eChapters

For many soil borne diseases, effective and economic control methods are not available. Several soil borne pathogens have developed resistance to many fungicides. The beneficial microbes are also affected by fungicides. Many fungicides pose health hazards by polluting food and water and cause direct toxicity of heavy metals and carcinogenic risk. In the context of growing worldwide awareness of environmental pollution and development of fungicide resistant strains of pathogens, biological method of suppression of disease causing organisms have been recognized as the safest approach for plant disease management.

Biological control of plant diseases is the suppression of populations of plant pathogens by living organisms. Amongst beneficial microorganisms, isolates can be selected which are highly effective against pathogens and can be multiplied on artificial media. Application of such selected and mass produced antagonists in high densities once or several times during a growing season is called “augmentative biological control”.

Mycorrhizal symbiosis is the most prevalent and important mutualistic association in plant kingdom. ‘Mycorrhiza’- the word means fungus - root, this association was reported by A.B.Frank in the year 1885. Nearly 95% of plant species form this mycorrhizal association. This serves as critical linkage between plant roots and soil and characterized by the movement of plant produced carbon to fungus and fungal acquired nutrients to plants. Through its hyphae this fungus absorbs the available P from soil and transports directly to root cortex. Mycorrhizal associations vary widely in structure and function. There are seven types of mycorrhizae (Table 1). These are endo (arbuscular), ecto, ectendo, arbutoid, monotropoid, ericoid, and orchidaceous mycorrhizae. Among them, endomycorrhizae and ectomycorrhizae are the most abundant and widespread

Introduction

Biofertilizers are microbial preparations containing living or latent cells of efficient strains of nitrogen fixing, phosphate solubilizing and cellulose decomposing microorganisms intended for seed or soil application and designed to improve soil fertility and plant growth by increasing the number and biological activity of beneficial microorganisms in the soil.

The objects behind the application of biofertilizers /microbial inoculants to seed, soil is to increase the number and biological / metabolic activity of useful microorganisms that accelerate certain microbial processes to augment the extent of availability of nutrients in the available forms which can be easily assimilated by plants. The need for the use of biofertilizers has arisen primarily due to two reasons i.e. though chemical fertilizers increase soil fertility, crop productivity and production, but increased / intensive use of chemical fertilizers has caused serious concern of soil texture, soil fertility and other environmental problems.Use of biofertilizers is both economical as well as environment friendly. Therefore, an integrated approach of applying both chemical fertilizers and biofertilizers is the best way of integrated nutrient supply in agriculture.

Biofertilizers, thus include i) Symbiotic nitrogen fixers- Rhizobium sp. ii) Non-symbiotic, free living nitrogen fixers- Azotobacter, Azospirillum etc. iii) Blue Green Algae (BGA) inoculants- Azolla-Anabaena, iv) Phosphate solubilizing microorganisms (PSM)- Bacillus, Pseudomonas, Penicillium, Aspergillus etc. v) Mycorrhiza vi) Cellulolytic microorganisms

Silage is preserved pasture. Fresh forage crops such as maize, grasses, legumes, wheat and lucerne can be preserved by ensiling. Silage is the green material produced by controlled fermentation of the green fodder crop retaining the high moisture content, which contain 20-40% dry matter.

Silage can be prepared from plants having thick stems and are generally not very suitable for hay making like sorghum, maize. Weeds can also be utilized along with main fodder crops for silage making. Silage making kills majority of weed seeds. It is highly palatable. The organic acids produced in the silage are similar to those normally produced in the digestive tract of the ruminants and therefore are used in the same manner.

Industrial microbiology is an important area of applied microbiology. In a broader sense, industrial microbiology is concerned with all aspects of business that relate to microbiology. It refers to the use of microorganisms in commercial enterprise. Cheap raw materials are converted to valuable products through the metabolism of microbes. Microbes for this purpose could be exploited in different ways.

Various commercial products of economic value made by microbes are :

Soil is the largest terrestrial ecosystem where a wide variety of relationships exists between different types of soil organisms. The associations existing between different soil microorganisms, whether of a symbiotic or antagonistic nature, influence the activities of microorganisms in the soil. Microflora composition of any habitat is governed by the biological equilibrium created by the associations and interactions of all individuals found in the community. In soil, many microorganisms live in close proximity and interact among them-selves in a different ways. Some of the interactions or associations are mutually beneficial, or mutually detrimental or neutral.

Plant Growth Promoting Rhizobacteria (PGPR) are root-colonizing bacteria that form symbiotic relationships with many plants. The name comes from the Greek rhiza, meaning root. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties (mutualism). They are an important group of microorganisms used in biofertilizer. Biofertilization accounts for about 65% of the nitrogen supply to crops worldwide. The term PGPRs was first used by W. Kloepper in the late 1970s and has become commonly used in scientific literatures. PGPRs have different relationships with different species of host plants. The two major classes of relationships are rhizospheric and endophytic. Rhizospheric relationships consist of the PGPRs that colonize the surface of the root, or superficial intercellular spaces of the host plant, often forming root nodules. Endophytic relationships involve the PGPRs residing and growing within the host plant in the apoplastic space.Some common examples of PGPR genera exhibiting plant growth promoting activity are: Pseudomonas, Azospirillum, Azotobacter, Bacillus, Burkholdaria, Enterobacter, Rhizobium, Erwinia, Mycobacterium, Mesorhizobium, Flavobacterium, etc.An ideal PGPR should possess high rhizosphere competence, enhance plant growth capabilities, have a broad spectrum of action, be safe for the environment, be compatible with other rhizobacteria, and be tolerant to heat, UV radiation, and oxidizing agent.

Microbes that live inside plant tissues without causing disease are termed as endophytes. Endophytes have been found in all parts of plant viz., seeds, roots, stems, leaves, fruits etc. Within these organs, there is evidence of endophytes living in apoplastic intercellular space within parenchyma tissues and xylem vessel apoplast. Endophytic bacteria have been isolated from both monocotyledonous and dicotyledonous plants, ranging from woody tree species to herbaceous crop plants. Further, a single plant may be a host for several different bacterial species. They are consistently present in the root, stem, leaf, fruit and tuber tissues of a wide range of agricultural, horticultural and forest species. All plants in nature harbor a diverse community of endophytic bacteria which can positively affect host plant growth.

