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Although the Rhabditids have been studied from last one century, the field level use of Entomopathogenic nematodes the members of Rhabditids for the management of insect pest was followed only after Dutky who practically used EPNs in 1937 on Apple Codling moth.

The taxonomy of Entomopathogenic nematodes of the order Rhabditidae is discussed in this lecture.

Basically we need to mount and prepare the slide for observation of various stages of EPN as detailed below in Table.1

I. In vivo culturing technique

In vivo culturing means culturing of entomopathogenic nematodes (EPN) in the host. Generally rice meal moth, Corcyra cephalonica and greater wax moth, Galleria mellonella are used as insect hosts for rearing EPNs.Insect hosts such as Spodoptera, Helicoverpa and Bombyx mori are also used for culturing Steinernema and Heterorhabditis but these insects need plant host especially leaves for culturing. Corcyra can be grown in broken cumbu grains and Galleria in honey comb or artificial diet.

Introduction

Entomopathogenic nematodes (EPNs) viz., Heterorhabditis, Neosteinernema and Steinernema have been recovered from many regions of the world and have been used commercially as biocontrol agents of insect pests. They are associated with mutualistic bacteria in the genus Xenorhabdus for Steinernematidae and Photorhabdus for Heterorhabditidae. The infective juvenile nematodes enter their host insect through natural openings (mouth, anus or spiracles), they release bacteria into the insect and cause rapid death of insect host (24-48 h). EPNs were used against many insect pests viz., rice leaf folder, Cnaphalocrocis medinalis (Srinivas and Prasad, 1991); tobacco cutworm, Spodoptera litura (Rajkumar et al., 2003); brinjal fruit borer, Leucinodes orbonalis (Hussaini et al., 2002) diamond back moth, Plutella xylostella (Singh and Shinde, 2002); Diaprepes abbreviates (Jenkins et al., 2007); Bemesia tabaci (QiuBaoli et al., 2008) and sugar beet beetle (Saleh et al. 2009).

Introduction

Agricultural production in India is constrained by number of biotic and abiotic factors. India is a tropical country and has a climate which is conducive for insect pests which are, one of the major constraints in agricultural production, processing and storage. Annual crop losses due to insect pests and diseases in India are estimated to be 18 percent of the agricultural output. Insect pests are widespread in all the agroclimatic zones and large quantities of insecticides are used to control these pests. Indiscriminate use of insecticides, lack of adequate knowledge on proper application techniques and dosages, and frequent use of broad spectrum insecticides are common in India. There is a need to identify suitable alternative methods for the management of insect pests and among them, biological control using viruses, bacteria, fungi and entomopathogenic nematodes is gaining greater attention world over.

Nematode families of Steinernematidae and Heterorhabditidae attracted most attention as they contain the entomopathogenic nematodes (EPN). Entomopathogenic nematodes are obligate parasites of insects that kill their hosts with the aid of bacteria (Xenorhabdus and Photorhabdus) carried in the nematode’s alimentary canal. The lethal obligate insect parasites are ubiquitously distributed without pathogenicity to mammals and beneficial non-target insects, and exempted from registration requirements. Currently there are about 98 valid EPN species belonging to Steinernema (79) and Heterorhabditis (19). Soil surveys conducted in divergent areas of Indian subcontinent have demonstrated high diversity of EPN across seasons, habitats, and geographic regions. There are several target pests in India that can be controlled with entomopathogenic nematodes and several workers have used EPN against cutworms, stem borer, white grubs, etc. under laboratory and field conditions and are compatible with existing IPM programmes.

Entomopathogenic nematodes (EPN) in the family Steinernematidae and Heterorhabditidae have enormous potential to control insect pests of economic importance. Since, chemical pesticides pose environmental hazards, these nematodes have received considerable attention as potential bio-insecticides because of their wide host spectrum, active host seeking, killing the host within 48 h, easy mass production, storage and application and are environmentally safe. One of the important criteria for a successful biocontrol agent is formulation of nematodes into a stable product, which has played a significant role in commercialization of these biological control agents. Apart from formulation field application and desired control of insect pests decides the success of EPN.

