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Carbon is the cornerstone of all life on Earth. Plant tissues and microbial cells contain approximately 40 to 50% of carbon on dry weight basis. Yet, the ultimate source is the CO₂ of the atmosphere. The CO₂ is converted into organic carbon 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.

Organic C in agricultural soils contributes positively to soil fertility, soil tilth, crop production, and overall soil sustainability. Changes in agricultural management can potentially increase the accumulation rate of soil organic Cd (SOC), thereby sequesteringCO₂fromtheatmosphere.Optimizingagricultural management for accumulation of SOC can result in the sequestration of atmospheric CO₂, thereby partially mitigating the current increase in atmospheric Carbon dioxide. In addition to the environmental benefits of soil C sequestration, Carbon dioxide consideration has also been given to the implementation of a C credit trading system which may provide economic incentives for C sequestration initiatives.

Changes in agricultural practices for the purpose of increasing SOC must either increase organic matter inputs to the soil, decrease’decomposition of soil organic matter (SOM) and oxidation of SOC, or a combination thereof. These practices include, but are not limited to, reducing tillage intensity, decreasing or ceasing the fallow period, using a winter cover crop, changing from monoculture to rotation cropping, or altering soil inputs to increase primary production (e.g., fertilizers, pesticides, and irrigation). Implementing practices that sequester C can reverse the loss of SOC that may have occurred under intensive cultivation thereby increasing SOC to a new equilibrium

Agriculture in India is one of the most important sectors of its economy. It provides livelihood to almost two thirds of the work force in the country and accounts for 18% of India’s GDP. About 43 % of India’s geographical area is used for agricultural activity and large number of production systems are in practice in different parts ofthe country. In pre-independence era (before the 1950s), agriculture in India was a system of harnessing nature for the sustenance of human beings, similar to the presently defined organic farming. Some countries have moved away from inorganic to organic farming yet it is not possible for India to depend entirely upon this system of farming due to large population. Total availability of plant nutrients from organic sources is projected to be 7.75 million tonnes by 2025 (Tandon, 1997) against the total requirement of37.50 million tonnes (Katyal, 2001) for 1504 million population (Sekhon, 1997). Thus nutrients supply through organic sources alone would result in deficiency of about 30 million tonnes of nutrients. Hence, it is imperative to use both organic and inorganic sources of plant nutrients enhance food production for the increasing population (Mahajan et al., 2007). In India the contribution of organic sources was 80-100 per cent in 1949-50 and reduced to about 32 per cent in 1993-94 and 20-25 per cent in 2004-05. So, it is imperative to enhance organic sources to meet total nutrient consumption. Following independence, rapid population growth in India placed great pressure on land and on these traditional farming systems; huge demands for food grains led to increased use of fertilizers and pesticides to boost production. Still a negative balance of about 8 million tones of NPK is foreseen in 2020, even if we continue to use chemical fertilizers, maintaining present growth rates of production and consumption. The most optimistic estimates at present, show that only about 25-30 per cent nutrient needs of Indian agriculture can be met by utilizing various organic sources. Many of the gains in production during the last 4-5 decades resulted from the “Green Revolution,” a campaign of technological interventions in agriculture widely adopted by farmers in developing countries. Expansion of irrigation to cover rain fed areas, popularization of hybrids/ transgenic varieties of crops, and use ofsynthetic chemical fertilizers and pesticides were the maj or technologies promoted. This paid rich dividends in India, quadrupling food grain production from 50 million metric tons in 1950-51 to 211 million metric tons in 2001-02, and enabling India to become self-sufficient in food grain. Now a second green revolution is also in the offing, to boost agricultural production and meet the estimated requirement of 337 million metric tons.

1. Introduction
Sustainable crop production describes farming practices that do not negatively impact the environment, biodiversity, natural agroecosystem, or the quality of the crops produced. Growing crops sustainably improves the system’s capacity to sustain long-term, steady, and uniform levels of food production and quality without raising the need for or increasing the demand for agricultural inputs necessary for effective management. Sustainable crop production deals with keeping the soil healthy with the integrated approach of maintaining organic matter, protecting biodiversity while reducing use of pesticides, ensuring food safety and food quality. It is an approach to grow food to feed our population without adversely affecting the natural ecosystem, soil and biodiversity, natural resources, environment etc. Since the soil is one of the key natural resources for food production, establishing and preserving soil health is crucial for agricultural sustainability and ecosystem function. The distinction between soil quality and health is quite narrow, and the phrases are sometimes used interchangeably. The “fitness for use,” “capacity of a soil to function,” or “the ability of the soil to serve as a natural medium for the growth of plants that sustain human and animal life” are some basic definitions of soil quality (Karlen et al., 1992; Pierce & Larson, 1993; Doran & Parkin, 1994; Acton and Gregorich, 1995; Karlen et al., 1997). Assessment of soil health by analyzing different associated attributes is necessary to monitor its status. However, soil health or quality evaluation focuses on the physical, chemical, and biological parameters which are highly interlinked to one another. All the parameters have their significant effect in governing overall soil health. This chapter will discuss the chemical parameters only which are very much important for the sustainable crop production and have very crucial role in determining the soil health. It will cover important chemical parameters, essential plant and soil nutrients, heavy metals, agrochemicals etc. which having significant effect on soil health determination

3.1. Soil colloids

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.

