eChapters

The water contents expressed under certain standard conditions are commonly referred to as soil moisture constants. They are used as reference points for practical irrigation water management. The usage of these constants together with the energy status of soil water gives useful knowledge. These constants are briefly explained below:

6.1 Saturation Capacity

Saturation capacity refers to the condition of soil at which all the macro and micro pores are filled with water and the soil is at maximum water retention capacity”. The matric suction at this condition is essentially zero as the water is in equilibrium with free water. Excess water above saturation capacity of soil is lost from root zone as gravitational water.

The soil water plays an important role in the physical, chemical and biological activities taken place in the soil which directly or indirectly affects the crop growth. The water content of the soil can be measured by different methods. These methods are gravimetric method, neutron scattering method and electrical resistance method.

Soil moisture stress in relation to crop and development of crop
Agronomic methods are supported with mechanical measures where land slope exceeds permissible limits and runoff gains erosive velocities. The following boxes explain the nature of agronomic measures which are essential in inter-bunded or terraced areas. These practices enhance the utility value of all kinds of mechanical structures.
1. Contour Tillage: All agricultural operations such as ridging, ploughing, harrowing, sowing, trenching, etc., are recommended to be done on the contour wherever possible or at least generally across the direction of the slope where holdings are very small. Even though the operation is very simple, it plays a major role in retarding the process of soil erosion through runoff. It also conserves soil, and due to increased time of concentration, more rainwater seeps through the soil profile to recharge ground water. Summer ploughing leaves the soil highly absorbent of initial rains.
2. Dead furrows: When all tillage operations are complete, it is advisable to leave a deep dead furrow at every 10 m interval. This should remain in position until the crop is harvested. Dead furrows aid in reducing the runoff velocity and they also conserve water.
3. Organic Matter: Indian soils are very poor in organic matter, especially in drought-prone areas: This can be improved by leaving the crop residue in situ (on the fields). Adding organic manures such as farmyard manure and compost every year as basal application to the soil improves the physical condition of the soil considerably.
Soil-Air, Soil-Temperature, and Soil-Moisture relationships are well balanced with the presence of organic matter. Organic matter improves the activities of soil microorganisms and also provides the much needed micro plant nutrients of all kinds, besides nitrogen, phosphorus and potash. Addition of large amounts of chemical fertilizers to dryland crops should be discouraged as it damages the soil due to:

6.1 Determination of Soil Moisture

6.1.1 Determination of Soil Moisture Content by Gravimetric Method (Direct method)

Traditionally, water content in soil is expressed as the ratio of the weight of water present in the soil to the weight of dry soil. When this ratio is multiplied by 100, it becomes the percentage of water in the soil sample on dry weight basis.

Equipments and Materials

Soil auger, aluminium moisture box with air tight lid, balance, oven and desiccator

5.1. Determination of Soil Moisture

5.1.1. Determination of Soil Moisture Content by Gravimetric Method (Direct method)

Traditionally water content in soil is expressed as the ratio of the weight of water present in the soil to the weight of dry soil. When this ratio is multiplied by 100, it becomes the percentage of water in the soil sample on dry weight basis.

Soil moisture is the water retained within soil pores and represents a critical link between precipitation, evapotranspiration, runoff, infiltration and groundwater recharge. It provides the principal water supply for plant growth and strongly influences crop establishment, nutrient availability and yield. This chapter discusses soil-moisture storage, field capacity, permanent wilting point, available water, infiltration, percolation and root-zone dynamics. It examines methods of measurement, estimation and monitoring and explains how soil texture, structure, organic matter, vegetation and weather influence moisture conditions. Effective soil-moisture management supports irrigation scheduling, drought mitigation, water-use efficiency and prevention of waterlogging, thereby strengthening agricultural productivity and environmental sustainability.

Introduction

Nitrogen constitutes 78% of the atmospheric gases and there is about 75,000 Mg N on each hectare of land. However, 99% of the atmospheric N is as N₂ and is inert and non-reactive, while the rest 1% or even less is the reactive-N. Reactive-N is linked with H, O and C and is responsible for the creation and sustenance of all life on the planet Earth. Nitrogen is the key nutrient that separates proteins from carbohydrates and fats, the three major macronutrients for humans. As a component of proteins, N is present in DNA and RNA, which carry the hereditary characters and then as the major constitute of the body muscles of humans (and animals). Plant growth and crop production are also very much dependent on the adequate availability of N in soil.

