eChapters

The leek is a centuries-old crop native to the eastern Mediterranean and the Middle East. The plant is linked to the onion, and its flavour is mild, sweet, and onion-like. Leek stalks are commonly used in European soups and stews, particularly as a side dish with potatoes, and can be cooked whole as a vegetable. It is a non-bulb producing onion family member farmed for blanched stem and leaves. It is eaten fresh or combined into salads, and it is cooked as a flavouring in soups and stews. It is not grown commercially in India, although it is a popular vegetable in a kitchen garden. The climate and soil conditions are the same as for onion.

Botany and taxonomy: The leek is a vegetable that is a cultivar of Allium ampeloprasum, often known as the broadleaf wild leek. 

Lettuce (Lactuca sativa), a member of the sunflower family Asteraceae, is India’s most popular salad vegetable. From a commercial standpoint, it is a tiny crop that is exclusively grown in kitchen gardens. However, because of the rising use of lettuce in continental hotels, the acreage and output of lettuce has expanded, making India the third largest producer. Lactuca indica, also known as Indian lettuce, is a lettuce species native to Asia that is found in Meghalaya and Sikkim and is used for medical purposes. Indian lettuce is a generic term for numerous Asian plants with leaves that vary in form and size. It is also known as the Goose vegetable, Tropical lettuce, Ku mak, Yao mak, Ku Mak Cai, Daun Panjang, and Sawi Rana.

Lima beans are flat green or light green beans. It is only utilised for the seed because the pods are coarse or woody. There are two kinds used in commercial canning: a climbing or pole variant and a dwarf or bush variation. Like other legumes used for seed, the lima bean (Phaseolus Iunatus) has a wide range in seed size, shape, and colour, as well as plant structure and primary applications. For freezing, canning, or fresh market usage, mature seeds are utilised like a vegetable. Lima’s have long been used as a grain legume when they are firm and dry. Peru is where they were discovered. The first domestication occurred in the Andes around 2000 BC, resulting in a large-seeded variety (Lima type), while the second occurred around 800 A.D. in Mesoamerica, resulting in a small-seeded variation (Sieva type).

In India, the ivy gourd is also called as small watermelon, little gourd, gentleman’s toes” and “Tindora/Tondli/Kundru.” This vine climbs aggressively and spreads quickly across fences, trees, shrubs, and other supports. Its natural range includes India and stretches from Africa to Asia.

Botany and taxonomy: This is a single-tendered perennial climber with glabrous leaves. The leaves are 6.5-8.5 cm long and 7-8 cm broad, with 5 lobes. It’s a dioecious species. Female and male blooms have three stamens and emerge from the axils on the petiole. Scarlet gourds are a genus of 25 species (Coccinia from the Greek, kokkinia or kokkinias - “red” or “scarlet”). It is found in SubSaharan Africa, as well as South and Southeast Asia (with one species, C. grandis), and has been imported to the New World. This is a single-tendered perennial climber with glabrous leaves. The leaves are 6.5-8.5 cm long and 7-8 cm broad, with 5 lobes. It’s a dioecious species. Female and male blooms have three stamens and emerge from the axils on the petiole.

Cucumis melo var. conomon or Oriental pickled melon is a veining annual vegetable native to China and India. In the summer, it produces yellow blooms, and in the fall, it harvests the fruit. The petals are crumpled because it belongs to the Cucumis genus.

Botany and taxonomy: The fruit’s outer surface is glossy and can be white or green, but it’s known as shiro-uri because when fully ripe, it turns white and is whiter than a cucumber. It’s a melon variety, but it’s used in cooking since it has a lighter sweetness and flavour, as well as a crisp texture when chewed. It contains vitamin C, vitamin B1, and carotene, however the majority of its constituents are water and low in calories.

Parsley is a popular culinary herb that is grown commercially as an annual plant for its attractive and aromatic leaves in many parts of the world. In Europe, parsley is the most generally farmed herb, and in the United States, it is the most commonly utilised herb. Rather than being a part of the diet, it is utilised as a garnish. The leaves’ oil can also be used to make sauces and spices.