ACTIVATION ENERGY: The energy required before the reaction of concern will proceed. Often, heating will supply the needed energy. Enzymes or other catalysts lower the activation energy of a reaction.

Soil Chemistry is the study of chemical composition of soil in relation to crop needs. It deals with the chemical constitution of the soil, the chemical properties and the chemical reactions in soils. Traditional soil chemistry focuses on chemical and biochemical reactions in soils that influence nutrient availability for plant growth, and potential environmental consequences associated with inorganic and organic fertilization. Soil chemistry has increasingly focused on soil genesis and the environment over the past few decades.

1. Central Fertilizer Quality Control & Training Institute (CFQC&TI) is situated at

a. Mysore           b. Karnal

c. Faridabad        d. Coimbatore

Soil Classification

Soil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use.

Soil classification is grouping of objects in orderly and logical manner into classes based on the properties of soils (differentiating characters) for the purpose of studying and identifying them. The soil individuals are grouped into classes of the lower category (soil series) which are grouped into classes of higher categories (soil orders). The lower categories are defined by large number of differentiating characters and higher category by a few differentiating characteristics.

For soil resources, experience has shown that a natural system approach to classification, i.e. grouping soils by their intrinsic property (soil morphology), behaviour, or genesis, results in classes that can be interpreted for many diverse uses. Differing concepts of pedogenesis and differences in the significance of morphological features to various land uses can affect the classification approach. Despite these differences, in a well-constructed system, classification criteria group similar concepts so that interpretations do not vary widely. This is in contrast to a technical system approach to soil classification, where soils are grouped according to their fitness for a specific use and their edaphic characteristics.

Classification means the process of ordering or grouping of objects having large number of population and wide variations into shorter one in a systematic manner with selected characteristics. Soils being the natural objects have no exception with other objects to classify them in a best possible way. Thus, soil classification refers to the systematic categorization of soils into various groups, classes or any simpler form based on the similar properties and other distinguishing characteristics. The main purposes of soil classification are to organize knowledge, easy remembering of the soil properties for each group and better understanding about the components and the relationships among individuals and various groups of soils. Various soil classification systems have been evolved during the past for categorizing the soils into various groups but the Soil Taxonomy has been now accepted as most scientific system of classification.

Soil Colloids A colloid, also known as colloidal solution or colloidal system, is a mixture in which finely divided particles (less than 1µm in size) known as dispersed phase are suspended in a medium, called dispersion medium. The particles must be dispersed in a medium and both the dispersed phase and dispersion medium may be solid, liquid or gaseous which leads to numbers of possible colloidal systems. In soil, the clay fraction (< 2µm in size) together with finer fractions of some organic matter forms the composition in a state of colloidal nature, known as soil colloids. The soil solids as dispersed phase predominates in the dispersion medium (soil solution). As such, colloids in a soil are referred to as ‘sol’. Clay colloids display the most reactive fractions of soil governing their distinct properties such as adsorption and exchange of ions, zeta potential, flocculation and other electrokinetic phenomenon.

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.

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.

Soil colloidal substances are composed of very fine particles (< 0.002 mm diameter) that undergoes two phase system in a heterogenous manner, where, one phase (disperse phase) is disperse in a fine state of subdivision in another medium called dispersion medium. The soil colloidal properties are affected by the physico-chemical and biological parameters of soils. The availability of plant nutrients are governed mostly by the surface charge characteristics of clay colloids.

5.0 Soil Colloids

Soil colloids being one of the active constituents determine the physical and chemical properties of the soil with the diameter ranging from 1 μm to 2 μm. The origin of the particles may be organic and inorganic constituents.

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”.

Soil conservation: it is using and managing land based on the capabilities of land itself, involving the application of the best practices to result in greatest profitable production without damaging the land Principles of soil conservation The aim of soil conservation is to obtain the maximum sustainable level of production from a given area of land whilst maintaining soil loss below a threshold level which theoretically, permits the natural rate of soil formation to keep pace with the rate of soil erosion. In addition, there may be a need to reduce erosion to control the loss of nutrients from agricultural land to prevent pollution of water bodies; to decrease rates of sedimentation in reservoirs, rivers, canals and ditches In the longer term, erosion has to be controlled to prevent land deteriorating in quality until it has to be abandoned and cannot be reclaimed, thereby limiting options for future landuse.

Q. 1 Which type of gully is formed on flat land?
(A) U-shaped  (B) V-shaped
(C) L-shaped   (D) T-shaped