Formulation of Entomopathogenic Nematodes

EPNs have been known since 1929 but they became commercially available only during 1990s.The first attempts at formulating EPN were initiated in 1979 but the shelf life was very limited (one month). Infective juveniles carried on moist substrates such as sponge, vermiculite and peat require continuous refrigeration to maintain their viability. While formulating nematodes, two things are necessary, a long shelf-life and easy application method. The simplest method is impregnating a moist substrate (such as sponge, peat, vermiculite, etc.) with nematodes. The sponge needs to be squeezed in water before application, to release the nematodes, whereas nematodes from other carriers could be applied directly to the soil. However, these formulations are labour-intensive, require continuous refrigeration and lack economy of scale and therefore can be used on small scale only.

Although the infective juveniles of entomopathogenic nematodes can be stored for several months in water in refrigerated bubbled tank, high cost and difficulties of maintaining quality preclude the deployment of this method. Therefore, nematodes are usually formulated into solid or semi-liquid substrates soon after they are produced.

The culture of colored and attractive fish in glass aquariums for aesthetic use is called ornamental fish culture. The beautiful, tiny fishes or ornamental fishes are peaceful in nature and suitable for keeping in captivity. These lovely fishes are usually kept in a glass made aquarium and decorated with toys, plants, ceramic structures etc. for beautification. It displays the attractive fish that live in a natural environment decorated with accessories while maintaining environmental parameters in tanks/aquariums by using aerators, heaters, filters, lights to control water movement, temperature, suspended organic matter, illumination etc. besides feeding. Keeping ornamental fish in a glass tank is a very old and popular hobby. More and more people are getting attracted to this hobby and due to growing interest in aquarium keeping; it has resulted in a steady expansion in its trade in more than 125 countries. Household aquariums are more popular; therefore, less than 1% of the global market for ornamental fishes belongs to the public aquaria sector. Most of the ornamental fish is available from developing countries in the tropical and sub-tropical regions. The international trade in ornamental fish breeding and culture provides employment opportunities for thousands of rural people in developing countries. Ornamental fish is becoming an important component in Indian fisheries too, along with food fishes, for both income and employment generation. As a result of advancements in breeding, transport and rearing technology, more and more fish species are being recognized as ornamental fish almost every year. The ornamental fishery is recognized by many developing countries for employment generation and livelihoods. Ecologically suitable culture systems must be developed by evolving micro, small and medium enterprises for sustainable growth.

India is one among the top ten mega-diverse countries of the world in terms of fish diversity (Dudgeon 2003). FishBase (Froese & Pauly 2016) has listed around 917 freshwater fish species (out of 2465 total fish species) as occurring in India. According to National Fisheries Development Board, Hyderabad, inland and marine waters in India possess a rich diversity of ornamental fish, with over 195 indigenous varieties reported from North-East Region and Western Ghats, and nearly 400 fish species from marine ecosystems. There are several other fish species which qualify to be called as ornamental fish. Considering enormous and diverse indigenous fish resources of the country, there is immense scope for India to become a potential candidate and a strong competitor in the international ornamental fish trade. Government of India has recognized ornamental fish sector as one of the thrust areas for generating employment opportunities and augmenting our foreign exchange earnings. It is estimated that the sector is directly and indirectly supporting about 50,000 of house-holds mainly in rural India. Large number of freshwater ornamental fish species is recorded from Indian waters and the majority of the global trade is based on freshwater exotic ornamental fishes including both classified and non-classified types.

Design and construction of farm including management is important with the advancement of cultural practices and increased involment of people. These are some common resources for breeding, rearing and display of ornamental fish.

Small scale commercial farms

Mazla/earthen vats

These are baked earthen containers and usually round in shape. These are used to keep the gravid female of livebearer fish for breeding. Its water holding capacity is 7.5-10.0 litre.

Ornamental Plants are a real attraction to the aquarium tank. While some plants are both ornamental and functional, people usually use the term “ornamental plants” to refer to plants which have no value beyond being attractive, although many people feel that the attractive value itself is quite enough. They provide optimal habitat for the fishes especially for the fry (baby fishes). It maintains the ecosystem of the aquarium tanks by providing good oxygen, food and also acts as a good hiding place for the baby fish. Mostly all aquatic plants are very easy to grow and care. They are always good for gold fish and other similar fishes. There is a huge variety of plants. Ornamental plants are grown for decoration, rather than food or raw materials. They are most often intentionally planted for aesthetic appeal. However, ornamental plants also serve some less obvious uses such as for the purpose of fragrance, for attracting wildlife and for cleaning the air. Ornamentals encompass a wide array of plants. Commonly, ornamental plants are grown for the display of aesthetic features including: flowers, leaves, scent, overall foliage texture, fruit, stem and bark, and aesthetic form. The concept of ornamental plants is used for decorative purposes in aquariums, gardens, home gardens, landscape design projects, squares, parks etc.