The potential of endophytic bacteria to fix nitrogen, promote plant growth and suppress plant diseases has renewed interest in such associations. It is advantageous to have the endophytic plant growth promoting organisms safely inside the plant, which could supply substantial amounts of nitrogen and other nutrients, growth hormones and other beneficial effects to the crop plants without the interference of the rhizosphere soil environment. Furthermore, plant growth promotion is often greater when it is induced by endophytes rather than by rhizosphere bacteria.

The spermosphere represents a short-lived, rapidly changing, and microbiologically dynamic zone of soil surrounding a germinating seed. It is analogous to the rhizosphere, being established largely by the carbon compounds released into the soil once the seed begins to hydrate. These seed exudations drive the microbial activities that take place in the spermosphere, many of which can have long-lasting impacts on plant growth and development as well as on plant health.

Rhizosphere

The term Rhizosphere was introduced by the German scientist, Hiltner in 1904. It is region of the soil, which is subjected to the influence of plant roots. It is the unique environment under the influence of plant roots. The roots of plants are not only involved in the uptake of mineral nutrients and water for plant growth, but they also release a wide range of organic compounds in the surrounding soil.

Pesticides are the chemical substances that kill pests and herbicides are the chemicals that kill weeds. In the context of soil, pests are fungi, bacteria, insects, worms, and nematodes etc. that cause damage to field crops. Thus, in broad sense pesticides are insecticides, fungicides, bactericides, herbicides and nematicides that are used to control or inhibit plant diseases and insect pests. Although wide-scale application of pesticides and herbicides is an essential part of augmenting crop yields; excessive use of these chemicals leads to the microbial imbalance, environmental pollution and health hazards. An ideal pesticide should have the ability to destroy target pest quickly and should be able to degrade nontoxic substances as quickly as possible.

The ultimate “sink” of the pesticides applied in agriculture is soil. Soil being the storehouse of multitudes of microbes, in quantity and quality, receives the chemicals in various forms and acts as a scavenger of harmful substances. The efficiency and the competence to handle the chemicals vary with the soil and its physical, chemical and biological characteristics.

Bioconversion refers to the recycling and reprocessing of organic waste materials using the system of biological decomposition or biodegradation into value added products. Composting is one way of conversion of organic waste into manures. It is defined as a method of solid waste management whereby the organic component of the solid waste stream is biologically decomposed under controlled conditions to a state in which it can be handled, stored or applied to the land without adversely affecting the environment.

Composting provides a partial solution to an issue of great concern in many communities. All around the world, landfills are filling up, garbage incineration is becoming increasingly unpopular, and other waste disposal options are becoming ever harder to find. Composting provides a way not only of reducing the amount of waste that needs to be disposed of, but also of converting it into a product that is useful for agriculture. Composting reduces waste, makes us less dependent on landfills, and decreases greenhouse gas emissions. Composting recycles essential nutrients back into the soil and promotes a prolific soil ecosystem.

Zinc is found in earth’s crust to the tune of 0.008%. However about 50% of Indian soils exhibit deficiency of zinc with a content much below the critical level of 1.5 ppm. Soil contains both available and unavailable form of zinc. Water soluble, exchangeable, organically complexed and armorphous iron oxide bound forms are considered available zinc and crystalline iron oxide bound and residual zinc are considered as unavailable. The available fraction constitutes around 8% of the total zinc which is too low for utilization by crops in zinc deficient soils.

Zinc is required by crops in microquantities for its growth. The use of high analysis fertilizer due to cultivation of high yielding varieties, enhanced cropping intensity and the decline in the use of organic manures result in the deficiency of trace elements particularly zinc. With the liberal application of nitrogen the demand for zinc increases producing deficiency symptoms. The critical limit of zinc in dry matter for different crops and levels less than 20 ppm suggests the probability of zinc deficiency and less than 15 ppm indicate sure deficiency.

Zinc is a limiting factor in crop production and application of ZnSO₄ is preferred because of its water soluble nature. The zinc sulphate, when applied soil, gets transformed into different forms like Zn (OH), zinc carbonate in calcium rich alkali soils ; zinc phosphate in near neutral to alkali soils of high phosphorus application; and zinc sulphide under reduced conditions.

Potassium (K) is among the most important essential macronutrients for plant growth. About 98% of the potassium in the earth’s crust exists in insoluble forms as rocks and silicate minerals, resulting in very low concentrations of soluble potassium in the soil for plant growth and development. Potassium is never incorporated in any organic compound and even in the plant system it remains as K⁺ ion. It does not form any gas that could be lost to the atmosphere. It does not cause eutrophication. When left in the ecosystem as such, it is not at all toxic.Potassium activates enzymes, maintains cell turgor, enhances photosynthesis, reduces respiration, helps in transport of sugars and starches, and is essential for protein synthesis. In addition to plant metabolism, potassium improves crop quality because it helps in grain filling and kernel weight, strengthens straw, increases disease resistance and helps the plant better to withstand stress.

Potassium in Soils

Soils usually contain potassium in larger amounts than any other nutrients. Total K content in soil is 30,000- 50,000 kg/ha in top layer of the soil. Nearly 98 % of K bound in the mineral form and 2 % is in soil solution. Generally, potassium in soils exist in four forms (often called fractions), their availability to crop roots is considerably different.

Microbial transformations of sulfur (S) are even more complex than those of nitrogen because of the large number of oxidation states of S and the fact that several transformations of S also occur abiotically. Although a number of oxidation states of S are possible, only three are significant in nature, 2 (sulfhydryl, R–SH, and sulfide, HS-), 0 (elemental sulfur, S0), and 6 (sulfate, SO₄ 2-). The bulk of S Earth’s S is in sediments and rocks in the form of sulfate minerals, primarily gypsum (CaSO₄) and sulfide minerals (pyrite, FeS2), but the oceans constitute the most significant reservoir of SO₄ ^2- in the biosphere. A significant amount of S, in particular sulfur dioxide (SO₂), enters the S cycle from human activities, primarily the burning of fossil fuels.

