Preface Soil is a finite resource and its loss and degradation is not recoverable within a human lifespan. Soils affect the food we eat, the water we drink, the air we breathe and our overall health and the health of all organisms on the planet. Without healthy soils, we would not be able to grow our food, or meet our fibre and fodder requirements. It is estimated that 95 per cent of our food is directly or indirectly produced on our soils. Since soil is one of the most complicated biological materials on our planet, it is imperative that we care for soil and nurture it the way it should be for the future generations. In this context, we all must have a basic understanding of the soil, its nature, properties and functions so that we will appreciate it and care for it. It is with this objective that this book is written. It has been attempted to present in a logical sequence all basic information about soils, from the definition of soil to how they are formed, the factors and processes governing their formation, their intrinsic properties, classification, functions, and the role they play in the global environment today and for a sustainable tomorrow. Emphasis on soil quality and soil health care has been laid as it is the order of the day. The 32 chapters of the book will provide a wealth of information for those looking to take their first deep dive into the realm of soils! I earnestly hope this book provides in simple, lucid style all necessary information that the students and teachers of Soil Science look for. Suggestions to improve the contents and presentation of the book are most welcome.

“SOIL” the term is derived from Latin word “solum” which means, “floor”. Soil has been a defining component of humanity since the beginning of civilization. It holds the clues of past cultures, it supports the web of life connecting all ecosystems, provides materials to build houses, and provides the base for our food, feed, fiber and even some biofuels. Soils are complex mixtures of minerals, water, air, organic matter, and countless organisms that are the decaying remains of once-living things. It forms at the surface of land and is the “skin of the earth.” Soil is capable of supporting plant life and is vital to life on earth. There are several general roles that soils play:

During the 19th century various hypotheses were advanced about the origin of earth. These hypotheses can be grouped into two general classes: 1. Monistic hypotheses: The pioneers of this hypotheses believed that only one mass or body was responsible for the creation of the solar system. The representative of this theory is Nebular hypothesis. The advocates of this hypothesis were Kant and Laplace. 2. Dualistic hypotheses: The advocates of this hypothesis believed that two separate masses or bodies were responsible for the creation of the solar system. The representatives of this hypothesis are Planetesimal hypothesis and Tidal hypothesis. The pioneer of Planetesimal hypothesis was the great English scientist T. C. Chamberlain. The Tidal hypothesis was advocated by another English geographer named James Jeans.

Rock A rock may be defined as “an aggregate of one or more minerals”. An aggregate of single mineral calcite gives rise to crystalline limestone. Granite is a combination of several minerals. Most rocks are hard and compact as granite and basalt, but some are loose and feebly aggregated as sandstone. Rocks possess certain characteristics such as structure, colour, specific gravity, cleavage and mineralogical make-up by which we can identify the rocks and also distinguish one from the other.

Mineral A mineral is defined as “a natural substance, usually inorganic, having a definite chemical composition and commonly a definite molecular arrangement which is usually expressed in geometric form”.

Rocks and minerals formed under a very high temperature and pressure, when exposed to atmospheric conditions of low pressure and low temperature, they become unstable and weather. Soils are formed from rocks through the intermediate stage of formation of regolith (parent material) which is the resultant of weathering.

Soil formation takes place in two consecutive stages: 1. The weathering of rocks into regolith 2. The formation of true soil from regolith The evolution of true soil from regolith takes place by the combined action of soil forming factors and processes. The first step is accomplished by weathering (disintegration and decomposition) and the second is associated with the action of soil forming factors which are known as pedogenic factors.