Aquatic plants predominantly grow in water. They vary greatly in type, with some being quite similar to common land plants while others are quite different. Aquatic plants can be commonly classified into four types: algae, floating plants, submerged plants and emerged plants. Aquatic plants live in water. This means that they require being submersed to survive or that they can only grow and thrive in water. Many aquatic plants can tolerate being emerged, meaning they’re out of the water for short periods of time, but require immersion in water for long-term survival.

There are some plants that simply can’t tolerate being emerged at all, but this is rare. These plants can only live when fully immersed in water. Most macro- algae require total submersion in water for the span of its entire life. Most aquatic plants, however, have roots underwater but can only grow or flower in a partially emerged state. All plants, aquatic and otherwise, require sunlight, soil, gases and water to survive. Plants make their own food via the process of photosynthesis; this means that they need a source of energy to drive this biochemical process, and the sun provides a perfect one. Plants can carry on for periods without sunlight, just as animals can live on stored fuel for a while in times of need. The soil offers a place for the plant’s roots to take hold.

Air contains large amounts of the carbon dioxide gas (CO2), plants need to power photosynthesis, but aquatic plants have evolved to draw in CO₂ that is dissolved in water in relatively small amounts. Finally, plants need water to combine with the CO2 to complete photosynthesis by generating oxygen and glucose.

Feed plays a vital role not only in growth but it also enhances the market value of ornamental fish. In natural waters, fish has access to a variety of natural food items, but their overall wellbeing depends upon the availability of their favorite food. However, in a confined system, nutritionally balanced supplementary feed is indispensable due to limitation on availability of natural live fish food organisms. In general, ornamental fish are fed with a variety of commercially available pelleted feeds, imported from countries like Singapore, Hong Kong, Korea and Thailand etc. Although they are very popular among ornamental fish hobbyists, the high cost of these feeds is not economical for commercial-scale application at farmer’s level. Further, in contrast to food fish in a pond, feeding ornamental fish in smaller culture units need high precision. Therefore, it is essential for an ornamental fish producer to possess technical know-how regarding nutritional requirements and feeding behaviour of different species in order to formulate farm-made feeds by using locally available low- cost ingredients.

Formulation of low cost and nutritionally balanced supplementary feed, using locally available ingredients, is the key to the success of ornamental fish culture in India. To reduce the cost of feed and make the ornamental fish production venture more profitable, the feed can be prepared at farm by using cost-effective and locally available quality ingredients (plant/animal-based).

Identification and selection of the feed ingredients are essential and it should be according to the fish species to be cultured. A number of feed ingredients may be locally available which can be used for feed formulation. Fish culturists have to identify and choose the best ingredients, depending upon its easy availability, nutrient status and cost, keeping in mind the nutritional requirement of ornamental fish (size, feeding habits, etc.) to be cultured.

Learning about water chemistry is often neglected by most of the aquarium owners. By understanding the concepts of water chemistry, they can considerably improve success rate in rearing healthy fishes. It is well known that the quality of water has a direct impact on the health of the fish. But many aquarium owners do not understand the basic internal chemistry of the water, nor do they understand how to correct it or safely adjust it. Until the basics of water chemistry is understood and some common water maintenance techniques are learnt, it will be difficult to maintain a healthy and safe environment for the fish in tank. The water quality is by far the single most important factor for the healthy fish, and the more is known, the better management is expected.

Fish depends for their basic necessities on the water in which they live. The most characteristic feature of an aquarium system is, therefore, the quality of the water available in tanks/aquarium. This water must be obtained from some source, pre-treated to make it suitable for the fish, delivered to the fish in sufficient quantities and maintained in good condition. The water supplied to an aquarium is not pure, but it contains dissolved and particulate materials, some are necessary for up keep of the fish and rests are harmful. Contamination may occur not only at source or from the animals, but often takes place within the aquarium from the materials used for its construction. The number of fish that can be maintained within the aquarium is said to be according to the quality and quantity of the water supplied to an aquarium. However, it is rarely the quantity of water per unit which limits the carrying capacity. The capacity is usually set by the requirement of dissolved oxygen and the accumulation of toxic metabolic products in the aquarium.