Hydrogen Sulfide and Sulfate Reduction

A major volatile S gas is hydrogen sulfide (H₂S). Hydrogen sulfide is produced from bacterial sulfate reduction or is emitted from sulfide springs and volcanoes. Although H₂S is volatile, different forms exist depending on pH: H₂S predominates below pH 7 and the nonvolatile HS- and S^2- predominate above pH 7. Collectively,H₂S, HS, and S₂ are referred to as “sulfide.” Sulfate-reducing bacteria are a large and highly diverse group and are widespread in nature. However, in anoxic habitats such as freshwater sediments and many soils, sulfate reduction is SO₄ ^2- limited. Moreover, because organic electron donors (or H₂, which is a product of the fermentation of organic compounds) are needed to support sulfate reduction, it only occurs where significant amounts of organic material are present.

Phosphorus is only second to nitrogen as an inorganic nutrient required by both plants and microorganisms. It stimulates growth of young plants, giving them a good and vigorous start. Phosphorus management and nutrition has both economic and environmental implications. Phosphate constitutes nearly 0.1% of the earth’s crust. They occur in soil in inorganic and organic forms. The organic phosphorus containing compounds are derived from plants and microorganisms and are composed of nucleic acids, phospholipids, lecithin, phytin and related compounds. In soil, 15-85 % of the total P is organic and soils rich in organic matter contain abundant organic P. The inorganic forms are derived from parent rocks or through fertilizers application and manuring with bone meal. They are soluble in water when present as phosphates of Na, K, Ca, Mg etc. Phosphorus exists mainly as apatites, with the basic formula M₁₀ (PO₄) ₆ X₂. Commonly the mineral (M) is Ca, less often Al or Fe. The anion (X) is either F^-, Cl^-, OH^- or CO^2- ₃. Diverse combinations of M and X results in 200 forms of P.

It is a major constituent of nucleic acids in all living systems essential in the accumulation and release of energy during cellular metabolism. This element is added to the soil in the form of chemical fertilizers, or in the form of organic phosphates present in plant and animal residues. In cultivated soils it is present in abundance (i.e. 1100 kg/ha), but most of which is not available to plants, only 15 % of total soil phosphorus is in available form. Both inorganic and organic phosphates exist in soil and occupy a critical position both in plant growth and in the biology of soil.

Fixation of elemental nitrogen in the atmosphere by the microorganisms through a reductive process into ammonia is called biological nitrogen fixation (BNF). Molecular nitrogen (N₂) is the major component (approximately 80%) of the earth’s atmosphere. The element nitrogen is an essential part of many of the chemical compounds, such as proteins and nucleic acids, which are the basis of all life forms. However, N₂ cannot be used directly by biological systems to build the chemicals required for growth and reproduction. Before its incorporation into a living system, N₂ must first be combined with the element hydrogen. This process of reduction of N₂ commonly referred to as nitrogen fixation (N-fixation).

N₂is transformed into reactive N via lightning discharge (5 teragrams (Tg) N / year; 1 Tg equals 1012 g) and by biological N fixation (100 -140 Tg N /year ). Conversion of dinitrogen to ammonia in the industrial process requires temperatures of 400 - 500^o C and high pressures (Haber-Bosch process). Biological nitrogen fixation is much more important as it contributes to more than 70 % of the input in to the world’s soil and water nitrogen. It has been estimated that 100 to 175 million tones of nitrogen are fixed annually by the process of biological nitrogen fixation.

The process of N₂ fixation is mediated by the enzyme, called nitrogenase (which reduces N₂ to ammonia). The overall reaction is as follows.

Soil microorganisms are the most important agents in the cycling or transformation of various elements (N, P, K, S, Fe etc.) in the biosphere; where the essential elements undergo cyclic alterations between the inorganic state as free elements in nature and the combined state in living organisms. Life on earth is dependent on the cycling of nutrient elements from their elemental states to inorganic compounds to organic compounds and back into their elemental states.

The microbes through the process of biochemical reactions convert / breakdown complex organic compounds into simple inorganic compounds and finally into their constituent elements. This process is known as “Mineralization”.

Mineralization of organic carbon, nitrogen, phosphorus, sulphur, and iron by soil microorganisms makes these elements available for reuse by plants. In the following paragraphs the cycling or transformations of some of the important elements are discussed.

Soil Organic Matter

Soil organic matter plays an important role in the maintenance and improvement of soil properties. It is a dynamic material and is one of the major sources of nutrient elements for plants. Soil organic matter is derived to a large extent from residues and remains of the plants together with the small quantities of animal remains excreta, and microbial tissues. Soil organic matter is composed of three major components i.e. plant residues, animal remains and dead remains of microorganisms. Soil organic matter comprises residues of plants and animals and these compounds occur in soil in close combination with inorganic substances. Animals and plant residues are made up of complex carbohydrates, simple sugars, starch, cellulose, hemicellulose, pectins, gums, mucilage, proteins fats, oils, waxes, resins, alcohols, aldehydes, ketones, organic acids, lignin, phenols, tannins, hydrocarbons, alkaloids, pigments, etc.

The organic matter subjected to microbial decay in the soil comes from several sources like plant remains, forest litter, incorporation of plant tissues, animal tissues, and excretory products. The chemistry of organic matter is very complex, and investigations of the transformations and the responsible organisms have therefore been extremely interesting. The organic constituents of the plants are commonly divided into six categories.

