Preface This book covers in detail the chemistry of soils, and various processes carried out in soils from chemistry point of view. With my teaching and research experiences that started from 1985, I do understand the problems of students studying the subject and accordingly I tried to prepare the book in such a manner that students may find the solutions to their all sorts of queries in soil science from chemistry point of view through a holistic and an up-to-date approach. The chapters have been arranged systematically, so that the readers will enter the domain of soil chemistry in a proper way step by step. Attempts have been made to incorporate the practical explanation and interpretation of issues dealt in each chapter. For better understanding of the subject for the readers, relevant references have been keyed into the text. Model questions, both subjective and objective type, have been included at the end of each chapter; so that students may be able to get an idea about the pattern of questions which are being asked during various competitive examinations. The book has been prepared following the recently revised course curriculum guidelines of Indian Council of Agricultural Research, New Delhi and most of the courses related to chemistry aspects of soil science have been dealt in this book.

1.1. What is Earth? The Earth is a layered body consisting of three parts: core, mantle, and crust. The radius of the Earth is nearly 6370 km of which crust, mantle and core share a depth of 40, 2850 and 3480 km, respectively (Figure 1.1). Exterior of the Earth contains the outer lithosphere which is surrounded (70% of surface) by water (hydrosphere). Solid and liquid sphere are surrounded by gas (atmosphere). The atmosphere is of 320 km above the lithosphere / hydrosphere. It is believed that there is a progressive increase in temperature with depth, if one goes to the core from the Earth crust. There is a rise of approximately about 1°F for every 64 feet depth on an average. Therefore, one could imagine that the interior of the earth is in molten condition.

2.1. Minerals Primarily, minerals are usually inorganic substances possessing a definite chemical composition and a definite molecular arrangement which is usually expressed in geometric form. The mineral matter is made up of various elements, combined to form compounds. Usually, minerals consist of more than one constituent elements. Quartz contains two elements, silicon and oxygen. Other minerals may contain many elements, such as amphibole consists of several elements that include sodium, calcium, magnesium, iron, aluminium, silicon, and oxygen. Some elements exist as such without forming compounds. They contain exceptionally one element. Graphite (C), diamond (C), and Sulphur (S). Minerals are naturally occurring crystalline solid where the atoms are arranged in a regular three-dimensional array. A crystal is a solid, formed by a repeating three-dimensional pattern of atoms, ions, or molecules & possessing fixed distances between the different parts. Minerals that do not grow in these regular or crystalline patterns are called amorphous solids. When we talk about ‘naturally occurring’, we exclude the laboratory-synthesized crystalline solids. Glass and amber are also naturally occurring solid, but they are not crystalline and thus, they are not considered as minerals, rather they are referred as amorphous solids.

3.1. What is weathering? Weathering may be defined as the transformation process of solid rocks, involving the disintegration and decomposition of rocks and minerals, into soil. There are different types of weathering: physical, chemical and biological. 3.1.1. Physical weathering In physical weathering the chemical composition of the weathered products remains same as that of the parental rocks. It is involved in fragmentation of rocks into comparatively smaller sizes. It does not transform the parental rock into a newer one that may differ from the parental rock in chemical composition. The process is totally physical in nature. But this is also true that physical weathering encourages chemical weathering, and both may be carried out simultaneously because physical weathering promotes more surface area of parental rocks for more interaction with agents of chemical weathering.

4.1. What is pH? pH is used to classify substance as acidic, neutral and alkaline, based on the prevalence of free H+ ions. Water molecules on autoionization dissociate to produce H+ and OH- in equal number, although degree of dissociation is very small because of its stability (about one molecule in 10 million is ionized at any one time). H+ ions later attached to un-ionized water molecule to give rise hydronium ions, (H3O)+

5.1. What is soil organic matter? Plant and animal residues which are not yet added to the soil are considered as organic materials. However, once they are added to soil, they become soil organic matter. Therefore, organic materials contributing to the soil matrix are referred as Soil Organic Matter (SOM). Sometimes SOM is also used to highlight only the non-living organic materials produced by living organisms. But SOM should be regarded as a broad term that includes biomass, partially decayed residue, and highly decomposed and transformed organic substances, humus. Agricultural soil contains nearly 1-5% of SOM, which is regarded as least amount in soil as compared to other three soil components, minerals (45%), air (25%) and water (25%). However, it contributes enormously to soil productivity. Soil productivity and fertility exhibit direct positive relationship with SOM content in soil. Plants and animals are considered as primary and secondary sources of soil organic matter, respectively. Plant residues and application of organic manures e.g., farmyard manure, compost or green manuring help to build up organic matter in soil and thus, enrich the soil with organic carbon and other plant nutrients. Under natural conditions, roots of grasses in grasslands, the leaves including shoot portion of the large trees in forest lands and after harvesting the crop remnants in agricultural field provide enough incorporation of organic materials in soil. SOM plays a vital role to maintain sustainability in agricultural production through producing healthy soils and thus, healthy crops.

