in construction of roof is obtained from soil. Pisolithic gravel is used for road construction. Rocky soil is used in final coating of dam walls. Raw material like soda lime and quartz are used to make glass window panes ad doors. The daily usage products like tooth paste, body creams & lotions and talcum powder requires clay minerals. Cooking gas, fuels are also the examples of the soil raw materials. In this way the life of the human community is highly depend on the raw materials acquired from the soil.

 

15.6.5 Resource for organic matter: Soil organic matter mainly composed of living soil organisms, decomposed plant and animal material that transformed into new organic substances and humus. It occupies 1-5% of solid portion of the soil and plays a major role in its structure and physico chemical and morphological properties. It is the main source of soil nitrogen and other nutrients like sulfur and phosphorous. It is an important source of energy for microorganisms. It controls the nutrient cycling, provides food for soil organisms. Soils with high organic matter have more water holding capacity with decrease in leaching of nutrients and erosion than the low organic matter containing soils. Along with the organic matter humus helps in adsorbing nutrients and water that available for plants and also to improve the soil structure.

 

15.6.6 Resource for biological habitat: Physico chemical process of soil was affected by the number of microbes and vertebrates present in the soil. They directly influence the soil fertility. Soil animals like mice and mile burrow the soil and redistribute the material of the soil from deeper layers to the surface. Earthworms, termites and ants also burrow and provide soil nutrients. Fungi help in decomposition of organic material and the formation of humus. Microbes like bacteria, algae, nematodes actinomycetes provides a gene pool in which actinomycetes provides earthy aroma when its rain and contains antibiotics for human consumption. The other antibiotics isolated from the gene pool are glycopeptides, tetracycline and cephalosporin. These microbes also help in developing the genetically modified crops. Rhizobium, nitrogen –fixing bacteria in legumes and cyanobacteria or actinomycetes in non-leguminous plants is useful in nitrogen fixation.

 

 

15.6.7 Resource for archeological materials

 

Soil is a natural physical and cultural heritage in terms of human revolution by storing archaeological remains. Soil properties are important in deciding the buried objects. Wooden objects decay due to the organic nature of the soil but can sustain in dry environment or under water. Metal objects like weapons, ornaments and coins sustain for a longer periods under aerated environment. Acid soils vigorously decompose iron artifacts. Objects like ceramics, pottery and glass sustain in all soil types. Fossils under the layers of soil provide the information about climate, agriculture and cultural variation of that particular region and time. The archeological evidences like bones, tools, metal sculptures and remains of pottery, utensils under the soil indicates about the environmental quality, paleontological characters of human civilization and soil age as well. These evidences provide the information to understand the culture of human civilization. For example, the existence of Indus Valley Civilization is evident at Mohenjo-Daro and Harappa by excavations.

 

 

15.8 Conclusion

 

 

Soil is an important non replenishable natural resource which provides life support, food production, recycling of essential nutrients on earth. Soil profile and soil structure shows the soil’s ability of self purification and neutralization. It is an interface between atmosphere and lithosphere that connects various organic, inorganic and biotic components. Soil is an important natural resource for minerals, organic matter, and raw material for industries that provides a flow of ecosystem services which are key factors to support human life and economic well-being. It is the human responsibility to conserve the soil in order to pass on to the next generation for their food, fodder and fuel security.

 

15.10 References and Suggested Further Readings

 

  1. Brady, N and Weil, R 1999, The nature and properties of soils, 12th Edition, Prentice-Hall Inc, Sydney.
  2. Fertilizer Industry Federation of Australia Inc. and CSIRO, 2006, Australian Soil Fertility Manual, CSIRO publishing and FIFA, Collingwood.
  3. Hans, J 1980, The Soil Resource Origin and Behaviour, Springer-Verlag, New York, pg. 10.
  4. McDonald, R, Isbell, R, Speight, J, Walker, J and Hopkins, M 1998, Australian soil and landsurvey field handbook, 2nd ed. CSIRO, Canberra.
  5. Twenhofel. W.H.1944,Soil-The most valuable Mineral ResourceIts origin,Destruction and Preservation.
  6. Stanjek H (1998) Pedogene oxide (Chap. 2.1.5.4). In Blume H-P, Felix-Henningsen P, Fischer WR,Frede HG, Horn R, Stahr K (Hrsg.) (1996 ff): Handbuch der Bodenkunde, ecomed, Landsberg; ab 2007. Wiley VCH, Weinheim