Soil structure and its classification

 Soil structure – classification

         Soil conditions and characteristics such as water movement, heat transfer, aeration, and porosity are much influenced by structure. In fact, the important physical changes imposed by the farmer in ploughing, cultivation, draining, liming, and manuring his land are structural rather than textural.

Definition:- The arrangement and organization of primary and secondary particles in a soil mass is known as soil structure. Soil structure controls the amount of water and air present in soil. Plant roots and germinating seeds require sufficient air and oxygen for respiration. Bacterial activities also depend upon the supply of water and air in the soil.

Formation and soil structure:-  soil particles may be present either as singles individual grains or as aggregate i.e. group of particles bound together into granules or compound particles. These granules or compound particles are known as secondary particles. A majority of particles in a sandy or silty soil are present as single individual grains while in clayey soil they are present in granulated condition. The individual particles are usually solid, while the aggregates are not solid but they possess a porous or spongy character. Most soils are mixture of single grain and compound particles. Soils, which predominate with single grains are said to be structure less, while those possess majority of secondary particles are said to be aggregate, granulated or crumb structure.

Mechanism of aggregate formation:-  the bonding of the soil particles into structural unit is the genesis of soil structure. The bonding between individual particles in the structural units is generally considered to be stronger than the structural unit themselves in aggregate formation, a number of primary particles such as sand, silt and clay are brought together by the cementing or binding effect of soil colloids. The cementing materials taking part in aggregate formation are colloidal clay, iron and aluminum hydroxides and decomposing organic matter. Whatever may be the cementing material, it is ultimately the dehydration of colloidal matter accompanied with pressure that completes the process of aggregation.

Colloidal clay:- by virtue of high surface area and surface charge, clay particles play a key role in the formation of soil aggregates. Sand and silt particles can not form aggregates as they do not possess the power of adhesion and cohesion. These particles usually carry a coating of clay particles; they are enmeshed in the aggregates formed by the adhering clay particles. Colloidal particles form aggregates only when flocculated. There is vast difference between flocculation and aggregation. Flocculation is brought about by coalescence of colloidal particles and is the first step in aggregation.

 Aggregation is something more than flocculation involving a combination of different factors such as hydration, pressure, dehydration etc. and required cementation of flocculated particles. The cementation may be caused by cations, oxides of Fe and Al, humus substances and products of microbial excretion and synthesis. Clay particles form aggregates only if they are wetted by a liquid like water whose molecules possess an appreciable dipole moment.

 Clay^--⁺water^--⁺cation⁺-^-clay^--⁺water^--⁺cation⁺-^-clay^-

The aggregation also depends upon the nature of clay particles, size and amount of clay particles, dehydration of clay particles, cations like calcium and anion like phosphate.

Fe and Al oxides:- the colloidal Fe oxide act as cementing agent in aggregation. Al oxide bind the sand and silt particles. These act in two ways. A part of the hydrdoxide acts as a flocculating agent and the rest as a cementing agent.

Organic matter:-  it also plays an important role in forming soil aggregates.

·       During decomposition, cellulosic substances produce a sticky material very much resembling mucus or mucilage. The sticky property may be due to the presence of humic or humic acid or related compounds produced.

·       Certain polysaccharides formed during decomposition.

·       Some fungi and bacteria have cementing effect probably due to the presence of slimes and gums on the surface of the living organisms produced as a result of the microbial activity.

Classification

 The primary particles- sand, silt and clay- usually occur grouped together in the from of aggregated.

Natural aggregated are called peds where as clod is an artificially formed soil mass. Structure is studied in the field under natural conditions and it is described under three categories

1)    Type – shape or form and arrangement pattern of peds.

2)    Class – size of peds.

3)    Grade – degree of distinctness of peds.

Types of structure: there are four principal forms of soil structure

plate-like:- in this type, the aggregates are arranged in relatively thin horizontal plates or leaflets. The horizontal axis or dimensions are larger than the vertical axis. When the unit/ layer are thick they are called platy. When they are thin it is laminar. Platy structure is most noticeable in the surface layers of virgin soils but may be present in the subsoil. This type is inherited from the parent material, especially by the action of water or ice.