The pedogenic processes are dynamic and extremely complex involving several chemical and biological reactions, usually operating simultaneously in a given area. One process may counteract another, or two different processes may work in tandem to achieve the same result. Different processes or combination of processes operate under varying natural environments. The combined interaction of various soil forming factors under different environmental conditions set a furtherance to certain recognized soil forming processes. Pedogenic processes include:

There are over 100,000 different types of soil in the world, each with its own set of characteristics. If any soil is dug deep, one can see that it is made of layers, or horizons. The horizons together form a soil profile. Each profile tells a story about the life of a soil. The development of soil profile is a constructive process where in disintegrated materials resulting from weathering of rocks and minerals get converted into a soil body. Soil Individual or Polypedon is a natural unit of soil that differs from its adjoining unit on the landscape in one or more properties. In modern soil taxonomy, polypedon is the unit of soil mapping. Apedon is the smallest volume recognizable in nature that can be called “a soil”.

Soil classification, or soil taxonomy as it is called, is grouping of soils in an orderly and logical manner based on soil properties (differentiating characters) for the purpose of studying and identifying them. The following are the purpose of soil classification: To organize our knowledge of soils To bring out and understand the relationships among the soils To remember the properties of soils To learn new relationships and principles in different soil groups

Physical properties of a soil greatly influence plant growth. The plant support, root penetration, drainage, aeration, retention of moisture and availability of plant nutrients are all linked with the physical condition ofthe soil. The physical properties of a soil which depend on the amount, size, shape, arrangement and mineral composition of its particles, also influence its chemical and biological behaviour.

Soil Structure Defined Soil structure is defined as the arrangement of individual soil particles with respect to each other into a pattern. Structure describes the gross, over-all arrangement ofthe soil solids. In other words, soil structure is the ‘architecture’ of a soil. Soils are composed of structural units, called “peds”. The peds consist of many individual particles, bound together by cementing agents like organic matter, clay and hydrous oxides of iron and aluminium. However, the strength of the bonds, the size and shape of the peds and the proportion of the soil particles involved in the peds differ considerably among soils.

Soil Consistence Defined Soil consistence is the strength with which soil materials are held together or the resistance of soils to deformation and rupture. Soil consistence is measured for wet, moist and dry soil samples. For wet soils, it is expressed in terms of both stickiness and plasticity. Stickiness is the ability of soil materials to adhere to other objects. Plasticity is the ability of the soil materials to change shape continuously under a constant pressure and to retain the impressed shape when the pressure is removed. Soil consistence may be estimated in the field using simple tests or may be measured more accurately in the laboratory.

Soil Density The density of a soil, as any other matter, is its mass per unit volume. However, in case of soil, because of its unique nature, the density can be expressed by two terms, viz., bulk density and particle density. Soil density is typically expressed in mega gram per cubic metre (Mg m-3) which is numerically equal to g cm-3.

Soil colour is the most obvious characteristic of soil. It has no direct effect on plant growth; however, it has indirect effect through temperature, moisture. Soil colour can indicate the climatic conditions under which it has developed and can be related to its parent material. It can also reflect upon its age, and the temperature and moisture characteristics of the climate. Thus, cooler regions tend to have grayish to black topsoils, due to the accumulation of humus. In moist warm regions, soils tend to be more yellowish-brown to red. Arid soils tend to be light in color (due to low organic content) and primarily express the color of their mineral content. Practically all colours occur in soils - white, red, brown, gray, yellow, black, etc. Predominantly soil colours are not pure, but mixtures. Frequently two or three colours occur, called as “mottling”.

15.1. Soil Temperature Soil temperature affects plant growth directly; influences soil moisture, aeration, structure, microbial activity, decomposition of plant residues and availability of plant nutrients. Intermediate temperatures, near optimum for each crop, are the desirable temperatures for crop growth. Extreme temperatures are undesirable, lest crop growth would be retarded and weeds and germs may be favoured. Moderate daily variations are desirable as they lead to soil moisture distribution and aeration. 15.1.1. Factors Affecting Soil Temperature The mean annual soil temperature is about 1°C higher than the mean annual air temperature. Soil temperature is influenced mainly by climatic conditions. The factors which affect the transfer of heat through atmosphere from the Sun also affect the soil temperature. The following are the factors that affect soil temperature:

Water is the elixir of life. It is required for the evapo-transpiration of growing plants. Water acts as a solvent and, together with dissolved nutrients, makes up soil solution from which plant roots derive nutrients. Soil water content controls soil air and soil temperature. 16.1. Forces of Water Retention The forces responsible for water retention in soils are adhesion and cohesion. Adhesion refers to the attraction of solid surfaces for water. Cohesion refers to the attraction of water molecules for each other. The force with which the soil particles retain water is defined in terms of pF scale (Schofield, 1935). pF is defined as “the logarithm to the base 10 of the numerical value of the negative pressure of soil moisture expressed in centimetres of water.”