One of the attractions of ornamental fish keeping is to multiply the fishes in captivity. Although techniques used for ornamental fish breeding are not much difficult, the breeding methods for specific ornamental fish species are closely guarded secrets. Farmers have operated almost entirely on their own, developing their own methods and rely on many years of experimentation. In India, the most important breeding centres for ornamental species are located in Kolkata, Mumbai, Chennai and Kochi. Ornamental fishes are broadly classified into two groups based on their breeding methods- Egg layer or oviparous and live bearer or viviparous species. Oviparous fishes lay floating, adhesive or non-adhesive eggs that may be scattered, laid in bubble nests or deposited on substrates or in shallow pits. The breeding and rearing of live bearers are easier than egg layers, which begin with handling the live bearers.

The indigenous ornamental fish which are very popular overseas, are being harvested from the wild and exported. This in the long run, may lead to depletion of stock due to over harvesting of our valued native fish species. Hence, breeding of native fish species is also equally important for a sustainable trade of ornamental fishery and to conserve the natural stock. Some fishes breed naturally with ease, whereas others are bred by artificial techniques like hand stripping and hypophysation. In nature, the fishes are stimulated by changes in environmental factors. These conditions can be simulated to some extent in the aquaria by providing specific biological requirements for spawning.

I. Breeding of Zebra danio (Brachydanio rerio)

Brachydanio rerio is commonly known as zebra danio. The zebra danio, as the name implies, is characterized by a number of bright blue or purple horizontal lines, which run from the head to the caudal fin. This pattern is repeated in the anal and caudal fins too. The flanks have prussian blue colouration and operculum has golden blotches. The dorsal fin of this species has an olive yellow base and there are blue gold markings on the anal fin.

As the ornamental fish industry expands throughout the world, genetic considerations have become more important in terms of production, sustainability and conservation. Genetic technologies are now being applied to increase food production in aquaculture through traditional and modern breeding programmes. Genetic markers can help to manage capture fisheries and distinguish farmed from wild stocks in mixed fisheries. Genetic technologies are also now finding application in post-harvest and trade to assist in product identification and traceability. Genetic improvement of fish has considerably increased the fish production from aquaculture. It includes selective breeding, hybridization, polyploidy, gene and nuclear engineering. For any genetic selection programme, induced breeding is the basic and initial step. Intensive selective breeding is practiced with various species of fish in perfecting appearance, shape and color so that new strains are produced.

Domestication

Most of the fish being widely grown in captivity are domesticated to some extent, but there are very few which have undergone genetic improvement. Just as terrestrial livestock and crops are bred to be not only productive, but productive in convenient proximity to humans, aquatic plants and animals are now undergoing these same pressures.

In comparison to agriculture crops, fruit plants and animals, aquaculture is much behind with regard to the domestication of the fish, especially ornamental fish. Never the less, a good number of ornamental fish have been domesticated which can be considered for selective breeding to improve their traits. The available technologies and tools could be used to remarkably speed up domestication and genetic improvement process.

Domestication implies much more than merely keeping wild animals in farms or homes. Domestication is a very long process, during which captive individuals become more adapted to captive environments. Domestication leads to permanent genetic modifications of a bred lin-eage, while taming or keeping wild fish in captive conditions is only conditioned behavioral modification of individuals. The entire life cycle of the domesticated fish species must be fully closed in captivity. Once full life cycle is completed in captivity, the process of domestication can proceed further.

The first truly domesticated fish species was certainly the goldfish, for which the domestication was initiated by the Chinese about 1500-2000 years ago.

Ornamental fishes are very prone to diseases due to their captive habitat. Fish health management is an important issue of concern at all the ornamental fish production facility. Proper water quality is required to be maintained in the aquarium for the good health of the fish. However, the chance of infection in these fishes is very high. Many limiting factors viz. water quality, over crowding, nutritionally imbalance food, rapid fluctuation in water temperature, lack of oxygen or poor husbandry practices influence the health status of the stock. The symptoms/signs indicate whether the fish is suffering from a particular disease or not. If the fish are observed to be swimming actively and there is no noticeable abnormality in the behaviour, it means there is nothing to worry about the health of the fish in the aquarium. On the other hand, diseased fish may look off colour, restless and/or behave erratically. In such a situation, the fish should be closely observed for visible infection and remedial measures should be taken immediately. Stock improvement in terms of disease prevention must be a top priority issue. In order to achieve healthy fish species, it is necessary to have a health management protocols in every farming practices.