Prism-like:-  the vertical axis is more developed than horizontal, giving a pillar like shape. Vary in length from 1-10 cm. commonly occur in sub soil horizons of arid and semi arid regions. When the tops are rounded, the structure is termed as columnar when the tops are flat/ plane, level and clear cut prismatic.

Block like:- all three dimensions are about the same size. The aggregates have been reduced to blocks. Irregularly six faced with their three dimensions more or less equal. When the faces are flats and distinct and the edges are mainly rounded it is called sub angular blocky. These types usually are confined to the sub soil and characteristics have much to do with soil drainage, aeration and root penetration.

Spheroidal (sphere like):-  all rounded aggregates may be placed in this category. Not exceeding an inch in diameter. These sounded complexes usually loosely arranged and readily separated. When wetted, the intervening spaces generally are not closed so readily by swelling as may be the case with a blocky structural condition. Therefore in sphere like structure, infiltration, percolation and aeration are not affected by wetting of soil. The aggregates of this group are usually termed as granular which are relatively less porous. When the granules are very porous, it is termed as crumb. This is specific to surface soil particularly high inorganic matter.

Definition and soil pedological and edaphological concepts

Soil science

Soil science is the study of natural resources on the surface of earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of spoil; and these properties in relation to the use and managements of soils.

     Sometimes terms which refer to branches of soil science, such as pedology (formation, chemistry, morphology and classification of soil) and edaphology (influence of soil on organisms, especially plants), are used as if synonymous with soil science. The diversity of names associated with discipline is related to the various associations concerned. Indeed, engineers, agronomists, chemist, ecologist, biologist, microbiologists, archaeologists, and specialists in regional planning, all contribute to further knowledge of soils and the advancement of the soil sciences.

Soil scientist have raised concerns about how to preserve soil and arable land in a world with a growing population, possible future water crises, increasing per capita food consumption, and land degradation.

Soil occupied the pedosphere, one of earth’s spheres that the geosciences use to organize the earth conceptually. This is the conceptual perspective of pedology and edaphology, the two main branches of soil science. Pedology is the study of soil in its natural setting. Edaphology is the study of soil in relation to sol-dependent uses. Both branches apply a combination of soil physics, soil chemistry, and soil biology. Due to the numerous interactions between the biosphere, atmosphere and hydrosphere that are hosted within the pedosphere, more integrated, less soil-centric concepts are also valuable. Many concepts essential to understanding soil come from individuals not identifiable strictly as soil scientists.

Definition of soil science:- ‘’The science dealing with soil as a natural resource on the surface of the earth, including pedology (soil genesis, classification and mapping), physical, chemical, biological and fertility properties of spoil and these properties in relation  to their management for crop production.’’

Soil science has six well defined and developed disciplines

Soil fertility - nutrient supplying properties of soil.

Soil chemistry - chemical constituents, chemical properties and the chemical reaction.

Soil physics - involves the study of physical properties.

Soil microbiology- deals with microorganisms, its population, classification, its role in transformations.

Soil conservation - dealing with protection of soil against physical loss by erosion or against chemical deterioration i.e excessive loss of nutrients either natural or artificial means.

Soil pedology - dealing with the genesis, survey and classification.

Definition and History of plant pathology

Definition:- it is the science which deals with the study of living entities and environment conditions that cause diseases in plants, mechanism by which these factors produce disease in plant, interaction between disease causing agent and diseased plants and method of preventing or managing disease and alleviating the damage it cause.

Symptom:-  external or internal reaction or alteration of a plant as a result of a disease  is called symptom e.g. wilting, curling, puckering.

Sign :- pathogen or its part or product (i.e. vegetable or fruiting structure of the pathogen) seen on a host plant e.g. rust, smut , spore etc is called sign.

Disease:- According to E.C. Stakman and J.George Harrar in 1957. A plant disease is the physiological disorder or structural abnormality i.e. deleterious to the plant or of its part or its products that reduces its economic value.