The phenomena related to the retention and movement of water in soils, its uptake and translocation in plants and its loss to the atmosphere are all energy related. The more strongly water is held in the soil, the greater is its energy. It means that if water is to be removed from a moist soil, work has to be done against the adsorptive forces. The water movement is always from a zone where the free energy of water is high (standing water table) to one where the free energy is low (a dry soil). This is called soil water energy concept. 17.1. Soil Water Potential The free energy of soil solids for water is influenced by: i) Matric force: the attraction of the soil solids for water (adsorption) which markedly reduces the free energy (movement) of the adsorbed water molecules.

Water can move in the liquid and gaseous phases: 1. Liquid water i) Saturated flow: Most pores are filled with water; water in this condition is tension free ii) Unsaturated flow: Pores are partially filled with air; the water is under tension. 2. Water vapour i) Diffusion: Water vapour moves by diffusion as a result of vapour pressure differences (partial pressure) ii) Mass flow: Water vapour flows in a mass with other gases of the system in response to differences in total pressure.

Soil Colloids A colloidal system is a two-phase heterogenous system in which one material in a very finely divided state is dispersed through the second phase. Solid in liquid (dispersion of clay in water) and liquid in gas (fog or clouds in atmosphere) are examples. The clay fraction of the soil contains particles less than 0.002 mm in size. Particles less than 0.001 mm size possess colloidal properties and are known as soil colloids.

Structure of Crystalline Clays Soil clay minerals are of two types: crystalline and amorphous. Crystalline clays are made up of sheets of octahedra and tetrahedra of O and OH. Clays with alternating sheets of aluminium octahedra and silicon tetrahedra, are called, “1:1 clays” or “diphormic clays”. A sheet of octahedra is sandwiched between two sheets of tetrahedra in 2:1 clays which are called as, “triphormic clays”. The building blocks of clay crystals are called “unit cells”. The geometry of octahedra has 3 voids in each half unit cell, equivalent to 6 negative valences. In order to nullify these 6 negative valences, 6 positive valences are needed. The 6 negative valences can be nullified by 3 divalent cations, one each in the three voids, or by 2 trivalent cations, leaving the third void empty. When trivalent cations such as Fe3+ and Al3+ fill two of the three voids, the clays are called, “dioctahedral clays”. When three divalent cations such as Fe , Mg and Mn fill up all the three voids, such clays are called, “trioctahedral clays”.

Ion Exchange Defined By ion exchange is meant “the reversible process by which cations and anions are exchanged between solid and liquid phases, and between solid phases, if in close contact with each other”. For exchange of cations the term “base exchange” is exclusively used.

Soil Reaction Soil reaction is an indication of the acidity or basicity of the soil and is measured in pH units. The pH scale ranges from 0 to 14, with 7.0 as neutral point. From pH 7 to 0, soil is increasingly acidic. From pH 7 to 14, soil is increasingly alkaline. Pure H2O ionizes to a very small degree, though to a measurable extent, so that it has the following equilibrium: both H+ and OH- are in concentrations of 10-7 moles L-1.