Botany and taxonomy: Parsley is a biennial that is cultivated and managed as if it were an annual. It’s the most popular herb for garnishing and flavouring. Parsley comes in two varieties: moss-curled and flat-leaf. Moss-curled produces a rosette of finely cut and tightly curled leaves. It develops as a biennial, with a rosette of tripinnate leaves 10-25 cm (4-10 in) tall, numerous 1-3 cm (0.4-1.2 in) leaflets, and a taproot that serves as a winter food store. It develops a blooming stem with sparser leaves and umbels of yellow to yellowish-green flowers in the second year. 

In the 14th century, the term “parsnip” was borrowed into Middle English as a modification of the Old French word “pasnaie,” which was derived from the Latin noun “pastinaca,” which meant “parsnip” or “carrot.” This history is still reflected in the scientific name for the parsnip, “Pastinaca sativa.” The parsnip is a native of Eurasia; it has been farmed by the Romans and has been used as a vegetable since antiquity, but there is some misunderstanding between parsnips and carrots in historical texts. Prior to the arrival of cane sugar in Europe, it was employed as a sweetener. Parsnips are carrot-shaped root vegetables that are cream-colored and belong to the parsley family. Although they can be eaten fresh, they are more commonly roasted, boiled, fried, or steamed. The thin, tan peel of parsnips is usually peeled before use, revealing white flesh beneath.

The pointed gourd (Trichosanthes dioica Roxb, Cucurbitaceae) is an Indian tropical vegetable crop. It’s called as parwal, palwal, or parmal in different parts of India and Bangladesh, and it’s one of the region’s most significant vegetables. Eastern Uttar Pradesh, Bihar, West Bengal, Assam, Odisha, Madhya Pradesh, Maharashtra, Gujarat, and some hilly portions of Telangana, Andhra Pradesh, and Tamil Nadu are among the states where the crop is commonly farmed. The entire cultivated area is 56 million hectares, while the total production is 798 million hectares (2018-19). Due to its well-established dioecism, perennial nature, and vegetative modes of multiplication, the pointed gourd is physically unique from other cucurbitaceous species.

As early as the 16th century, rhubarb was farmed and marketed for therapeutic purposes. According to History of Fruit, by the 19th century, rhubarb had acquired prominence as a food and vegetable source, with a redder stem and sweeter flavour. Rhubarb is a native of Central Asia, and knowledge of its therapeutic properties dates back five thousand years in China. It was also widely used by Arabs, Greeks, and Romans in antiquity. It came as no surprise to Marco Polo when he discovered it in China in 1271.

Botany and taxonomy: Rhubarb refers to the fleshy, edible stalks (petioles) of Rheum species and hybrids (culinary rhubarb) of the Polygonaceae family, which are cooked and eaten. Rhubarb refers to the entire plant, which is an herbaceous perennial with short, thick rhizomes. The big, triangular leaves are inedible due to high quantities of oxalic acid and throne glycosides. 

Spinach is estimated to have originated in ancient Persia some 2,000 years ago, and was transported to India and ancient China as the “Persian vegetable” via Nepal in 647 AD. The Saracens introduced spinach to Sicily around 827. Spinach (Spinacia oleracea), also known as vilayati palak in Hindi, is a leafy vegetable that grows well in cool or temperate climates. Vilayati spinach is dioecious, unlike beet leaf spinach, which is hermaphrodite. Its production is restricted to household gardens in mountainous regions of India, and it is consumed as a cooked vegetable.

“Every year, 26,688,150 tonnes of spinach are produced over the world.” With a production output of24,484,507 tonnes per year, China is the world’s greatest spinach grower. The United States of America is in second place, producing 323,620 tonnes each year. “Spinach isn’t grown in India.”

Momordica dioica, also known as bristly balsam pear, prickly carolaho, teasle gourd, and kantola, is a flowering plant of the Cucurbitaceae/gourd family that is commonly known as spiny gourd or spine gourd. It is consumed as a vegetable throughout India and parts of South Asia. It is valuable commercially and is exported and utilised locally. The fruits are sometimes served with meat or fish and are prepared with spices or fried. Underground tubers are used to proliferate it. Small leaves, small yellow flowers, and small, dark green, spherical or oval fruits characterise this plant. It is dioecious, which means there are separate male and female organisms.

In India, Indonesia, Malaysia, China, and tropical Africa, it is widely grown. In India, there are four species of bitter gourd that belong to the genus Momordica.