Spatial variability in soil parameters including nutrients has been attributed to the parent material, topography, landforms, cropping pattern, fertilization and manuring practices.. Blanket nutrient recommendations further widen the variability and enhance the risk of soil degradation in terms of soil organic carbon and nutrient mining, acidity, green house gas emission, water and environmental pollution. Unfavorable economics on account of blanket recommendation, adversely influence farmers to increase investments in new crop management technologies. This results in low farm productivity and poor soil health, that jeopardize food security and agricultural sustainability.

Introduction

Soil test-based fertility management is an effective tool for increasing productivity of agricultural soils that have a high degree of spatial variability. However, major constraints impede wide scale adoption of soil testing in most developing countries. In India, these include the prevalence of small holding systems of farming as well as lack of infrastructural facilities for extensive soil testing (Sen et al., 2008). Under this context, GIS-based soil fertility mapping has appeared as a promising alternative.

Introduction

Organic matter in soil is the key to soil health. SOM improves many physical, chemical, and biological characteristics of the soil, including water holding capacity, cation exchange capacity, pH, buffering capacity, and chelating of micronutrients. Furthermore, well decomposed SOM improves soil structure by increasing aggregation, enhances biological activities in the soil, slowly releases nutrients, and suppresses some diseases. A loss of SOM can lead to soil erosion, loss of fertility, compaction, and general land degradation.

Introduction

The soil is the largest terrestrial pool of organic carbon, about 1150 Pg (1Pg = 10¹⁵ g) compared with about 700 Pg in the atmosphere and 600 Pg in land biota (Lal and Kimble,1997). Organic carbon level of soils reaches a fixed equilibrium that is determined by a number of interacting factors such as precipitation, temperature, soil type, tillage, cropping systems, fertilizers, the type and quantity of crop residues returned to soil, and the method of residues management. In the temperate climate, it may take about 50 years for the organic carbon of soils to reach a new equilibrium level following a change in management. However, this period is much shorter in a semiarid and arid climate and also in tropical climate like India.

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.

Soil organic matter is a key component of soil fertility, biological activity, aggregation and carbon storage. This chapter examines the origin, transformation and management of organic matter in agricultural soils, with particular attention to irrigated agroecosystems. It discusses the decomposition of organic matter under aerobic and anaerobic conditions and explains the major factors influencing decomposition rates. Microbial activity plays an important role in transforming organic materials and regulating nutrient cycling within the soil. The chapter introduces humus, humic substances and non-humic substances and explains interactions between clay and humus that influence soil properties. The relationship between soil organic matter and plant growth is also considered, highlighting its contribution to nutrient availability, soil structure and biological processes. Methods for maintaining soil organic matter and conserving soil carbon in agroecosystems are discussed along with factors controlling soil carbon levels. The chapter therefore establishes the importance of organic matter management for improving soil health, carbon conservation and the long-term sustainability of irrigated agricultural systems.

Soil organic matter makes up only a few per cent of most soils, but it has a great deal of influence on soil properties and on agricultural productivity.

2.1. Quantity of Organic Matter

Based on the content of organic matter, soils are identified as mineral or organic. Mineral soils dominate most of the world’s cultivated land and may contain anything from a trace to 30 per cent of organic matter. Organic soils are naturally rich in organic matter, chiefly due to climatic reasons. Although they contain more than 30 per cent organic matter, it is precisely for this reason that they are not vital cropping soils.

Plants are the generators or the primary source of organic matter while animals are the transformers- secondary source of organic matter. In agriculture, plants add 10–30 percent of organic matter through their roots stubbles and leaf fall and exudations. The excreta of livestock, the organic wastes of urban and rural residential areas include the garbage and sewage and sludge. The by products of agro based industries viz. bagasse, vinasse, pressmud from sugar industries, the coir dust from the coir industries, dust of cotton from ginning industries, the fruit and food processing industries wastes.