Electrical conductivity (EC) is the ability of a material to transmit electrical current. In the soil, the EC is the ability of the soil solution to carry an electrical current and is a measure of the total amount of soluble salts present in the soil. Pure water is a poor conductor of electricity and increases in soluble salts water result in proportional increases in the conductivity. 23.1. Sources of Soluble Salts in Soil The soluble salts in soils consist mostly of the cations sodium, calcium, and magnesium and the anions chloride and sulfate. Constituents that ordinarily occur only in minor amounts are the cation potassium and the anions bicarbonate, carbonate, and nitrate. The original and, to some extent, the direct source of all the salt constituents are the primary minerals found in soils and in the exposed rocks of the earth’s crust. While the mean chlorine and sulfur content of the earth’s crust is 0.05 and 0.06 percent, respectively, sodium, calcium, and magnesium each occur in the range of 2 or 3 percent. During the process of chemical weathering, which involves hydrolysis, hydration, solution, oxidation, and carbonation, these constituents are gradually released and made soluble. When carbon-di-oxide, of atmospheric or biological origin, dissolves in water, bicarbonate ions are formed. Water containing carbon-di-oxide is an active chemical weathering agent that releases appreciable quantities of the cation constituents as the bicarbonates. The concentrations of carbonate and bicarbonate ions are a function of the pH of the solution. Appreciable amounts of carbonate ions can be present only at pH values of 9.5 or higher.

Soil organic matter (SOM) is one of the most complex materials existing in nature. Besides the organic constituents in undecayed plant and animal tissues, SOM contains both living and dead microbial cells, microbially synthesized compounds and an unending array of derivatives of these materials produced as the result of microbial activity. Soil organic matter contains most of the naturally occurring organic compounds. Some components of SOM are undoubtedly unique to the soil environment, particularly those involving inorganic-organic complexes.

The Carbon Cycle The element that serves as a corner stone for cell structure is carbon. Plant tissues and microbial cells contain approximately 40 to 50% of C on dry weight basis. Yet, the ultimate source is the CO2 of the atmosphere. The CO2 is converted into organic C by the action of photoautotrophic organisms (higher plants and green algae). It has been estimated that the vegetation on earth’s surface consume some 1.3 x 1014 kg of CO2 per annum which is about V20 of the total CO2 supply of the atmosphere or 1/1000 of that in the ocean. Soil carbon constitutes the largest terrestrial pool of carbon.

The synthesis of humic substances is the least understood and the most intriguing aspect of humus chemistry. Studies on this subject are of enduring and continued research. Several pathways have been identified for the formation of humic substances during the decay of plant and animal remains in soil, the main ones being shown in Fig. 26.1.

To millions of different organisms around the world, soil is the habitat. A handful of soil can contain billions of soil micro-organisms. One gram of a fertile soil can harbour up to one billion bacteria. Only 5% of what is produced by green plants is consumed by animals, but 95% is consumed by microorganisms. Soil organisms are generally grouped into microfauna/microorganisms and macrofauna. The main soil microorganisms are bacteria, fungi, and protozoa. The macro-fauna includes oligochaeta, arthropods, mollusks and nematodes. These organisms play an essential role in soil formation and soil environment. 27.1. Bacteria Bacteria are the major single-celled microorganisms in the soil ecosystem that are responsible for many important processes. There are many different types of bacteria and most of them are aerobic, requiring oxygen from the soil atmosphere. However, there are bacteria that live without oxygen and other types can live both with and without oxygen. The growth of these bacteria is limited by the food that is available in the soil. They decompose plant materials as well as other organisms’ wastes that contain plant nutrients in an unusable form and convert them into a form that can be used by plants. One such essential nutrient that is converted by bacteria is nitrogen. Nitrogen-fixing bacteria are able to use the nitrogen (N2) that exists in the atmosphere and convert it through cellular processes into ammonia (NH3). Nitrifying bacteria mix ammonia with water to create ammonium ion (NH4+) and nitrate (NO3-), which can be used by other organisms.

Although plants can be grown to maturity in aerated nutrient solutions and other media, virtually all land plants, including those on which we depend for food, fibre and fuel, grow in soil. The soil has three-fold function to perform with respect to crop growth: physical, chemical and biological.