All organisms require the six elements CHNOPS in sizable quantities, and that these minerals may occur in different forms. Depending on whether the local environment is aerobic or anaerobic, the exact chemical form of any of these elements may change quickly and drastically. Example: : If the soil becomes waterlogged and anaerobic, sulfate will be converted into hydrogen sulfide and nitrate into nitrogen gas or ammonia (via anaerobic respiration). As the water recedes and the soil becomes aerobic again, the ammonia is oxidized back to nitrate in stages. All of this (and many other processes) occur largely because of the activities of soil microbes. Without these microbes, life as we know it would not occur because necessary elements would remain tied up in unusable chemical forms and gradually be removed by sedimentation. To maintain the level of these components it is essential that they undergo a regular process of recycling. Different microorganisms bring about this process of recycling through various transformations.

Carbon Cycle

The most important element in the biological realm and substance that serve as the cornerstone of the cell structure is carbon. It constituents about 40-50% of all living organisms, yet the ultimate source is the CO₂ that exists in a perennially short supply, only 0.03% of the earth’s atmosphere, which undergoes a cyclic change from an oxidized to reduced state. The concentration of carbon in living matter (18%) is almost 100 times greater than its concentration in the earth (0.19%). Hence, living things extract carbon from their non-living environment. For life to continue, this carbon must be recycled. Carbon exists in the nonliving environment as carbon dioxide (CO₂) in the atmosphere and dissolved in water (forming HCO₃") carbonate rocks (limestone and coral = CaCO₃) deposits of coal, petroleum, and natural gas derived from once-living things dead organic matter, e.g., humus in the soil. Carbon enters the biotic world through the action of autotrophs: primarily photoautotrophs, like plants and algae, that use the energy of light to convert carbon dioxide to organic matter through photosynthesis and to a small extent, chemoautotrophs - bacteria and Archaeans that do the same but use the energy derived from the oxidation of molecules in their substrate.

Soil fertility depends on the physical, chemical, and biological properties of the soil. Among the biological properties, soil enzymes are used as an index of soil fertility. Soil enzymes respond to changes in soil management faster in comparison to other soil variables, and also catalyze the reactions necessary for the life processes of microorganisms in soils. The biochemical reactions are brought about by the catalytic action of enzymes. Soil enzymes include amylase, phosphatase, urease, cellulase, protease, glycosidase, and arylsulfatase. They play a role in organic material decomposition, the transformation of organic matter, nutrient cycling, nitrogen fixation, detoxification of pesticides, xenobiotics, etc., and thus regulate the ecosystem.

Depending on their location, enzymes can be extracellular or intracellular. Intracellular enzymes are found in the cell’s cytoplasm or bound to the cell walls of living and metabolically active cells. Extracellular enzymes released into the soil and are “permanently” immobilized on clay and humic colloids via ionic interactions, covalent bonds, hydrogen bonding, entrapment, and other mechanisms. There are two kinds of soil enzymes.

Soil microbial biomass is an important parameter linking the plants to soil. It comprises about 2-3% of the total organic carbon in the soil and recognized as an important source of nutrients to plants. It is also a dynamic component of the soil associated with several functional properties of the terrestrial ecosystem. It represents the fraction of the soil responsible for the energy and nutrient cycling and the regulation of organic matter transformation.

Microbial biomass is a small but labile and living component of soil organic matter that is involved in most bio-geo-chemical processes in the terrestrial ecosystem. It interacts with crop productivity by regulating nutrient availability as a source; determining soil carbon storage- sink; and contributing to the atmospheric CO₂ from respiration. Thus it is the main agent that controls the flow of carbon and cycling of nutrient elements.

There is a close relationship between soil microbial biomass, decomposition rate, and nitrogen mineralization. It is also positively correlated with grain yield in organic farming and also contributes to soil structure and stabilization. Thus, there is the immense importance of microbial biomass in soil fertility as well as in agriculture.

Soil microorganisms (Flora & Fauna) are just like higher plants depends entirely on soil for their nutrition, growth and activity. The major soil factors which influence the microbial population, distribution and their activity in the soil

Several environmental conditions affect the density and composition of the microflora and frequently alter their activities in soil. Primary factors include soil moisture, aeration, temperature, pH, organic matter and inorganic fertilizers. Lesser variables are the secondary factors, which include crop rotation, season, soil depth, cultivation practices etc.

I. Primary Factors

Soil moisture

Soil moisture is one of the important factors influencing the microbial population. Water is the major component of protoplasm; an adequate supply must be available for vegetative development of organisms. But, when it becomes excessive, proliferation is suppressed due to limitation in gaseous exchange and lowers the availability of O₂ supply creating an anaerobic environment. Most of the organisms prefer a moisture percentage between 20 and 60 per cent. Many bacteria and fungi can adjust themselves to different moisture conditions. Under dry conditions, the bacteria may form spores that can resist the drought conditions. The fungi may sporulate or form chlamydospores to tide over adverse conditions. The protozoans also may form cysts and can survive under dry conditions. Actinobacteria are the chief group of organisms that prefer dry conditions. It is believed that the concentration of nutrients is diluted and also the aeration is very much limited at moisture and hence only the anaerobic and microaerophilic organisms can develop better.

Soil is a bioorganic - mineral complex of the loose surface layer of the earth’s crust. Soil microbiology is the science that deals with the millions of microscopic forms of life found in the soil. It is stated that one hectare - foot (one hectare to a depth of 30cm) contains 2 to 10 metric tons of living microorganisms. Soil microorganisms are in a constant struggle for existence and are believed to be in a state of dynamic equilibrium. Without microorganisms, the soil would be an inactive geological mass incapable of supporting plant life and without plant life, animal life would not be possible. This soil population consists mainly of bacteria, fungi, actinobacteria, algae, and protozoa. The soil ecosystem includes these microbial groups as well as the inorganic and organic constituents of a given site. The collections of cells represented in the community are considered as distinct populations. All the inhabitants of the particular locality make up the community.

Types of Microorganisms in Soil

Living organisms both plants and animals, constitute an important component of soil. The pioneering investigations of several early microbiologists showed for the first time that the soil was not an inert static material but a medium pulsating with life. The soil is now believed to be a dynamic or rather a living system, containing a dynamic population of organisms/microorganisms. Cultivated soil has relatively more population of microorganisms than the fallow land, and the soils rich in organic matter contain much more population than sandy and eroded soils. Microbes in the soil are important to us in maintaining soil fertility and productivity, cycling of nutrient elements in the biosphere, and sources of industrial products such as enzymes, antibiotics, vitamins, hormones, organic acids, etc. At the same time certain soil microbes are the causal agents of human and plant diseases.