Carbon may be present in soil in four different forms:

1. Carbonates: Mainly as CaCO₃ and MgCO₃, active CO₃ ^2- and HCO₃ ^- ions and gaseous CO₂.
2. Highly condensed elemental organic form: Charcoal, graphite.
3. Completely decomposed resistant organic residues: Humus.
4. Undecomposed and partially decomposed organic residues.

The latter three fractions constitute the total organic carbon of which last two are chemically active form.

Carbon may present in soil in four different forms:

1. Carbonates: Mainly as CaCO₃ and MgCO₃, active CO₃^2- and HCO₃^- ions and gaseous CO₂.
2. Highly condensed elemental organic form: Charcoal, graphite.
3. Completely decomposed resistant organic residues: Humus.
4. Undecomposed and partially decomposed organic residues.

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.

Soil organic matter is any material produced originally by living organisms (plant or animal) that is returned to the soil and goes through the decomposition process. At any given time, it consists of a range of materials from the intact original tissues of plants and animals to the substantially decomposed mixture of materials known as humus. Soil organic matter (SOM) is the organic component of soil, consisting of three primary parts including plant residues and living soil organisms, decomposing (active) organic matter, and stable organic matter (humus). Most soil organic matter originates from plant tissue.

4.0 Soil organic Matter

The soil organic matter (SOM) is a storehouse of multi-nutrient elements, composed mainly of carbon, hydrogen and oxygen and small amounts of other elements like nitrogen, phosphorous, sulphur, potassium, calcium , magnesium and organic residues coming out from the decomposed plant or animal products (Figure 4.1). The SOM may have the recently incorporated materials or age-old decomposed materials or with living flora or fauna in the organic matrix. Soil organic matter is one of the important components determining soil quality indices and also for enriching the fertility status of soil for production maximization. The nutrient retention capacity , water holding capacity, degradation of pollutant elements, carbon sequestration, soil structure and aggregate stability are affected by the presence of SOM.

ABSTRACT

Microorganisms play a key role for the proper running of the ecosystem. They help in nitrogen fixation, which is unique and most necessary property found only in this group. They help in decompositions, mineralization, nutrient cycling, phosphate solubilization, soil aggregate formation, symbioses, parasitism and many other important works. A great emphasis has been given by many scientists to conserve the microbial diversity. Developing human civilization has a great negative impact not only on the above ground biodiversity but also on the belowground biodiversity. This article is an overview, what important and crucial role nitrogen fixers play for maintaining soil and plant health. Nitrogen fixer not only fixes atmospheric nitrogen but also produces growth promoting hormones which are very important for plant growth. 

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.

ntroduction

Millions of living organisms present in soil make it a living and a dynamic system. Under a microscope, it reveals a complex array of soil particles and pore spaces filled with air and water. In these pore spaces that plant roots and millions of organisms develop, which are ranging from sub-microscopic to macroscopic in size. The physical and chemical characteristics of soil determine the nature of the environment in which vast number of soil microorganisms especially soil microorganisms are found. Living organisms in soils include both macro (earthworms, rodents, termites etc.) and microorganisms (bacteria, fungi, actinomycetes, algae and protozoa). These organisms not only help in the development of soils but also carry out a number of transformations, facilitating the availability of nutrients to the plants.

Pasteurization: The heat treatment given to soil to free it of harmful microorganisms or pathogens and pests is called pasteurization. It kills weed seeds also.

Pasteurization of greenhouse plant growing media

Greenhouse growing medium may contain harmful pests such as disease causing organisms, nematodes, insects and weed seeds. It is advisable to decontaminate it by heat treatment or by treating with volatile permissible chemicals.

11.1 Determination of Penetrability of Soil

Penetrability of a soil is usually assessed by the penetration resistance, which a measure of soil strength and thereby the soil compaction. Penetration resistance is the capacity of the soil in its confined state to resist penetration by a rigid object. Besides soil strength, the penetration resistance also depends on the size, shape and orientation of the axis of the penetrating probe. The penetrating probe may be a finger, pencil, stick, root or any specially designed object having a specified geometric shape and a device to measure the resistance as it is pushed into the soil.

Soil penetrability is intimately related to agriculture because of its adverse impact on draught requirement during ploughing, root development and crop yield, to civil engineering due to its relation to settlement, stability and ground water flow.