Botany and taxonomy: It is an annual climber vine with woody, thick stems that can grow up to 20 metres long, large, heart-shaped leaves, large, creamy to pale yellow flowers that are single, ovoid, large, dark green fruits that can grow up to 20 cm long and weigh up to 1.5 kilogrammes, periphery with smooth thorns, yellow turned orange when ripe, and black seeds. The leaves, petals, and seeds of this plant are edible and can be eaten as a vegetable.

• Useful plant parts: Fruits, leaves, flowers, tuberous root and seeds.
• Medicinal uses: To treat Ulcers, lumbago, fractures, bronchial asthma and bleeding piles.

These beans are mostly found in China, Korea, Japan, India, Sri Lanka, Madagascar, and Indonesia, and are not frequently cultivated for commercial purposes. Sword bean pods can reach a length of over a foot and a width of one to two inches. It is grown as a minor vegetable for local use throughout South and Southeast Asia, particularly in India, Sri Lanka, and Myanmar. Sword bean is typically planted as an annual vegetable crop because it is a perennial climbing legume. Sword bean is grown for its soft pods, which can be eaten raw or cooked.

Pigeon pea seeds discovered in historical tombs suggest it was farmed in Egypt circa 2000 BC. Vavilov (1939) concluded that India is the primary origin of the cultivated pigeon pea due to the wide diversity of germplasm available. It’s a crucial location in rain-fed agriculture. It is grown on 4.92 million hectares worldwide, including 3.58 million hectares (72.7 percent) in India alone. Although the crop is planted in 22 nations, it is only cultivated in considerable areas in a handful of them. Its seeds have become a widespread food in Asia, Africa, and Latin America since its domestication in the Indian subcontinent at least 3,500 years ago. It is an important source of protein for the Indian subcontinent’s people and is consumed on a big scale in South Asia. India is a major producer of pigeon peas. Kenya, Malawi, Uganda, Mozambique, and Tanzania are major producers of pigeon pea on the African continent.

Equatorial Asia is home to winged beans (Geneva). On atrip to Thailand, Baker Creek’s founder, Jere Gettle, first observed flying beans. Because the winged bean is a tropical crop from the tropics, it takes a long time to flower, setting pods right before frost in our Missouri environment. The winged bean (Psophocarpus tetragonolobus) is a tropical herbaceous legume plant also known as cigarillas, goa bean, four-angled bean, four-cornered bean, manila bean, princess bean, asparagus bean, and dragon bean. From the Philippines and Indonesia to India, Burma, Thailand, Vietnam, and Sri Lanka, winged bean thrives in hot, humid equatorial climates. It’s well-known, yet it’s only farmed in limited quantities in Southeast Asia and Papua New Guinea.

Around the world, nearly one billion people live along the 3,12,000 Km long coastline. The Indian coastline runs around 8,129 Km distributed along nine coastal states, two groups of islands and two union territories. The coastal ecosystem of India forms a very valuable resource community, supporting the livelihood security of several million rural poor and also contributing the national economy in a large measure. The latest estimates by National Bureau of Soil Survey and Land Use Planning (NBSS & LUP), Nagpur, India, using Global Assessment of Soil Degradation (GLASOD) guidelines indicate that the coastal area in the country is spread over an area of 11.8 million hectares (Mishra, 2005). Yadav et al. (1983) have reported about 3.1 M ha and 2.1 lakh ha of coastal salinity in India and Tamilnadu respectively.

The coastal salt affected soils are low in productivity due to various soil fertility constraints that affect the crop growth and yield. The high salt content &/or high pH cause changes in solubility, availability and efficiency of plant nutrients, besides deteriorating the physical condition of soil. The soil biology (microbial population and enzymatic activities) also severely affected in these salt affected soils. The bio-resources are known to improve soil physical, chemical & biological properties of the soil. The maximization of rice production and considerable improvement in quality and composition of rice in coastal salt affected soil with the use of bio-resources have been reported by several research workers.

It is generally observed that N, P and Zn content in soil decreases with increasing soil salinity. Nutrient imbalances created by continuous use of chemical fertilizers, particularly nitrogenous fertilizer alone, combined with sub optimal rates of other nutrients or total omission of other nutrients are the primary causes for unsustainable yields of crops in saline soil. A sustainable approach for nutrient management in salt affected soils should therefore include balanced use of organic, inorganic and biological sources of nutrients (Bandyopadhyay and Rao, 2001). Organic amendments are known to facilitate the reclamation of sodic soils and better nutrition of crops (Anand swarup, 1992).