History of Soil Microbiology

There is enough evidence in the literature to believe that microorganisms were the earliest of the living things that existed on this planet. Man depends on crop plants for his existence and crop plants in turn depend on soil and soil microorganisms for their nutrition. Scientists from the beginning studied the microorganisms from water, air, soil, etc. and recognized the role of microorganisms in natural processes and realized the importance of soil microorganisms in the growth and development of plants.Thus, microorganisms have been playing a significant role long before they were discovered by man. Today, the soil is considered to be the main source of scavenging the organic wastes through microbial action and is also a rich storehouse for industrial microflora of great economic importance.

Unlike soil science whose origin can be traced back to Roman and Aryan times, soil microbiology emerges as a distinct branch of soil science during the first half of the 19^th century. Some of the notable contributions made by several scientists in the field of soil microbiology are highlighted in the following paragraphs.

Anton Von Leeuwenhoek (1673) discovered and described microorganisms through his own made first simple microscope with a magnification of 200 to 300 times. He observed minute, moving objects which he called “animalcules” (small animals) which are now known as protozoa, fungi, and bacteria. He for the first time made the authentic drawings of microorganisms (protozoa, bacteria, fungi).

Soil represents a medium or substrate in which numerous microorganisms live and bring about a great variety of processes that are responsible for the continuation of the cycle of life in nature. The numerous living forms which spend their life in the soil, ranging from submicroscopic forms to the lower animal forms. With the growing recognition of the numerous processes carried out by the microorganisms in the soil there gradually emerged a branch of microbiology, which came to known as soil microbiology. It is a branch of soil science concerned with soil-inhabiting microorganisms and their functions and activities.

Since soil microbiology concerns soil microorganisms and their processes it is closely associated with soil biochemistry. Medical bacteriologists are interested in the soil as a medium for the growth and survival of disease-producing organisms. Agricultural chemists are also interested in the soil processes that result from the activities of microorganisms. General bacteriologists, zoologists, botanists are interested in a certain special group of organisms found in the soil. Recently, soil microbiology has expanded to include the study of the role of soil microorganisms in genetic engineering, in the biological control of pests and diseases, the degradation of pollutants, production and destruction of radioactive gases and its transfer. Thus microbial participation in several important processes emphasizes that soil microbiology has become a global science.

Salt affected soils occupy 962.2 m ha globally which accounts for 33% of the arable land. In India 8.373 m ha is affected with salts. Of which 2.359 m ha are alkali soils, 3.829 m ha are saline soils and 2.185 m ha coastal saline soils. Reclamation of these soils provides additional area for food grains production and employment.

SOIL MANAGEMENT PRACTICES

Good soil management is the basic for deriving greater benefits from the fertilizers used in crop production. Some of the good soil management practices are

Good tilth is the first feature of good soil management. It means a suitable physical condition of the soil and implies in addition a satisfactory regulation of soil moisture and air.

The maintenance of soil organic matter which encourages granulation is an important consideration of good tilth.

Tillage operations and timings should be adjusted as to cause the minimum destruction of soil aggregates. Good tilth minimizes erosion hazards.

The choice and sequence of adaptable crops or crop rotation are other very important considerations. These are related to climate, particularly rainfall and its pattern of distribution and the characteristics of the soil profile, including drainage and extent and duration of available soil moisture. A proper sequence of crop varieties greatly influences soil conditions. It is more realistic to evolve cropping patterns and land management practices according to land capability. Cropping patterns chosen and management practices adopted should aim at soil and moisture conservation for efficient nutrient and moisture utilization.

Integrated plant nutrition system conceptualized by food and agriculture organization is the maintenance or adjustment of soil fertility and of plant nutrient supply to an optimum level for sustaining the desired productivity through optimization of benefits from all possible sources of plant nutrients viz. organic manures, fertilizers, crop residues, compost or nitrogen fixing crops etc. in an integrated manner.

Biofertilizers act as complementary and supplementary sources of plant nutrients. Biofertilizers help in increasing the biologically fixed atmospheric nitrogen and enhancing phosphorus availability to the crops. Among the biofertilizers for increasing nitrogen supply, nitrogen fixing bacteria (Rhizobium, Azotobacter and Azospirillum), blue green algae (BGA) and Azolla are important. The availability of phosphorus is improved by phosphorus solublizing microorganisms (PSM) and mycorrhizae (VAM).

Rhizobium is of vital importance to various pulses and some of the oilseeds and fodder legumes as 80–90 per cent of their nitrogen requirement is met through biological nitrogen fixation (BNF). Rhizobium fixes atmospheric nitrogen ranging from 50–60 kg per ha in groundnut, 100–300 kg per ha in alfalfa. In pulses it is estimated at 50–110 kg/per ha. Field studies indicated a yield increase of 14–53 per cent due to Rhizobium inoculation. Nitrogen left over the demand of legumes in combination with the nitrogen present in roots of the legumes will enhance the soil fertility. Consequently the nutrient needs of the subsequent crop from applications will be reduced.

Green manure is defined as fresh organic matter/undecomposed plant material/ real organic matter, natural plant food added to the soil for the purpose of supplying plant nutrients.

CLASSIFICATION OF GREEN MANURES

Nonlegumes: Rape seed, buck wheat, niger, rye, oats and maize

Legumes:

Grain legumes: cowpea, mungbean, pigeon pea, soybean, khesari (Lathyrus sativus)

Dual purpose legumes: cluster bean (Cyamopsis tetragonoloba). pillipesara, Sesbania speciosa.

Non grain legumes: sunnhemp, dhaincha, pillipesera (Phaseolus trilobus), stylosanthes and desmodium.

Farmyard Manure is the manure prepared by using the mixture of dung and urine soaked litter by microorganisms.