Soil penetrability is a measure of the ease with which an object can be pushed or driven into the soil. Any device designed to measure the resistance to penetration may be called penetrometer. There are several types of penetrometers: (i) pocket penetrometer, (ii) proctor penetrometer, (iii) cone penetrometer, and (iv) split-spoon penetrometer. Here, we will discuss about two most commonly used penetrometers viz. pocket penetrometer and cone penetrometer used for agricultural purposes.

Crusting soils

What is soil crusting?

Soil crusting refers to formation of a compact thin layer having high bulk density and high penetration resistance at the soil surface when dries after dispersion of fine particles and clogging of pores due to beating action of rain drop and slaking of irrigation water.

In arid and semiarid regions, crust formation occurs due to the destruction of soil aggregates. Crust formation occurs in all types of soils except sand. It is characterized by high bulk density, low non capillary pores, low hydraulic conductivity and aeration and high penetration resistance.

Introduction

Soil aeration status involves the rate of such ventilation, as well as the proportion of pore space filled with air, The composition of that soil air and the resulting chemical oxidation or reduction potential in the soil environment. Water provides the base for food, nutritional and environmental security. Soil water plays a significant role in controlling the energy balance of the soil and its overlying micro-environment by modifying its radiation exchanges and thermal properties. Soil temperatures affect plant and microorganism growth and also influence soil drying by evaporation. It is a central maximum of ecology that “everything is connected to everything else.” This interconnectedness is one reason why soils are such fascinating and challenging objects of study. In this chapter we shall explore three aspects of the soil air, water and temperature.

This chapter presents laboratory procedures for measuring the major physical properties that regulate soil behaviour, water availability, aeration and root development. It begins with particle-density determination using a pycnometer and proceeds to bulk density, soil texture, moisture content, total pore space, porosity and water-holding capacity. Each practical exercise provides the underlying principle, required apparatus, procedural steps, calculations and relevant precautions. The hydrometer method is introduced for estimating the proportions of sand, silt and clay. Methods for evaluating moisture retention and pore relationships are also described. Collectively, these measurements help characterize soil structure, compaction, drainage, water storage and its suitability for crop production.

Soil physics is the study of soil’s physical properties and processes. Soil physics deals with the dynamics of physical soil components and their phases as solid, liquids, and gases. Physical properties of soil include texture, structure, density, porosity, consistency, temperature, colour and water content. The physical properties depend on the amount, size, shape and arrangement and mineral composition of its particles , kind and amount of organic matter and the volume and form of its pores and the way they are occupied by water, air at a particular time.

1. The smallest size soil separate is

a. Sand      b. Silt

c. Clay        d. Loam

ABSORPTION ACTIVE: Movement of ions and water into the plant root as a result of metabolic processes by the root, frequently against an electrochemical potential gradient.

The interaction between soil and plants is a complex and dynamic process that plays a vital role in determining plant growth, productivity, and nutrient uptake. This abstract explores the intricate relationship between soil properties, root morphology, and nutrient availability, highlighting key mechanisms involved in nutrient uptake by plants. Soil serves as the primary reservoir of nutrients essential for plant growth, providing essential elements such as nitrogen, phosphorus, potassium, and micronutrients. Plant roots play a critical role in accessing these nutrients, with their morphology and distribution influenced by soil texture, structure, and nutrient availability. Various mechanisms, including root exudation, mycorrhiza associations, and ion exchange, facilitate nutrient uptake by plants, enhancing their resilience to environmental stresses and optimizing nutrient use efficiency. Understanding the dynamics of soil-plant interaction and nutrient uptake is essential for sustainable agriculture, guiding the development of management practices that promote soil fertility, crop nutrition, and ecosystem resilience.