                  According to Agrios (2005) “Disease is a malfunctioning process. He defined it as the series of visible and invisible responses of plant cells and tissues to a pathogenic organism or environmental factor that result in adverse change in the form, function or integrity of plant and may lead to the partial impairment or death of plant part or entire plant’’  

History of plant pathology

Year period	scientist	contribution
1871	kuhn	Used carbon disulfide to control sugar beet cyst nematodes
1667	Hooke	First to observe spores of rust fungus with the help of compound microscope in england
1884	H.C. Gram	Gram staining
1885	Millardet	Discovered Bordeaux mixture for the control of downy mildew of grapevine and later, used against many diseases including late blight of potato.
1743 AD	Needham	First plant parasitic nematode- Anguina tritici on wheat.
1886	E.J. Butler	Father of Modern plant pathology-detailed studies on Indian fungi and disease for 20 years in ‘The imperial agri. Institute’ at Pusa.
1875	Robert Koch	Establishment pathogenicity rules known as ‘koch’s Postulates; known as the ‘Father of microbial techniques for his outstanding contributions in microbiology
1892	Lwanowski	Demonstrated that TMV could pass through the filters which retain bacterial cells; stated that viruses are smaller than bacteria.
1918	E.J. Butler	Father of modern plant pathology in India-authored ‘fungi and disease in plants; a monograph on pythiaceous and allied fungi; scientific study of several plant diseases
1950	J.C. Luthra	Developed solar treatment of wheat seeds to control loose smut in india
1963	Vanderplank	Establishment epidemiology as a major factor in plant disease development in book plant diseases; Epidemics and control, also reported the occurance of vertical and horizon resistance, in plants against pathogens.
1966	Kassanis	Discovery of satellite virus.
1967-1971	T.O. Diener and Raymer	First to report viroid as a causal agent of potato spindle tuber diseases.

 Modification of legs

Legs :- One pair of legs is located on each of the segments. Legs are important organs of mobility. A typical leg consists of six segment ; the subcoxa is constituted by the pleurites and not a distinct segment; the coxa is the base; the trochanter articulates with the coxa but is fixed with the next segment; the femur is the largest part of the leg; the tibia is only a slender shaft; the tarsus is divided into 2-5 segments and the terminal pretarsus. The tarsus often has pad-like plantulae or pulvilli.

Different segments of the legs are modified in various insect groups, depending upon the mode of life, their habits and the speed of their movement.

Various modifications of legs:-

Saltatorial type- Saltatorial type of legs are found in the grasshopper, Poekilocerus pictus. In this case, the femur is greatly enlarged and init consists strong muscles. Tibia is elongated and is spined dorsally; when it strikes against the ground surface the inner takes a leap in the air.

Raptorial type- Raptorial t ype of legs are the modified fore-legs in the super family Mantoidea. These legs re adapted for catching and holding the prey in between the greatly enlarged and ventrally grooved femur and the blade like curved tibia. The forelegs in the other Embioptera are adapted for web spinning. The metatarsus of foreleg is swollen to accommodate the silk gland, out of which comes the silk thread.

Notatorial types of legs:- The notatorial legs are typical of the giant water bug, Belostoma indica. These legs are flattended and are marginally fringed with dense hair, forming oar-like structures.

Ambulatorial or running type:- Ambulatorial or running type of legs are typical of the cockroach, Periplaneta Americana. The legs are cylimdrical, with well developed coxae and well defined tarsomeres.

Scooping type:- The scooping type of legs are found among dragonflies (sub-order Anisopetra): the legs are long having rows of stiff bristles along the inner margins.

Grasping type:- In the grasping type of legs, found in male giant diving beetle. Dytiscus marginalis, the basal tarsomeres are enlarged, forming circular disc-bearing suction cups on the inner surface. The bladder-footed legs are the characteristic feature of the order Thysanoptera, the common example being the onion thrips, Thrips tabaci ; in these legs the distal tarsomeres bear vesicle to provide a firm hold on to the surface on which the insects feed.

Fossorial type:- Fossorial legs are adapted for digging into the ground and are typically represented as the forelegs of the male cricket, Gryllotalpa spp. In this case, the tibia and tarsi are flattened and are shaped like shovels for digging the soil.