6.1. What is colloid and colloidal system? A colloidal particle may be defined as a substance which is microscopically dispersed evenly throughout another phase and is possessing a size of 10-7 to 10-3 mm and representing a larger surface area per unit mass. Thus, a colloidal system refers to a two-phase system in which small particles of one phase which might be of solid, liquid or gas is suspended evenly in the second phase of solid, liquid or gaseous material. These colloidal systems may be of solid in liquid or solid in gas or liquid in liquid or liquid in gas. Unique examples of liquid in liquid colloidal system includes the fat droplets dispersed in water of milk. When you put floor disinfectant liquid in water, it forms a white emulsion. Thus, emulsion becomes a colloidal system of liquid in liquid. Likewise, in fog, water droplets are suspended in gaseous air. So, fog is an example of a colloidal system, constituting of liquid dispersed in gas. When we refer smoke, it contains carbon particles (solid) suspended in air (colloidal system of solid and gas). However, smog is composed of smoke and fog. In case of soil colloidal system, soil colloids, which are of tiny particles (<0.001 mm size) and may be of organic and inorganic nature, are suspended in soil solution (water) and referred to as sol. Colloidal chemistry, which involves in the surface reactivity and charge characteristics of colloids, plays a pivotal role in total soil chemistry.

7.1. Introduction Like all other living organisms such as humans, animals, and microorganisms, plants also need elemental nutrients for their growth and development. They assemble the nutrients from soil, water and air. However, to obtain economical crop yield, nutrients are also supplied to the crops from external sources such as chemical fertilizers and manures and using biofertilizers. It is the present-day need to handle the soil fertility status in a more realistic and careful way to obtain maximum crop harvest. Under poor management practices, soil may become infertile and loose its importance in agricultural use. In modern days, huge food demand for the burgeoning human population does not permit to show luxury to use only fertile soil. Moderately fertile soil or even degraded soil should also be managed properly so that they may be used to grow crops. For this, plant nutrients status of the soil plays a significant role along with other soil properties. For sustainable soil fertility management, parameters such as crop to be raised, its nutrient demand at various growth stages, current nutrient status of the soil, amount of nutrient application based on the soil testing results etc are important to be considered. Balanced nutrition to the plants is the prerequisite to obtain maximum benefit. If nutrients supplied to the plants are low in quantity than their requirement, crop production is lowered down. Similarly, if the nutrients are provided in more amount in the soil or unbalanced way through fertilizer or manure applications, total crop yield is suffered. It is a very common example that excess of phosphorus leads to zinc deficiency in most crop plants and it has been observed that excess N is responsible for potassium starvation in potato. Again, excess of iron, manganese, zinc, copper shows plant toxicity. Addition of nutrients to soils in excess also causes environmental pollution when the excess nutrients are lost from soil by leaching, surface run-off and volatilization. Unscientific way of soil fertilization leads to increasing cost of cultivation and imparts adverse effect to the environment instead of doing any good. Therefore, one should have a clear knowledge on soil fertility to obtain a sustainable productivity.

8.1. What is an ion exchange reaction and its application in soil system Ion exchange is a reversible chemical reaction between two substances, either between two electrolytes or between an electrolyte solution and a solid complex as well as between two solid phases (if in close contact with each other), where ions of equal charge may be interchanged in equivalent proportion. Organic colloids and inorganic micelles (clays) are sites of ion exchange. Soil contains silicate clay minerals which exhibit charges due to isomorphous substitution as well as due to the pH dependant charges for ionisation of hydrogen and hydroxide functional groups. Organic matter present in soil also contributes to the pH dependent charges due to the presence of various active functional groups. Thus, the organic and inorganic colloids present in soil bear positive or negative charges. Electric charge on soil particles is neutralized by equivalent amount of oppositely charged exchangeable counter ions, held by coulomb forces and van der Waals forces. Ca+2, Mg+2, H+, K+, Na+, NH4 +, Al+3, Fe+3, Fe+2, Mn+2, etc. are the common cations present in soil where Ca+2 serves as the dominant ion. However, Na+ and Al(OH)2 + are the dominant cations found in the exchangeable phase in strong alkali and strong acid soils, respectively. SO4 - 2, Cl-, HCO3 -1, NO3 - etc. are the most common anions found in soil to neutralize positively charged soil colloids.