Introduction
Soil is considered as a thin layered living entity over the earth’s surface, poses a profound implication on the solidity of ecosystems. It act as a transient nutrient sink for the plants and plays a crucial role in the crux of biogeochemical cycling of Earth’s elements which is being carried out by distinct microbial assemblages present in the soil profile. Soil is the most diverse niche for the microbial communities in which large proportion of taxa exists at a very low correlative abundance. Despite the extensive importance of this entity it has been snubbed lately with the various undesirable activities done by human kind. Therefore, the scientific understanding of soil origin is critical. Scientifically speaking, formation of one inch of soil almost took 15 years if the parent materials are soft whereas hard parent material might takes hundreds of year for its development. Soil horizon involves the arrangement of mature soil in a series of zones. Each horizon encompasses the distinct texture and composition that varies with different kinds of soils. Soil profile is the cross sectional view of the horizons in a soil. The O horizon is the topmost layer of soil, mainly consists of partially decomposed leaves, animal wastes, fungi and other organic materials. A horizon is looser than other deeper layers of soil, mainly dark in colour and considered as the uppermost layer of the soil consisting of partially decomposed organic matter known as humus. Generally, in the upper layers of soil the roots of most plants exist. As long as these layers are supported by vegetation, it acts as a storehouse of water which gets released in a trickle throughout the year. Plant roots grow in the soil and absorb nutrients and water which are required during the process of photosynthesis that helps in the growth and development of plants. Soil contains essential nutrients which are being adsorbs by the plants and is beneficial to all living organisms such as herbivores and omnivores depends on plants for their food, then these being eaten up by carnivores and omnivores in the food web, so nutrients that are absorbed from the soil is reached to all strata of living entity. Soil is a niche for different organisms as its top layers harbors a large population of bacteria, fungi, earthworms and many other small insects that

Soil pollution happens when we destroy the thin layer of healthy and productive soil, which covers the earth’s surface. Most of the world’s food is produced using this resource and if we don’t protect it the world’s farmers will be unable to grow enough food to support the worlds people.
 
Healthy soil depends on bacteria, fungi, and small animals (such as worms) to break down wastes in the soil and release its nutrients which help plants grow. The over use of fertilizers and pesticides can limit the ability of soil organisms to process wastes, which in turn makes the soil less productive or in the worst case scenario useless or even poisonous. 

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.

Pollution

Pollution is an undesirable change in physical, chemical or biological characteristics of air, water or land caused by excessive accumulation of pollutants (substances causing pollution).

Kind of pollution

Pollution can be classified in many ways:

Soil pollution is defined as presence of xenobiotics (e.g. chemical compounds, radioactive elements) that alters the soil properties viz., both chemical, physical and biological properties of soil. Soil pollution, including heavy metals, may be of natural origin, like volcanic eruptions, atmospheric lightning, animal excrements or ore leaching. Nevertheless, human activity and mostly chemical industry, mining and metallurgy, as well as municipal waste management and traffic emissions are the main source of environmental pollution.

Life on Earth is rooted in soil. Soil is not just a substratum anchoring the plants and giving a base for all terrestrial animals. It is the largest terrestrial ecosystem, and one of the most wonderful ecosystems on Earth. It inhabits most microorganisms. Microbial populations flourishing within the soil have greater biomass than all the human beings, elephants, and whales put together. It is due to the microorganisms that the soil constitutes the most active ecosystem on Earth.

The health of the soil is crucial for the health of all the plants growing on it and for all the animals dependent on it for their inhabitation and nourishment. Soil health is also critical for the well-being of numerous microorganisms, detritivores, and several other organisms. Soil is composed of all essential nutrients/ minerals, air, water, and organisms in a proportion that strikes its ecological balance and maintains its structure and functions revealed through its productive potential.

Soil, like all other ecosystems, is itself a dynamic ecosystem. Its composition keeps changing but natural inbuilt mechanisms help strike a balance and recover 

Introduction

Productivity has increased as a result of the significant industrialization that has occurred in India during the past 40 years. However, it has also had detrimental effects on the ecology, particularly in India where soil contamination is progressively becoming more severe. In addition to the effects of man and land use, soil is constantly adapting changes in environmental conditions. A characteristic of soil pollution is the decrease in soil productivity brought on by the presence of soil pollutants. Some changes in the soil will be temporary and reversible, while others will be permanent. Pollutants in the soil have a negative impact on the physical, chemical, and biological characteristics of the soil and decrease its production. Soil pollution is a result of a variety of factors, including pesticides, fertilizers, organic manure, chemicals, radioactive waste, abandoned food, clothing, leather goods, plastics, paper, bottles, tins, and corpses. Industrial waste contains chemicals including iron, lead, mercury, copper, zinc, cadmium, aluminum, cyanides, acids, and alkalis, among others, which either directly or indirectly enter the soil.