Clinging type:- The clinging type of legs are found in the human louse, Pediculus humanus; here a padded projection in present at the distal end of tibia and the pre tarsus is modified into an enlarged claw-shaped structure which fits on the pad. The forelegs of honeybee, Apis spp. are adapted for antennae-cleaning by a tibial spur which overlaps the bristled notch. The middle pair of legs in honeybees are modified for pollen brushing; there are transverse rows of bristles on the inner side of metatarsi, which are used for brushing the heap. The pollen-collecting legs are also found in honeybees; these are the hind legs in which tibia are modified to form cavities (pollen baskets), fringed with spines on the outer surface, and a concave pollen cavity fringed with spines at the proximal end of metatarsus.

Insect mouth parts

INSECT MOUTH PARTS

Mouth parts of insects vary to a great extend among insects of different groups depending up to their feeding habits. They are mainly of two types viz. Mandibulate (feeding mainly on solid food) and haustellate (feeding on liquid food).

1.    Biting and chewing types:- e.g. cockroach and grasshopper. It primitive type of mouth part and consists of the following parts.

 i.            Labrum:- (upper lip) it is flap like, bi-lobed and attached to the clypeus by an articular membrane. It is movable. It covers the mouth cavity from above. It helps to pull the food in to the mouth. It holds the food in position so that mandibles can act on it. It forms the roof of the pre-oral food cavity.

ii.            Labrum-epipharynx:- inner surface of the labrum is referred to as epipharynx. It is frequently membranous and continuous with the dorsal wall of pharynx. It is an organ of taste.

iii.            Mandibles :- there is a pair of mandibles. They are the pair of jaws. They are also called as primary jaws or true jaws. Mandibles articulate with the cranium at two points. They are heavily sclerotised. They are toothed on their inner border. There are two types of teeth. Distal are sharply pointed and are called incision or cutting teeth and proximal teeth are called molar or grinding teeth. They act transversely to bite and grind the food into small fragments.

iv.            Maxillae :- They are paired and more complicated then mandibles. They are called secondary jaws or accessory jaws. At proximal end the first sclerite cardo joins the maxilla to head. The second sclerite is called strips which articulates with cardo. Stripes carries a lateral sclerite called palpifer which bears a five segmented antennae like maxillary palp. On the distal end of the stripes, there are two lobes. The outer lobe is called galea and inner lobe is lacinia which is toothed. Maxillae direct the food into the mouth. They hold the food in place when the mandibles are in action. They act as auxiliary jaws and assist in mastication of food. Sense organs connected with the perception of touch, smell and taste are abundantly found in palpi.

v.            Hypopharynx:- it is tongue like organ. It is located centrally in the pre-oral cavity. Salivary gland duct opens through it.

vi.            Labium or lower lip:- it is a composite structure formed by the fusion of two primitive segmented appendages. It bounds the mouth cavity from below or behind. It forms the base of the pre-oral cavity. It consists of three median sclerites viz., sub-mentum (large basal sclerite), mentum (middle sclerite) and pre-mentum (apical sclerite). On the lateral side of the pre-mentum there are two small lateral sclerite called palpifer bearing three segmented labial palpi. Distally pre-mentum bears two pairs of lobes. The other pairs of lobes is called paraglossa and inner pair of lobes, glossae. Both pairs when fused are called ligula.

2.    Piercing and sucking/ hemipterous/ bug type: e.g. plant bugs.

Labium projects downwards from the anterior part of the head like a beak. Beak is four segmented and grooved throughout its entire length. At the base of the labium there is a triangular flap like structure called labrum. Labrum is neither involved in piercing nor sucking. It functions as a protective covering for the four stylets found with in the groove. Both mandible and maxillae are modified into long slender sclerotized hair like structure called stylets.  They are lying close together and suited for piercing and sucking. The tips of the stylets may have minutes teeth for piercing the plant tissue. The inner maxillary stylets are doubly grooved on their inner faces. When these are closely opposed they form two canals viz., food canal and salivary canal through sap and saliva are conducted respectively. Saliva contains enzymes or toxins that can distort plant cell wall to permit the stylets to penetrate down and reach phloem for sucking the sap. Both palps are absent.