Good farmyard manure is one

ZINC (Zn)

Chandler et al., (1932) deduced the requirement of zinc by plants while working solution for little leaf or “rosette” on peach. Zinc plays significant role in assimilation of NO₃ and for synthesis of tryptophan. Involved in biosynthesis of indole acetic acid which is responsible for f lowering and fruiting, it is an activator of several enzymes (triphosphate dehydrogenase and tryptophan synthetase), involved in photosynthesis and nitrogen metabolism.

Deficiency results in poor growth of the terminal bud. Decrease in stem length resulting in resetting and whorling of leaves. Zinc deficiency disorder in rice is called khaira. Common when zinc availability is < 0.7 ppm. White bud in maize and malformation of fruits, reduction in water uptake. Interveinal chlorosis often with necrosis and pigmentation and reduced leaf size in broad leaved crops known as little leaf and malformation of leaves.

The crops which are known to respond for zinc application are maize, rice, soyabean, castor, onion, barley, wheat, potato, sorghum and cotton

CALCIUM

Calcium is important in imparting structure and permeability to cell membrane. Deficiency produces general breakdown of membrane structure. Calcium enhances the uptake of NO₃ ^– N and interrelated with NO₃ ^– metabolism. It regulates cation uptake, required for cell elongation and cell division. Deficiency causes failure of terminal buds and apical tips of roots to develop. Blossom end rot of tomato, bitter pit of apples are due to calcium deficiency. Calcium is beneficial in imparting good soil structure.

It is immobile in plant. Very little translocation of calcium in the phloem is observed. Hence there is poor supply of calcium to fruits and storage organs. Downward translocation of calcium is also limited. Hence roots fail to enter the low calcium soils.

Glauber J. R. (1604–1670) in Netherlands first proposed that the salt peter (KNO₃) was the principle of vegetation. He obtained large increases in crop yield by addition of salt peter.

Mineral soils contain 0.04–3.0% K. Total potassium content of soils range from 3000 to 100000 kg per ha in the upper 0.2 m of soil profile. Of this total potassium approximately 98 percent is bound in mineral form whereas 2% in soil solution and exchangeable phases.

ROLE OF POTASSIUM

It is Involved in activating the enzymes (as many as 80) responsible for carbon dioxide reduction and required for synthesis of adenosine triphosphate. Hence deficiency of potassium reduces crop growth and yields.

Liebig (1840) on the basis of soil and plant analysis deduced the phosphorus requirement of plants. J.B. Lawes conducted series of field experiments to improve the fertilizer value of common sources of phosphorus such as bone meal, apatite, rock phosphate. Lawes (1843) produced super phosphate by treating apatite mineral containing phosphorus with sulphuric acid. He also stated that the phosphorus present in bones of higher animals can be made available by treating with sulphuric acid.

Phosphorus is the second most deficient nutrient in the world. Annual application of phosphorus fertilizers to crops is 36 million tons of which

1.32 m tones are consumed in India. Most of the arable soils in tropical and subtropical regions are deficient in phoshphorus, but the response of crops to phosphorus is uncertain. Phosphorus concentration in plants ranges from 0.1 to 0.4 per cent.

Greek scientist Theophrastus (300 BC) advocated legume rotation. As early as 1750 AD it was known that plants require Nitre. Louis Pasteure in 1860 AD stated the possibility of atmospheric nitrogen fixation by microorganisms. Hellriegel and Willfarth (1886) strengthened the views of Louis Pasteure by his observations on the irratic behaviour of legumes to nitrate nutrition. Beizerink (1890) isolated the organism involved in nitrogen fixation. The systematic field experiments conducted at Rothemstead experimental station showed that the plants respond to inorganic nitrogen.

ROLES OF NITROGEN IN CROPS

Plants contain 1-5 per cent nitrogen by weight. It is a structural component of chlorophyll, aminoacids and proteins. Plant proteins contain 16 per cent nitrogen (6.25 × %N = protein). Plants are dependent on protein for their propagation. Cereals possess a threshold protein level below which grain will not form.

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.

Concept of nutrient availability: The ions in the soil solution are available to plant provided they can reach an active root or it can reach them. The concentration of ions in the soil solution at any given moment is however; usually low, except immediately after the application of water soluble fertilizers.

Most of the nutrients are taken up by the plant in the form of cations which are positively charged. Available cations either in the soil solution or held against leaching by negatively charged clay or humus particles are easily taken up by the plants. Other plant nutrients notably phosphorus, boron and molybdenum are taken up by the plant as anions which are negatively charged. The concentration of anions in the soil solution is usually low because they become converted to new less soluble forms e.g., soluble phosphate ions forms less soluble iron or aluminium phosphates. Minerals upon weathering and organic matter upon mineralization release nutrient ions.

Physical factors

However, much readily available nutrients a soil may contain various physical features of the soil make it impossible for the plant roots to absorb it. It then becomes positionally unavailable. These features include hard pans, undesirable soil structures, presence of hard clods, dry soil horizons and water logged situation.

Low soil cation exchange capacity leads to less retention of calcium, magnesium, potassium, copper and zinc.

ESSENTIAL ELEMENTS FOR PLANT GROWTH

Arnon (1954) set three criteria to state an element is essential to plants. They are:

1. The plant must be unable to grow normally or complete its life cycle in the absence of the element,
2. The element is specific and cannot be replaced by another and,
3. The element plays a direct role in metabolism.

Accordingly there are 16 elements essential for plant growth. They are carbon, hydrogen and oxygen called primary nutrients, calcium, magnesium and sulphur called secondary nutrients; iron, zinc, manganese, copper, boron, chlorine and molybdenum are known as micronutrients.