Introduction
Historian “Donald Worster” tells us that fertilizers are not a replacement for rich soil. The soil is a resource for which there is no substitute. Time and climate have an impact on how soils develop as well. In humid and warm climates, soils develop more
quickly than in excessively dry or cold regions. In 15 years, under perfect temperature circumstances, soft parent material may develop into 1 centimeter of soil. It could take hundreds of years for a hard parent material to turn into soil in unfavorable climate conditions.
Definition
The land’s thin layer of soil is made up of a combination of minerals, organic matter, living things, air, and water that work together to support the growth of plants. The source material’s transformation into soil was influenced by a number of elements.
The accumulation of persistent poisonous compounds, chemicals, salts, radioactive elements, or disease-causing agents in soil that negatively impacts plant growth and animal health is known as soil pollution.

1.1 INTRODUCTION

This volume is concerned with the chemistry of soil processes. These are rather arbitrarily divided and discussed in the first part of this book in terms of mass flow and diffusion, precipitation, cation, anion and ligand exchange, adsorption, translocation of metals, and oxidation and reduction. The way in which these processes operate to determine the fate of different substances added to soils is described in the remaining part of the book. In this initial chapter an attempt is made to show how the individual chemical processes described later combine with physical and biological processes to give soils their ability to support plant life, and to accept the detritus of the world and to convert it to harmless constituents of the soil mass.

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 physical properties
The physical properties of soil, particularly its mechanical behavior, significantly impact its suitability for plant growth. Factors like plant support, root penetration, drainage, aeration, moisture retention, and nutrient availability are all influenced by the soil’s physical condition. These properties also affect the soil’s chemical and biological activities.
A soil’s physical properties are determined by the quantity, size, shape, arrangement, and mineral composition of its particles, as well as its organic matter content and pore spaces.
Important Physical Properties of Soils
1. Soil Texture
2. Soil Structure
3. Surface Area
4. Soil Density
5. Soil Porosity
6. Soil Color
7. Soil Consistence
Soil Texture
Definition
Soil texture refers to the relative proportions of different-sized particles, specifically the percentage by weight of sand, silt, and clay. It’s a fundamental soil property that is difficult to alter. Soil textural determinations typically exclude particles larger than 2 mm in diameter (stones and gravels). While larger particles can affect land management and tillage, they contribute minimally to water-holding capacity (WHC) and nutrient storage/supply.

Introduction

Intensive production of agricultural crops has contributed to decline in soil quality, leading to lower crop productivity and farm profitability. Major causes of this decline are soil compaction, surface crusting, low organic matter, increased pressure and damage from diseases, weeds, insects, and other pests, as well as a lower density and diversity of beneficial soil organisms. These constraints have increased the interest of farmers in assessing the health status of their soils and in implementing sustainable soil management practices. Soil health is critically important to sustainable agricultural productivity and environmental well being. Healthy soils provide a range of environmental services including water infiltration, habitat provision and profitable and sustainable agriculture.

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.

Soil health

Soil health is defined as the continued capacity of soil to function as a vital living system, by recognizing that it contains biological elements that are keys to ecosystem function within land use-boundaries. These functions are able to sustain biological activity of soil, maintain the quality of air and water as well as promote plant, animal and human health.

The characteristics of a healthy soil depends on the physical, chemical and biological properties of soils which directly or indirectly influence crop production.

Soil quality and soil health are terms that are discussed in agricultural community. Both these terms encompass a complex, dynamic concept, different to define and hard to measure. The term soil quality and soil health are often used interchangeably in the scientific literature, with scientists in general preferring soil quality and producers preferring soil health. Soil health may be considered as the state of soil at a particular time equivalent to the dynamic properties in short term. In contrast, soil quality may be considered for a particular purpose over a longer time scale, equivalent to static soil quality. In a broader way soil quality may refer to both permanent and static soil properties. (Fig. 11.1).