3.    Piercing and sucking/ dipterous/ mosquito type:- e.g. female mosquito mouthparts of female mosquito consists of an elongate labium which is grooved forming a gutter which encloses six stylets. The stylets are composed of labrum- epipharynx (enclosing the food canal), two maxillae and two mandibles. Both the ends of maxillary stylets and mandibular stylets are seeing like and suited piercing flesh. The stylets are inserted into host’s skin by a strong downward and forward thrust of body. Both mandibles and ,maxillae are reduced in male and they feed on plant nectar and juices of decaying fruits. Female pierces the skin of human being into which it insects saliva containing an anticoagulant (to keep the blood flowing without clotting) and an anesthetic (to keep the victim unaware of the bite) and sucks up the blood. Labium does not pierce but folds up or back as stylets pierce. Maxillae palpi are present.

4.    Chewing and lapping type:- e.g. honey bee.

Labrum and mandible are as in biting and chewing type of mouth parts. But mandibles are blunt and not toothed. They are useful to crush and Sharpe wax for comb building; ingest pollen grains and other manipulative functions. Maxillolabial structure are modified to form the lapping tongue. The tongue unit consists of two galea of maxillae, two labial palpi and elongated flexible hairy glossa of labium. The glossa terminates into a small circular spoon shaped lobe called spoon or flabellum which is useful to lick the necter.

5.    Rasping and sucking:-  e.g. Thrips

Mouth cone consists of labrum, labium and maxillae. There are three stylets derived from two maxillae and left mandible. Right mandible is absent. Stylets are useful to lacerate the plant tissue and the oozing sap is sucking up by the mouth cone. Both maxillary palpi and labial palpi are present.

6.    Mandibulosuctorial type:- e.g. grub of antlion mandibles are elongate sickle shaped and grooved on the inner surface. Each maxilla is elongated and fits against the mandibular groove to form a closed food canal. The body of the insect victim is pierced by the opposing mandibles and fluids are extracted.

7.    Sponging type:- e.g. house fly.

The proboscis is fleshy, elbowed, and retractile and projects downwards from head. The proboscis is differentiated into basal rostrum and distal haustellum. The proboscis consists of labium which is grooved on its anterior surface. Within this groove lie the labrum-epiphraynx (enclosing the food canal) and slender hypopharynx (containing the salivary canal). Mandibles are absent. Maxillae are represented by single segmented maxillary palpi. The end of the proboscis is enlarged, sponge like and two lobed which acts as suction pads. They are called oral discs or labella. The surfaces of labella are transverse by capillary canals called pseudo tracheae which collect the liquid food and convey it to the canal. Labella function as sponging organs and are capable of taking exposed fluids. These insects often spit enzyme containing saliva into solid foods to liquefy them.

8.    Siphoning type:- e.g. moths and butterflies

Mouth parts consists of elongate sucking tube or proboscis. It is formed by two greatly elongated galeae of maxillae which are zippered together by interlocking spines and hooks. Galeae are grooved on their inner surface and when they are fitting together closely they form a suctorial food canal through which the nectar is sucked up. The proboscis is coiled up like watch spring and kept beneath the head when it is not in use. By pumping of blood into galeae, the proboscis is extended. The other mouth parts are reduced or absent except the labial palpi and smaller maxillary palpi

Different types of insect eggs

DIFFERENT TYPES OF EGGS

1.   .Eggs of chrysopa:- They are laid on leaves but raised on stalks which may be few mm  in height. The stalk is produced as a viscous material which fly in  the air

2.   .Eggs of locust/grasshopper:- The eggs are elongated and 8mm long. They are laid in a pod made of cement like frothy secretion. They look like a grain of rice. The pod is held under the ground.

3.   .Eggs of cockroach:- Eggs are laid in glad stone bag like  ootheca . They are somewhat enlarged.

4.   .Eggs of head louse:- The eggs are laid in hairs 200-500 eggs in a day laid by a female. The eggs are 0.9 mm long and are attached by their posterior ends by a hard cementing secretion to the hairs.

5.   .Eggs of musca and drosophila:- Eggs are laid in food . The  eggs are creamish white and a millimeter long.

6.   Eggs of nepa and ranatra:- Eggs of nepa and ranatra lay eggs in floating  vegetation. Respiration horns remains out of water.