9.1. Introduction Soil biology is a branch of science that studies the soil inhabiting organisms in relation to their ecology, functions, behaviour and impact on various soil processes and plant growth. Soil fertility is directly dependant on humus content as well as the presence of microorganisms in soil, although surface soil does not contain more than 0.5% by weight of living organisms. However, such a small portion of soil constituent as compared to other organic and inorganic constituents plays a significant and major role in transformation of plant nutrients from their unavailable to available form from organic wastes, soils and fertilizers. They have a direct role in improving soil tilth, providing an appropriate environment for root growth, decomposing organic residues to form humus and in turn releasing nutrients for plants use, releasing many plant growth promoters, fixing inert atmospheric nitrogen in plant usable forms and they promote many other important processes without which plants cannot survive. Absence of living organisms in soil leads to its infertility. An understanding of microbial processes in soil environment, keeping in view of soil nutrient status, carbon sequestration, soil health and quality, overall sustainability in agriculture and also global warming in respect of greenhouse gas emissions (nitrous oxide and methane) through agricultural practices, is very important in regulating nutrient release at different plant growth stages. If we are in a state to understand how various biological processes influence various changes in soil environment or in the opposite way that how different soil conditions alter the soil biota in number as well as in their functions and diversity, it is easier to implement soil management practices that promote to gain more benefits. Microorganisms are ubiquitous in nature. Possessing a diverse habitat, they are found everywhere in soil, air and water at a varied temperature and pH. One may find the highest genetic diversity of life within the domain of microorganisms. But unfortunately very few are known to us, so far only 1% of the microbial species on the earth are culturable. However, it is believed that soil of one gram may contain hundreds of millions to billions of microbes which may represent several thousand microbial species. In respect of the population, bacteria top the list followed by actinomycetes, fungi, algae and protozoa in accordance with a decreasing numerical order. Out of three important soil fertility components viz. physical, chemical and biological; the biological component is least understood due to its wide complexity.

10.1. Introduction The soils possessing characteristics that are not suitable and economical for crop growth are termed as Problem soils. These characteristics may be of physical, chemical and biological. For cultivation of crops in these soils, it is necessary to follow specific management interventions and proper reclamation measures. Frequently we come across another term, ’degraded soil’. Degraded soil is also a problem soil. However, there exists a hair-line difference between problem and degraded soil. Problem soils refer to the soils in which the inherent soil characteristics, obtained from its genesis, pose problems for their optimal use, while degraded soils are developed due to anthropogenic activities by unwise management interventions. 10.2. Types of problem Mainly there are four types of problem that may exist in soils: physical problems, chemical problems, biological problems and nutritional problems as a result of physical, chemical and biological constraints in soils.

11.1. What is acid soil? Acid soils refer to those soils that are characterized with the presence of higher concentration of H+ in soil solution and at exchangeable sites or characterized with its high proton donating capacity such as aluminium (Al3+) on hydrolysis gives rise to proton. Both H+ and Al3+ are responsible to contribute soil acidity. Al3+ causes soil acidity indirectly by generating H+ through its hydrolysis, while H+ contributes directly. Therefore, due to more abundance of H+, they exhibit low soil pH and low base saturation (Ca2+, Mg2+, Na+ and K+). Such soil is considered as problematic soil because plant growth is affected adversely under acidic conditions and they suffer root injuries when grown in acid soils. Besides, soil acidity interferes in the soil nutrients availability to plants, making them either deficient to plants (such as phosphorus, calcium, magnesium, molybdenum) or toxic to plants for their more availability. Under acidic condition, Al, Mn and Fe are found in more soluble form. Thus, they are taken up by the plant in such a higher level that they become toxic to plants. Phosphorus becomes immobile due to the formation of insoluble compounds with aluminium and iron under acidic conditions. Ca, Mg and Mo also become unavailable in acid soil and show their deficiency symptoms. Soil acidity also affects soil microbial growth and thus, in turn, affects the nutrient dynamics in soil. Soil acidification is induced heavily under intensive agricultural practices such as increasing leaching due to more irrigation practices, addition of fertilizers, build-up of soil organic matter and many other possible ways.