Experiments at Central Soil Salinity Research Institute (CSSRI), Kamal, India showed that alkali soils are not always high in available P and rice and wheat significantly responded to addition of P (Pathak et al., 1975).

The availability of phosphorus in soil either native or added is limited by fixation and precipitation mechanisms. The organic acids released during decomposition of organic matter influences the soil pH, form stable complexes with cations responsible for P fixation and thus increases the availability (Tiwari et al., 1980).

The use of bulky organic manures (Maurya and Ghosh, 1972; Chaudhary et al., 1981) and green manures (Prabhakar et al., 1972; Sharma et al., 2000) has been found very beneficial for P management of saline and alkali soils as these improve the soil physical condition and the availability of soil phosphates.

Potassium availability in most of the coastal salt affected soils of India is rich except when the soils are low in clay content (Bandyopadhyay et al., 1985). Plants grown under high salinity may show K deficiency due to the antagonistic effect of Na on K absorption and / or disturbed Na / K ratio. Under such conditions application of K is also known to reduce the adverse effect of salinity and sodicity on crop growth (Qadar, 1995). Singh et al. (1980) reported that the addition of FYM increased the K concentration in the soil. Swarup (1987) reported that the incorporation of green manure increased the available N, P and K status of the soil. Available P and K remarkably increased to a tune of 30 and 44 per cent respectively by the application of coir waste (Santhi et al., 1991).

Secondary nutrients

Secondary nutrients includes Calcium, magnesium, and sulfur. Crop plants requires secondary nutrients in moderate quantity for crop growth and yield. Their requirement by plant is less than the macro nutrients. Sulphur and Magnesium requirement of crops are very similar to the Phosphorus needs. Calcium is required in higher amount than Phosphorus. Calcium and magnesium are mostly deficient in acid and alkali soils. In India S deficiencies are found in almost 40 per cent of the cultivated soil. Under intensive cropping system, the use of higher amount of nitrogenous fertilizer without secondary and micro nutrients and lesser use of organic manures are the major reason for the secondary and micro nutrient deficiency in soil.

The bio-resources application in the soil is known to improve the biological properties of the soil. Several earlier reports show a spur in microbial populations or increased microbial biomass carbon and the enhanced enzymatic activities with the incorporation of bio-resources in soil (Perucci, 1990; Goyal et al., 1993; Bonde et al., 2004).

Microbial populations

Indigenous microbial populations in soil are of fundamental importance for an eco system functioning, through determining nutrient cycling, organic matter decomposition and energy flow (Doran and Zeiss, 2000).

In the global scenario, the increasing energy crisis and consequent escalation of chemical fertilizer prices along with increasing risk associated with soil degradation have reviewed interest in bio-resources for attaining soil health and sustainable crop production. The use of organic manures, bio-fertilizers and chemical fertilizers will augment the efficiency of combined application of nutrients to maintain a high level of soil productivity and rice production (Thakuria et al., 1991; Prasad et al., 1995). Several experimental evidences showed that the combination of various bio-resources with chemical fertilizers improved the rice production in sustainable way (Roy and Jha, 1987; Subba Rao, 1995; Mahapatra et al., 1997; Suresh et al., 2001).

Organic manures

Anand Swarup (1992) stated that application of green manures along with rice husk increased the concentrations of N, P, Zn, Mn and Fe and decreased the Na concentration of rice in a reclaimed sodic soil. Kolambe and Patel (1994) reported that application of organic amendments along with phosphorus increased the uptake of phosphorus at different growth stages of rice.

Talashilkar and Chavan (1997) reported that incorporation of 10 t of Glyricidia ha^-1 and application of 100 per cent RDF recorded the highest total N uptake by rice over three successive years. Mishra and Sharma (1997) found that application of 100 per cent NPK + 10 t FYM ha^-1 resulted in highest total uptake of NPK by rice. Mahapatra etal. (1997) revealed that application of 80 kg N ha^-1 through green manure alone or through combined use of green manure and urea increased the total uptake of N by rice over application of fertilizers alone.

The uptake of Mg by rice plant was highest when fertilizer was applied in combination with vermicompost (Jadhav etal, 1997). Pattanayak etal. (2001) also reported the highest Ca uptake in the treatments that received green manuring of dhaincha. Sriramachandrasekharan (2001) who reported the highest Fe uptake of rice with green manuring of Sesbania aculeata.