Ornamental fish breeding and trade provide excellent opportunities for employment and income generation. One of the most critical determinants of the success of the trade is the problem of delivering a quality product to the market. The know-how of collection, critical handling, packaging methods are essential for serving the successful transport of live tropical fish to their final destination. However, a major impediment in expanding the trade of indigenous fishes is the poor roads/communication from most of the collection/landing centres to the town, non-availability of oxygen packaging at remote collection centres, difficulties in transportation of the collected fishes to the agent/stockist. Transportation of live fish and shellfish from areas of collection to destination or from areas of farming to the destination is an important activity of ornamental fish industry.

Proper collection, packing and transportation may help in increasing trade to a greater extent. Ornamental fishes destined for marketing, both domestic and export, have to use modern post-harvest technology or modifications of the current post-harvest technology to improve the post-harvest and post-shipment survival that is critical to the industry. It is in this context that the significance of packing and transport come into the picture. To overcome the problem of high mortality of fish species at different stages during transportation, it becomes essential to evaluate the most suitable and ideal condition for transportation. At present, the mortality rate during fish catching, collection, transportation is very high. The claim by the importers due to DOA (Death on Arrival) from India is on the higher side compared to the consignments from other developing countries. This is due to the lack of appropriate technology for fish packing & transportation/ shipment. Similarly, since the airfreight charges are very high, the exporters have no idea of the optimum number of a particular live fish to be packed in a container to reduce transportation cost. Such information is lacking for Indian fishes. The success of ornamental fish transportation depends on the following steps:

The purpose of managing a fishery is to maintain the resource so that it is renewable and therefore sustainable. Responsible aquaculture practices rely on sustainable production systems, to minimize impacts on the natural environment, and support resource conservation. In other words, the harvest of fish from the wild or their domestic culture, if performed with sound foundations of ecological and economic principles, can be sustainable and self-reliant commercial industry. In traditional subsistence fisheries, fishermen use primitive and inefficient gear to capture most ornamental fish. However, the supply of aquarium fish is not inexhaustible, and signs of over-fishing are becoming apparent in localized areas. With high demand and pricing of many beautiful species, ornamental fish are being harvested at greater volumes and higher rates, threatening the viability or sustainability of the fishery. Most endemic and sensitive species are restricted to very narrow specific habitats. Their survival, affected by physical over-exploitation for the aquarium trade, may be further hit by habitat alteration. No comprehensive studies have been carried out on the requirements of these endemic or sensitive exported species. In the absence of suitable impact studies, it is not possible to predict the impact of habitat alteration on these species. Exporters target more colorful varieties and since their ecological significance has not been studied, what long-term effect such selective exploitation will have on genetic diversity cannot yet be defined.

Apart from legislation that can be effectively implemented, eco-physiological and population studies of a quantitative nature are urgently needed to advise on the collection, maintenance and transport conditions that need to be followed by exporters to safeguard collected stocks from unnecessary mortality. Exporters are eager to learn and would be receptive to receiving appropriate, scientifically formulated, well-meaning practical advice. Studies should be targeted towards this end since it seems unlikely that the export trade can at present be voluntarily modulated on the basis of conservation requirements. Such a strategy could only become feasible after an adequately robust ecological database has been compiled, which would necessarily require time.

An effective management strategy needs to address not only aquarium-trade related matters but also policy and other matters in an integrated approach if we are to be hopeful of sustaining the ornamental fish industry in the long-term.

Ornamental fish culture has made giant strides in many countries in recent years. The industry is growing in leaps and bounds with involvement of even increasing number of aquarium hobbyists. A lucrative export market and high domestic demand have made ornamental fish industry a potential source for income generation. The range of species of ornamental fish available to the hobbyists is very large, with estimates of up to 1,500 species. As popularity of ornamental fish keeping has gained momentum, the need to transfer ornamental fishes from the resource abundant places to resource deficient places resulted in the marketing of ornamental fishes. In order to assess the marketing opportunities for ornamental fish, it can be classified as:

1. Choose the most appropriate option with a tick mark

i) Floating plants are required for breeding

a) Guppies

b) Angel fish

c) Swordtail

d) Gourami

ii) Breeding traps are generally used for

a) Egg-scatters

b) Egg-depositors

c) Livebearers

d) Nest-builders

“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 19^th 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.