Our production systems are at a pivotal stage in terms of meeting consumer demand for affordable food while improving sustainability. Current intensive crop production practices designed to maximize yield have created an unstable, fragile, and non-sustainable production system. Such systems are prone to reduced soil quality, frequent insect and disease resurgence, emergence of resistant weed species, reduced food quality, and detrimental effects on the health and well-being of our communities. With increasing awareness among consumers of produce quality, nutritional value, methods of crop production and effects of production techniques on the environment, there has been a strong move to improve the quality of our production systems. Grower response to this change has been supportive. Growers have shown interest in developing resilient and stable production systems through the adoption of production practices that enhance soil health, crop productivity, and improve long-term farm sustainability and profitability. Such practices emphasize the use of natural processes within farming systems, often called ‘ecologically sound’ practices which build resilience through synergies and complementarities within the field, the farm, and across our landscape and communities.

Sustainable horticulture is nothing but a contemporary approach to horticulture that emphasizes organic/natural approaches and allows an integrated meshing of ideas with synthesized chemicals used sparingly or relegated to the “last resort.” High Tech & Low Tech are encouraged to create a long-term viable result. Short-Term techniques that impact the environment negatively and thwart the biodynamic continuum are rejected.

Introduction

With India’s population to catapult to a gigantic figure of 1.63 billion by 2050, it rests upon the shoulders of the country’s dwindling land resources to cater to the food demand to the surging human race with an eco-friendly manner. This requires a mammoth effort from the food producing machinery of the country-an effort which has to be inclusive of the technological innovation and extension in this field and to the fullest possible extent. Coping with the short-run challenges to food security posed by food price volatility is indeed a daunting task. But the serious concern is the long-term challenges of avoiding a perpetual food crisis under conditions of global warming. The decline of contributions of Indian Agriculture to the National GDP and an apparent stagnation of the growth rate in agriculture, non remunerative agricultural system, ever increasing population pressure, shrinking land resources, global warming and climate change, indiscriminate use of agricultural inputs, over exploitation of natural resource base and environmental quality have made the situation more vulnerable towards food security. In this backdrop, agricultural knowledge and technology accumulation, transfer, application, and diffusion are key to sustainable development and food security in an eco-friendly manner. The knowledge and technology revolution is critically different from the past industrial revolution in that it is based upon a shift of wealth creating assets from physical things to intangible resources based on knowledge and technologies. Thus, effective management and transfer of knowledge and technologies are believed to be the most critical capability of individuals, organizations, and nations in the globalized 21st knowledge society. Extension could play a pivotal role in fostering sustainability through its educational programs but there has been a growing realization that traditional extension models have not been sufficiently effective in promoting socialization of sustainable agricultural practices. Since sustainable agriculture is a knowledge-intensive system, it requires a new kind of knowledge, which differs from other forms on the basis of conventional agricultural practices. In fact, conventional extension system cannot accomplish sustainability in agriculture; because today’s agricultural extension must consider environmental implications, social issues, and overall economic growth within the agriculture sector. In such a situation, the present conceptual discussion shall be envisaging the dictum and destiny of evolving extension approaches by cherishing a clear focus on extension management perspectives towards sustainable development and environment protection and management so as to the well established extension approaches can be restructured and applied in the niche of extension management towards sustainability.

Introduction to sustainable livestock

Sustainable development has been defined by FAO as “the management and conservation of the natural resource base, and the orientation of technological and institutional change in such a manner as to ensure the attainment and continued satisfaction of human needs for present and future generations. In other words, the development that meets up the needs of the present without compromising the ability of future generations to meet their own needs. Such sustainable development (in the agriculture, forestry and fisheries sectors) conserves land, water, plant and animal genetic resources, is environmentally non-degrading, technically appropriate, economically viable and socially acceptable”. (FAO Council, 1989). Hence, sustainable livestock farming is “the efficient production of safe, high quality products, in a way that protects and improves the natural environment, the social and economic conditions of farmers, their employees and local communities, and safeguards the health and welfare of all farmed species”.

Introduction

Weeds are the most costly category of agricultural pests, causing more yield losses and added labor costs than either insect pests or crop disease. Weeds are a constant fact of life in annual row crops, vegetables, and other horticultural crops. In India, the manual method of weed control is quite popular and effective. The usage of herbicides in India and elsewhere in the world is increasing due to possible benefits to farmers. At the same time, the continuous use of herbicides of the same weed shift, herbicide resistance in weeds and environmental pollutions. Application of herbicides also kills species of bacteria, fungi and protozoa thereby upsetting the balance of pathogens and beneficial micro organisms. The complexity of these situations has resulted in a need to develop a holistic sustainable eco-friendly weed management programme throughout the farming period (Gnanavel and Natarajan, 2014). Sustainable development of agriculture is concerned about the conservation of land, water, plant and animal genetic resources, is environmentally non-degrading, technically appropriate, economically viable and socially acceptable. Sustainable weed management is the use of weed control methods that are socially acceptable, environmentally benign and cost-effective.

Introduction to sustainable disease management

Plants and pathogens are involved in ongoing interactions over millennia that have been modified by coevolutionary processes to limit the spatial extent and temporal duration of disease epidemics (Zhan et al. 2015). These interactions are disrupted by modern agricultural practices, such as intensified monoculture using high yielding varieties. Agricultural intensification and monoculture coupled with globalization disrupt the coevolutionary dynamics of plant-pathogen interactions which escalating the evolutionary rate of pathogens and increasing the risk of plant disease epidemics. These activities, when supplemented with high resource inputs, create unique conditions which is conducive to widespread plant disease epidemics and rapid evolution of pathogen. Disease management requires a significant shift to include ecoevolutionary principles in the design of adaptive disease management strategy for minimizing the evolutionary potential of plant pathogens by reducing their genetic variation, stabilizing their evolutionary dynamics, and preventing dissemination of pathogen variants carrying new infectivity or resistance to pesticides (Zhan et al. 2014). As environmental and ecological issues continue to impact on agriculture, all the crop-specific and regional-specific, Good Agricultural Practices (GAP) to be developed for crop production, must be economically feasible, ecologically sound, environmentally safe and socially acceptable. Numerous non-chemical methods for management of crop diseases such as pathogen-free seeds, disease resistance, crop rotation, plant extracts, organic amendments and biological control agents are considered less harmful than chemical pesticides and therefore, offer great potential for application in modern agriculture (Marshal et al. 2008). No single method can provide satisfactory management of crop diseases. Integration of all the effective and eco-friendly measures in accordance with the dynamics of the agro-ecosystem management would be the best strategy for efficient plant disease management.