Debele et al. (2001) reported that the application of FYM enriched with 10% rock phosphate + gypsum with PSB and PSF @ 500 g inoculums t^-1 of FYM increased the nutrient uptake of maize. The higher nutrient uptake of 227 kg N ha^-1, 59.5 kg P ha^-1 and 227 kg K ha^-1 recorded with the application of enriched FYM @ 2 t ha^-1.

The term soil organic matter (SOM) includes any material that is produced originally by soil organisms (plant or animal) that is returned to the soil and undergoes decomposition process. At any point in time, it consists of an array of materials, ranging from the intact original tissues of plants and animals to the substantially decomposed mixture of materials known as humus (Fig.1.1). Most soil organic matter originates from plant tissues. Plant residues contain 60 - 90 per cent of moisture and the remaining dry matter consists mostly of carbon, oxygen, hydrogen, with smaller amounts of sulphur, nitrogen, phosphorus, potassium, calcium and magnesium. Though present in small amounts, these nutrients are of great significance from the standpoint of soil fertility.

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

2.1. Quantity of Organic Matter

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

Soils vary greatly in their organic matter content. For all practical purposes the organic matter content of the soil parallels the soil N content. The C/N ratio of soil organic matter generally falls in the range of 10 to 12. Because of the ease with which Kjeldahl determination can be made, N is often used as an index of soil organic matter content.

3.1. Composition of Organic Matter in Soil

Organic matter constitutes 1 to 6% of the topsoil weight of most upland soils. In a typical grassland soil, 5 to 6% of organic matter may be present in the top 15 cm. Top soils with less than 1% organic matter are mostly limited to desert regions. On the contrary, the organic matter content of soils in low, wet areas may be as high as 90% or even more. Soils with more than 12 - 18% organic carbon (approximately 20 - 30% organic matter) are called organic soils.

Carbon is the cornerstone of all life on Earth. Plant tissues and microbial cells contain approximately 40 to 50% of carbon on dry weight basis. Yet, the ultimate source is the CO₂ of the atmosphere. The CO₂ is converted into organic carbon by the action of photoautotrophic organisms (higher plants and green algae). It has been estimated that the vegetation on earth’s surface consume some 1.3 x 10¹⁴ kg of CO₂ per annum which is about V₂₀ of the total CO₂ supply of the atmosphere or ¹/₁₀₀₀ of that in the ocean.

Soil is the product of various soil forming factors acting on the parent material over a period of time. The parent material is usually devoid of organic matter and consists of variable quantities of sand, silt, clay and carbonates. High temperatures and adequate moisture promote mineral transformations, but these effects are magnified by living organisms and the organic substances they produce. The biosphere and the organic substances they produce are directly or indirectly responsible for the following:

1. Decay of organic matter by microbes leads to the formation of CO₂ which is quantitatively the most important “aggressive” weathering agent because of its tendency to form carbonic acid with water [CO₂ + H₂O → H₂CO₃].

Soil organic matter is among the most complex materials existing in nature. In addition to the organic constituents present in undecayed plant and animal tissues, soil organic matter contains living and dead microbial cells, microbially synthesized compounds and an endless array of derivatives of these materials produced as the result of microbial activity. Soil organic matter probably contains most of the naturally occurring organic compounds. Some components of soil organic matter are no doubt distinctive to the soil environment, particularly those involving inorganic-organic complexes.

6.1. Composition of Soil Organic Matter

The composition of soil organic fraction is as follows:

1. Components of organic residues undergoing decomposition

Soil is the fundamental resource in the agricultural production systems and monitoring its fertility is critical to the sustainability of agriculture. Plant nutrients undergo different transformation processes through which they are converted from unavailable to available forms. Nutrient cycling thus involves the transformation and availability of nutrients and is mediated by microbial action. Understanding the dynamics of nutrient and soil organic matter transformations requires an understanding of the biological, chemical, and physical processes involved.

7.1. Soil Nitrogen

The synthesis of humic substances is the least understood and the most intriguing aspect of humus chemistry. This has been one of the intensely- researched subjects for a long time. Formation of humic substances takes several pathways during the decay of plant and animal remains in soil and the four important ones are shown in Fig. 8.1.