12.1. Introduction All soils invariably contain soluble salts. However, physico-chemical and biological soil properties start degrading when excessive amount of salts are accumulated in soil, particularly in rhizosphere, due to several anthropogenic activities and environmental conditions, and crop production is severely affected. Then we refer those soils as salt-affected soil. The need of intensive agriculture to keep pace with the increasing world population for their food requirement necessitates to bring more agricultural land area under irrigation. However, irrigation system without taking enough drainage provision and poor water quality and its poor management may make the situation worst due to the development of salt-affected soils that ensue unproductive soil and low crop yield. Crops grown on salt-affected soils suffer from high osmotic stress, nutritional disorders due to nutrient (N, Ca, K, P, Fe, Zn) deficiency, specific ion toxicities, poor soil physical conditions and as a consequence, a reduced crop productivity. Although it is considered that the rocks and minerals on weathering lead to salt accumulation in the soil, but the salt-affected soils are not truly originated from the accumulation of such salts in situ. On the contrary, occurrence of salt-affected soils is due to receiving salts that come from other locations with water. Sea water inundation in coastal belt, irrigation with poor quality water including saline groundwater, poor drainage system, irrigation adopting unscientific ways leading to waterlogging, shallow water table etc. contribute heavily toward development of salt-affected soils. Salt-affected soils may occur under almost all climatic conditions. Their distribution, however, is relatively more extensive in the arid and semi-arid regions compared to the humid regions because of poor precipitation to leach soluble salt adequately. Since evapotranspiration greatly exceeds precipitation in these climatic conditions, the soluble salts move upward with water due to capillary action and are left behind on the surface soil and the rhizosphere, once water is evaporated. The commanding ions of accumulated salts in affected soil are sodium, potassium, magnesium, calcium, chlorides, sulphates, carbonates and bicarbonates. The nature and properties of salt-affected soils are so diverse that they require specific approaches for their reclamation and management. Therefore, it is necessary to know their origin, formation, nature, chemistry and properties in detail to classify them into groups for their proper management to maintain sustainable agriculture.

13.1. Introduction Water is chemically benign. However, its interference in free gas exchange in soil becomes detrimental to crop growth when soils are subjected to waterlogging or plants are submerged with water.Waterlogging of soil refers to a soil condition when the soil attains a moisture regime to such an extent that oxygen is not available in the pore space in enough quantity for plant roots to respire adequately. On the contrary, detrimental gases such as carbon dioxide, methane, hydrogen sulphide, ethylene etc. accumulate in the root zone and adversely influence the growth of plants. Thus, anaerobic condition prevails in waterlogged soil causing depletion of oxygen and increase of CO2 in root zone, which severely affects aerobic micro-organisms while harmful organisms proliferate and restrict the plant growth. Waterlogging leads to nutrient deficiencies or nutrient toxicities, root tip decomposition and eventually, reduced growth or death of the plant. Physical, chemical and biological activities in the soil are also disturbed due to low temperature. However, this is important to note that plants differ among themselves in their demand for oxygen. Even oxygen demand for same plants differs with its growth stage. Therefore, it is well understood that there is no universal level of soil oxygen that can identify waterlogged conditions for all plants. Mostly waterlogging prevails in the land due to non-permeability of the soil material, the presence of an impervious layer in soil horizon, or a high-water table.

14.1. Introduction Environmental pollution primarily deals with five types of pollution: soil pollution, water pollution, air pollution, sound pollution and light pollution. However, soil, water and air pollutions are considered very important as the pollutants from the environmental compartments may be cycled within themselves, enter the food web and interfere the biological processes adversely of any living species. Soil supplies nutrients and water to plants which, in turn, act as the nutrient supplier to animals and humans. Thus, maintenance of soil quality is considered prerequisite for growth of plants, animals and humans. If soil, water and air are polluted through various mechanisms, living components in the environment are suffered adversely. Soil, water and air pollutions are associated with the build-up of soil contaminants such as toxic organic and inorganic compounds, petroleum hydrocarbons, radioactive materials, salts, disease causing agents etc. above the permissible limit, so that their presence will cast adverse effects on physicochemical and biological properties of soil which in turn exert negative impact on crop growth and on any non-targeted organisms. Common soil contaminants include heavy metals, pesticides, petroleum hydrocarbons, fertilizers if used in excessive amount, and many others. Contamination of the soil, air and water results from intensification of farming activities. Primarily nutrients (particularly nitrogen and phosphorus), sediment, wastes, pesticides and salts are regarded as agricultural nonpoint source pollutant. Realistically, most of the pollutants originate from anthropogenic activities, although there exists many naturally occurring mineral components that may exhibit toxicity towards living system at higher concentration. Land disposal of municipal and industrial wastes, automobile emissions, mining activity, and application of fertilizers and pesticides to agricultural field cause a continuous accumulation of heavy metals and other pollutant components in soils. These pose threats of environmental degradation via soil erosion, loss of biodiversity, climate change etc.