A static soil property relates to natural characteristic of soil (texture) which are derived from pedogenic processes. These properties cannot be easily amended. The dynamic properties are readily related to management practices. The dynamic component is of most important for the growers because it allows the soil to come to its full potential.

Introduction
Soil quality is a critical concept in environmental science, agriculture, and land management. It refers to the ability of soil to perform its essential functions, which include supporting plant growth, regulating water flow, cycling nutrients, filtering pollutants, and providing habitat for microorganisms. Healthy, high-quality soil is fundamental for the productivity of ecosystems and the sustainability of agricultural practices. The quality of soil is influenced by a combination of physical, chemical, and biological factors. These include soil texture, structure, organic matter content, pH, nutrient availability, and the presence of beneficial organisms like bacteria and fungi. When these factors are balanced, soil can support healthy crops and ecosystems. However, soil degradation—caused by practices such as overuse of chemical fertilizers, deforestation, erosion, and pollution—can severely reduce its quality, leading to lower agricultural yields, water contamination, and loss of biodiversity. Assessing soil quality involves monitoring several key indicators, such as organic matter levels, soil texture, compaction, water infiltration rates, and the presence of key nutrients. Organic matter is especially important, as it helps to bind soil particles, retain moisture, and provide a food source for beneficial microorganisms. The chemical aspect of soil quality is equally vital, encompassing the availability of nutrients such as nitrogen, phosphorus, and potassium, as well as the presence of any harmful contaminants like heavy metals. The biological dimension of soil quality relates to the diversity and activity of organisms within the soil. A rich microbial community enhances nutrient cycling, helps decompose organic matter, and promotes plant health. Earthworms, for instance, play a critical role in aerating the soil and facilitating water infiltration. Soil quality is not a static property; it can change over time depending on land management practices. Sustainable practices such as crop rotation, conservation tillage, the use of organic amendments, and maintaining ground cover can improve and maintain soil quality. Conversely, unsustainable practices like excessive tillage, monoculture, and the overuse of agrochemicals can degrade soil, making it less productive and more prone to erosion. In

17.1 Determination of Soil pH

The soil pH value is the measure of hydrogen or hydroxyl ion activity in the soil solution and it indicates whether the soil is acidic, neutral or alkaline in reaction. The pH is the most important property of soil as it controls the availability of plant nutrients, microbial activity and physical condition of soil. Since crop growth is affected both under very low (strongly acidic) and very high (strongly alkaline) soil pH, reclamation of these soils are necessary. Soil pH in fact indicates the extent of active acidity (activity of hydrogen ion in soil solution) of soil. Depending upon the purpose of measurement and soil condition, soil pH is measured in several soil-water or soil-salt solution ratios. The saturated paste is used for identifying specific soil problems like degree of acidity or alkalinity. For making fertilizer recommendation, 1: 2 soil-water suspension is generally adopted. To mask the variability in salt content of the soil, the pH is measured in soil-CaCl₂ suspension (1:2 soil-0.1 M CaCl₂ solution). The soil pH value is independent of dilution over a wide range of soil-salt solution ratio, thus more reproducible than those obtained with soil-water suspension.

10.1. Determination of Soil pH

The soil pH value is the measure of hydrogen or hydroxyl ion activity in the soil solution and it indicates whether the soil is acidic, neutral or alkaline in reaction. The pH is the most important property of soil as it controls the availability of plant nutrients, microbial activity and physical condition of soil. Since crop growth is affected both under very low (strongly acidic) and very high (strongly alkaline) soil pH, reclamation of these soils are necessary. Soil pH in fact indicates the extent of active acidity (activity of hydrogen ion in soil solution) of soil. Depending upon the purpose of measurement and soil condition, soil pH is measured in several soil-water or soil-salt solution ratios. The saturated paste is used for identifying specific soil problems like degree of acidity or alkalinity. For making fertilizer recommendation, 1: 2 soil-water suspension is generally adopted. To mask the variability in salt content of the soil, the pH is measured in soil-CaCl₂ suspension (1:2 soil-0.1 M CaCl₂ solution). The soil pH value is independent of dilution over a wide range of soil-salt solution ratio, thus more reproducible than those obtained with soil-water suspension.

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.