Fig. 8.1: Mechanisms for the formation of soil humic substances

The classical theory by Waksman advocated humic substances as modified lignins (Pathway 1), but a vast majority of current crop of investigators favour a mechanism involving quinones (Pathways 2 and 3). Including sugar-amine condensation pathway (Pathway 4), all the four pathways must be considered as likely mechanisms for the synthesis of humic and fulvic acids in nature. It is absolutely possible that these four pathways may operate in all soils, though not to the same extent or in the same order of importance. 

Practically every aspect of the chemistry of heavy metals in soil is related to the formation of complexes with organic matter. While monovalent cations (Na+, K+, etc.) are held mainly by simple cation exchange through the formation of salts with COOH groups (RCOONa, RCOOK), multivalent cations (Cu²+, Zn²+, Mn²+, etc.) form coordinate linkages with organic molecules.

A schematic diagram showing organic matter reactions involving metal ions in soils is given in Fig. 9.1. The metals present in the solution phase as charged species and as soluble metal-organic complexes (MChe), are shown to be influenced by the activities of higher plants and microbes, both of which serve as sources of water-soluble ligands for complex formation; some metals are held in insoluble organic complexes and are non-leachable and relatively unavailable to plants.

The chemistry of humic substances is the least comprehended in the field of Soil Science. The many important functions of humus will remain obscure if the structural chemistry of the humic and fulvic acids is not known. A variety of functional groups have been reported in humic substances. However, considerable disagreement still exists in respect of the exact amounts present and, in some cases, proof of existence is lacking.

Some important structural groups of organic molecules are given below:

Understanding the mechanisms controlling the adsorption of organic molecules on clay minerals is of significance in many branches of science and industry. The interaction between clay minerals and organic compounds elicits interest that emanates from the prospect that the adsorption of the organic matter fraction in the soil on the mineral particles would render physical stability to soil aggregates.

Organic compounds tend to interact with clay minerals by:

• adsorption process at the external surfaces
• adsorption process at the external and internal surfaces

Extensive studies have shown that much of the humified material in soil is finely bound to colloidal clay. However, it is uncertain to what proportion of the clay surface in any given soil is coated by organic substances as this will depend on the organic matter content and the type and amount of clay.

The ways in which organic substances are retained in soil are as follows:

1. As insoluble polymeric complexes of humic and fulvic acids.
2. As polymeric complexes of humic and fulvic acids bound together by di- and trivalent cations such as Ca²+, Fe³+ and Al³+.

Organic matter is of immense importance in forming good aggregates in a wide range of soil types, particularly those representing Mollisols, Alfisols, Ultisols and Inceptisols. Organic matter is somewhat less important in the Oxisols where hydrous oxides may play apredominant role. These relationships are depicted in Fig. 13.

Fig. 13.1: Relationship between OM and the formation of soil aggregates

Aggregates do not directly influence plant growth, but they alter the physical and chemical environments in which plant roots grow through their effects on porosity, aeration, moisture retention, etc., when well-irrigated, even fine- textured soils permit adequate exchange of gases with the atmosphere. The polysaccharides of soil have received the greatest attention in producing stable aggregates, but other organic substances bonded to clay through association with Fe or Al may also be involved.

It is well known that organic matter plays a unique role in the binding of pesticides in soil, and that this phenomenon is usually the most important cause for interaction of pesticides in the soil environment. Fulvic or humic acids are most commonly associated with the binding interactions. Binding can take place with either the original pesticide or a transformation product, the reaction being induced by abiotic or biotic agents (microbial or plant enzymes). The reactions or processes involved are apparently the same as those responsible for the formation of humic substances, i.e., for the humification process.

Humic substances are the most widely-present, natural, complexing ligands occurring in nature. They are found not only in soils, but in natural waters, sewage, compost heaps, marine and lake sediments, peat bogs, carbonaceous shales, coals and other miscellaneous deposits.

The fate of natural organic matter, especially humic substances, has over recent decades attracted increasing attention of scientists representing various disciplines. Humic substances constitute about 25% of the total organic carbon on the earth. These are a group of naturally occurring complex molecular structures into which biomolecules derived from plant and animal residues are transformed progressively through biotic and abiotic pathways, by aggregation and accumulation processes

Humic substances, the natural organic materials, represent a mixture of relatively small organic components, which form supramolecular structures held together by dispersive forces such as n - n and van der Waals’ interactions. Identification of the best analytical method for complete characterization of humic substances is still being discussed. Humic substances differ in molecular weight, elemental composition, acidity and cation exchange capacity and often are classified into three major fractions according to their solubility as humic acids, fulvic acids and humins.