Introduction

Chemical pesticides play an important role in plant protection in India and most of the developing countries of the world. These synthetic organic insecticides provide spectacular control of insects. Chemical control is one of the effective and quicker methods in reducing pest population. However, over reliance and injudicious use of pesticides resulted in a series of problems in the agricultural ecosystem mainly, the development of resistance in insects to insecticides, pest resurgence, outbreak of secondary pests into primary nature, environmental contamination and residue hazards, destruction of natural enemies of insect pests etc. All these problems aggravate the pest problem in agricultural crops. Thus, the importance of eco-friendly pest management techniques is being realized more and more in the recent past. The modern concept of pest management is based on ecological principles and involves the integration of different control tactics into a pest management system. This is not altogether a new concept. It was practiced before the advent of modern chemicals. Integrated Pest Management is an ecological approach in which utilization of all available techniques of pest control to reduce and maintain the pest population at levels below economic injury level. The UN’s Food and Agriculture Organisation defines IPM as the careful consideration of all available pest control techniques and subsequent integration of appropriate measures that discourage the development of pest populations and keep pesticides and other interventions to levels that are economically justified and reduce or minimize risks to human health and the environment. IPM emphasizes the growth of a healthy crop with the least possible disruption to agro-ecosystems and encourages natural pest control mechanisms. A well-defined Integrated Pest Management (IPM) is a program that should be based on prevention, monitoring, and control which offers the opportunity to eliminate or drastically reduce the use of pesticides, and to minimize the toxicity of and exposure to any products which are used. So, IPM is an eco-friendly approach which encompasses cultural, mechanical, biological and need based chemical control measures.

Introduction

During the past 60 years, conventional agriculture has become increasingly dependent upon petroleum-based. Chcmically-synthesized fertilizers and pesticides for crop protection and to supply plant nutrients. Certainly, these energy intensive technologies have contributed greatly to this Nation’s agricultural productivity. However, sharply escalating production costs associated with the increasing cost and uncertain availability of energy i.e., fuel and fertilizers, have generated considerable interest in less expensive and more environmentally compatible production alternatives such as organic farming (USDA, 1980). The apparent decline in soil productivity throughout the world from excessive soil erosion, nutrient runoff and loss of soil organic matter; the impairment of environmental quality from sedimentation and pollution of natural waters by agricultural chemicals and the potential hazards to human and animal health and food safety from heavy use of pesticides, have also stimulated interest in organic farming systems of food production.

Use of organic wastes and residues in agriculture

Most countries have traditionally utilized various kinds of organic materials to maintain or improve the fertility, and productivity of their agricultural soils. However, several decades ago organic recycling practices were largely replaced with chemical fertilizers which were applied to high yielding cereal grains that responded best to a high level of fertility and adequate moisture, including irrigation. Soil cultivation was also intensified to improve weed control and seedbed conditions. Consequently, the importance of organic matter to crop production received less emphasis, and its proper use in soil management was neglected, or even forgotten. As a result of this and failure to implement effective soil conservation practices, the agricultural soils in a number of developed and developing countries have undergone serious degradation and decline in productivity.

Agroforestry: At a Glance

Agroforestry as a landuse system is gaining wide importance and acceptance in the current global scenario to mitigate global climatic conditions. It is a highly efficient system of sustenance for the rural population with ample livelihood opportunities. The benefits from agroforestry is immense for socio-economic as well as in ecological perspective. From the vivid ecosystem services like climate regulation, carbon sequestration, soil productivity and biodiversity conservation to food security and sustainable livelihood enhancement, agroforestry envisages immense services. The implication of the agroforestry system and practice varies from area to area. Traditional agroforestry can be modified by implementing different species combinations suitable for vivid regions thus enhancing the long term productivity. An important specification of agroforestry is with a minimal labour input maximum production and utilization of resource could be attained. National Agroforestry policy, implemented is a big step forward for new agroforestry initiatives in the country. Conduction of region specific agroforestry studies and policy implementations with involvement of rural populations will be an effective breakthrough approach for a sustainable future.

Biodiversity is a magnificent contribution of nature on our planet. The great variety of life forms on earth has catered for man’s needs over thousands of years. This diversity of living creatures which forms a support system has been used by each civilization for its growth and development. Those that used this “bounty of nature” carefully and sustainably survived. Those that overused or misused it disintegrated. The attempt to classify and categorize the variability existing in nature for over a century has led to an understanding of its organization into communities of plants and animals. This information has helped in utilizing the earth’s biological wealth for the benefit of humanity and has been integral to the process of development. This includes better health care, better crops and the use of these life forms a raw material for industrial growth which has led to a higher standard of living for the developed world. However, this has also produced the modern consumerist society, which has had a negative effect on the diversity of biological resources upon which it is based. The diversity of life on earth is so great that if we use it sustainably we can go on developing new products from biodiversity for many generations. This can only happen if we manage biodiversity as a precious resource and prevent the extinction of species.

The sustainable management of biodiversity is one of the most important challenges facing humanity. Biological diversity, abbreviated to biodiversity, refers to the variety of life forms at all levels of organization, from the molecular to the landscape level. These are the legacy of billions of years of evolution, shaped by natural processes and increasingly, by the activities of humans. Losing biodiversity has eroded the basis for social and ecological resilience, which has resulted in reducing the capacity for adaptive responses in a rapidly changing world. However, human activities still continue to have a large impact on the environment and on biodiversity. Agricultural, urban and infrastructural developments and climate change has exacerbated biodiversity loss. As a result, ecosystem functions and services are affected with serious consequences. The Sustainable Development Goal should be devoted to “protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss”. It is high time to realize the linkages between the severity of global biodiversity loss and degradation of ecosystems and the negative impact that this situation have resulted on food security, nutrition, access to water, health of the rural poor and people worldwide.