Soil pollution indicates the presence in the soil of any chemical or substance out of place and/or present at an abnormally higher concentration so as to adversely affect any non-targeted organism. Soil pollution often cannot be visually perceived or directly assessed, thus rendering it a latent danger. The Status of the World’s Soil Resources Report identified soil pollution as one of the main soil threats affecting global soils and the ecosystems services provided by them. Concerns about soil pollution are growing in every region. Recently, the United Nations Environmental Assembly (UNEA-3) adopted a resolution calling for accelerated actions and collaboration to address and manage soil pollution. This consensus, achieved by more than 170 countries, is a clear sign of the global relevance of soil pollution and of the willingness of these countries to develop concrete solutions to address the causes and impacts of this major threat. 29.1. Sources of Soil Pollution The main anthropogenic sources of soil pollution are the chemicals used in or produced as byproducts of industrial activities, domestic, livestock and municipal wastes (including wastewater), agrochemicals, and petroleum- derived products. These chemicals are released to the environment accidentally, for example from oil spills or leaching from landfills, or intentionally, as is the case with the use of fertilizers and pesticides, irrigation with untreated wastewater, or land application of sewage sludge. Soil pollution also results from atmospheric deposition from smelting, transportation, pesticide applications and combustion of many substances as well as radionuclide fallouts from atmospheric weapons testing and nuclear accidents. New concerns are being raised about emerging pollutants such as pharmaceuticals, endocrine disruptors, hormones and toxins, among others, and biological pollutants, such as micropollutants in soils, which include bacteria and viruses.

Soils play a vital role in the health of ecosystems in the world. Soils connect with air, water, rocks and organisms and are responsible for a multitude of functions in the global ecosystem services. These functions include providing habitat for most living things, air composition and quality, temperature regulation, carbon and nutrient cycling, water cycling and quality, and natural waste recycling. Mankind cannot survive without these soil functions. 30.1. Habitat Soils are the environment in which plants grow. They provide heat, nutrients and water to nurture plants to maturity. These plants, together with other plants and organisms, create ecosystems. These plants then provide valuable habitat and food sources for animals, bacteria and other things. Ecosystems depend on the soil and soils determine the nature and kind of ecosystems.

Soil quality is the capacity of the soil to function, within its natural or managed ecosystems, to sustain plant and animal productivity, maintain or enhance water and air quality, support human and animal health and habitation. This is not limited to agriculture, though most work and evaluation has occurred on agricultural lands. Soil health refers to the condition of the soil and its potential to sustain biological functions, maintain environmental quality and promote plant and animal health. Healthy soils are the key to food security and sustainable future. They support sustainable food production, mitigate and adapt to climate changes, filter water, enhance resilience to floods and droughts and so much more.

India is endowed with varied types of soils and this could be attributed varied relief features, landforms, climatic realms and vegetation types in the subcontinent. In ancient times, soils used to be classified in India into fertile and sterile soils. Later, the inherent characteristics and external features of soils such as texture and colour, were used to classify the soils. Based on texture, soil types were identified as sandy, clayey, loam, etc. In terms of colour, they were red, yellow, black, etc. Scientific surveys of soils in India have been conducted by various agencies since independence. All India Soil Survey Organization, established in 1956, carried out detailed soil surveys in the catchment areas of major river valley projects like the Damodar Valley. Later, the National Bureau of Soil Survey and the Land Use Planning (NBSSLUP) ofthe Indian Council of Agricultural Research, established in 1976, made extensive studies on Indian soils and classified them as per the USDA SoilTaxonomy

Multiple Choice Questions 1. The father of Pedology is: (a) Sorenson (b) Hilgard (c) Dokuchaev (d) Kononova 2. The science dealing with the laws of origin, formation and geographic distribution of soil as a body in nature is: (a) Pedology (b) Edaphology (c) Geology (d) Paleontology 3. The study of soils from the standpoint of higher plants is called: (a) Edaphology (b) Soil Taxonomy (c) Geology (d) Pedology 4. The most dominant element on earth’s crust is: (a) Carbon (b) Oxygen (c) Silicon (d) Hydrogen 5. The most dominant metal in earth’s crust is silicon: (a) Carbon (b) Iron (c) Aluminium (d) Silicon