The nuclear magnetic resonance spectroscopy (NMR) has led to major advances in the studies on the nature and chemical composition of soil organic matter. The technique has the potential for characterizing organic matter in the intact soil and its size fractions without the need for extraction and fractionation. Furthermore, it can provide information on the compositional changes in crop residues, peat and litter of the forest soils during biodegradation and humification.

The study of soil organic matter is becoming increasingly important by the day as world agriculture attempts to ensure sustainability of soils, while at the same time striving to enhance food production to feed an ever-burgeoning population. The use of green manures and pasture leys, the return of residues and additions of organic amendments to the soil are often resorted to in an attempt to increase soil organic matter which immensely benefit the physical, chemical and biological make-up of the soil.

Managing soil organic matter is of paramount importance in sustainable crop production. Human interventions always lead to a decrease in soil organic matter content and biological activity. Therefore, even for maintaining moderate levels of organic matter in soils, sustained efforts should be in place. Such sustained efforts should include adopting high-residue crops in rotation and ensuring regular return of residues to soils.

It is all the more difficult to maintain the organic matter content in well aerated soils and soils of hot and arid regions, because in these soils added materials decompose rapidly. However, in comparison, it may not be so difficult to maintain the organic matter levels in fine-textured soils with restricted aeration and in soils of cold temperate regions.

19.1. Measures that Decrease Soil Organic Matter Content

Methods and procedures for the qualitative characterization and quantitative analysis of different constituents of soil organic matter are presented in this Chapter.

20.1. Moisture

The soil sample should be in an air-dry condition after mixing, sieving and grinding. When stored in a moisture-tight container, the free H₂O content should not vary appreciably during storage. While most determinations can be done on air-dry samples, for certain purposes such as obtaining summation values, a moisture content value is required.

ntroduction

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

Introduction

The microorganisms exist as single cell or cluster of cells capable of orderly growth, metabolism and reproduction in two identical daughter cells. These includes cellular organisms {protozoa, algae and fungi (Eukaryotes), bacteria and archaea (Prokaryotes)} and viruses (non cellular entities). The study of microorganisms contribute to understanding of :

1. Basic biological sciences - It acts as a tool for providing the basic information for life processes. The discovery of DNA, RNA, proteins and enzyme functions all came from microbial study.
2. Applied biological sciences - Medicine, agriculture, industry, plant and animal pathology, immunology, plant and animal nutrition, biotechnology all have origin from the understanding of the cell function and metabolism of microorganisms.

Introduction

The term actinomycetes, coined in the late 1800s (actins, meaning ‘ray’ and myces, meaning ‘fungus’) is a misnomer. Because actinomycetes have a considerably smaller aerial mycelium than fungi and can produce a large number of asexual spores called conidia, which give the colonies a ‘powdery or chalky’ look.

The actinomycetes constitute a specialized group of soil bacteria that occur in soils throughout the world. The actinomycetes are prokaryotes confirming cellular structures and functions of true bacteria and their morphological similarity to the fungi (Eukaryotes). Between bacteria and fungi, actinomycetes are a transitional category. On their hyphae, most actinomycetes produce single, pairs, or chains of asexual spores.The diameter of the hyphae varies from 0.5 to 1.0 µm. 

Fungi are a diverse group of multicellular eukaryotic organisms with wide variation in vegetative and reproductive morphologies and diverse life cycles. They are more abundant, on a mass basis, in soils than any other group of microorganisms; their biomass ranges from 500 to 5000 wet kg ha^-1 (Metting, 1993). Fungi inhabit almost any niche containing organic substrates, so that they are active participants in ecosystems as -

• degraders of organic matter
• agents of disease as well as beneficial symbionts - lichen (algae and fungi), mycorrhizal association

Introduction

Soil algae are not as numerous as bacteria, actinomycetes and fungi. Furthermore, these are photosynthetic microorganisms. Recent work has led to a more complete knowledge of the ecology and importance of terrestrial algae. These algae are found abundant in the habitats in which moisture is adequate and light is accessible.

Classification

Soil algae are classified into four major classes:

1. Cyanophyceae (Blue green algae)
2. Chlorophyceae (Green algae)