7.   Eggs of pieris :- The eggs are spherical and sculptured.

8.   Eggs of earwings:- Female lay a cluster of eggs in under ground chamber and mount guard over them. Eggs are regularly picked up and cleaned by female to remove any fungal spores. The eggs are spherical and white in color.

Types of larvae

TYPES OF LARVAE

Larvae hemimetabolous insect resemble the immature from of these insect are called as nymph to distinguish them from adults .However the most conspicuous difference between hemi or holometabolous larvae is in the development of wings. Larvae amongst holometabolous insect are modified as:-

a.   Protopod larvae:- These larvae develop early in their embryonic development and have the internal system

 poorly developed. They have no legs and segmentation is very poor eg. Parasitic forms of order hymenoptera.

b.   Polypod larvae:-In this type of larvae in addition to the thoracic legs , abdominal prolegs are also present .It is poorly sclerotized and is a relatively inactive from living in close contact with its food eg. Lepidoptera and tenthredinidae.

                                                                              i.            Types of larvae in lapidoptera:-  in lapidoptera larvae are named on the basis of prolegs on different abdominal segments . these are called as caterpillar ,looper ,

·       Caterpillar :-  The larvae has five pairs of prolegs on 3^rd 4^th 5^th 6^th and 10^th abdominal segments eg. Larvae of cabbage butterfly.

·       Semilooper:- The larvae has three pairs prolegs on 5^th ,6^th and 10^th abdominal segment eg. Cabbage semiworm.

·       Looper:- The larvae of looper has two pairs of prolegs on 6^th and 10^th segment eg.  measuring worm.

                                                                           ii.            Tenthredinidae:-The larvae of family tenthredinidae is generally called as pseudocaterpiller. The larvae has six to eight pairs of prolegs in abdominal eg. mustard sawfly larva

                                                                        iii.            Campodeiform larvae:- These larvae have prognathus  head .The body is well sclertized and flattened dorso-ventrally eg. Nauroptera, trichoptera,  and some coleoptera.

                                                                         iv.            Scarabaeiform larva:- These types of larvae are usually short legged , inactive which remain burrowing in wood or soil. The larvae is fat with poolly sclerotized thorax abdomen with C-shaped body eg. Some coleopteran and scrabaeidae.

                                                                            v.            Eucephalous larvae:- They have well sclerotized head capsule. Eg .nematocera , coleopteran.

                                                                         vi.            Hemicephalous larvae:- These type of larvae have reduced head capsule which can be retracted within the thorax . eg. Tipulidae , brachycera.

Insect life cycle

INSECT LIFE CYCLE

Metamorphosis in insects:- the process of hatching from the egg varies greatly in the different groups. After hatching , the insect undergoes growth and during this process, it sheds its skin one or more times. This process is known as moulting and ecdysis, where as the skin is termed as euviae. The interval between the ecdysis are known as stage or stadia, while the farm assumed by an insect during a particular stadium is termed by an insect during a particular stadium is termed as instar. When an insect hatches it is said to be in its first instar. At the end of this stage the first moult occurs and the insects then assumes its second instar and so on. The final instar is the fully mature from which is known as the adult. So one of the most characteristics features in insect development is the marked changes in bodily from and transformation of a newly hatched young insect to the adult stage by a process which is known as metamorphosis. In other words , the young insect, after hatching often passes through a series of moults, undergoes marked changes in body from the  transformation from a young ton fully development and growth is known as metamorphosis.

Type of metamorphosis:- All the insects do not undergo the same series of changes. So on the basis of degree of changes in the body form, the order of insects are often divided into three main groups:-

a.   Ametabolous insects.

b.   Hemimetabolous insects.

c.    Hemimetabolous insects.

Ametabolous:- Ametabolous  are primitive , wingless insect very slight changes are brought about during post embryonic development at each moult and moulting continue even after the adult stage is reached .The newly hatched young insect looks like the adult but is of small size and sexually immature eg. Silverfish, springtails.

Hemimetabolous:-  This term is applied to members of the lower order which pass through a simple metamorphosis. It is often described as direct or incomplete metamorphosis. Pupal stage is absent and the immature insects are referred to as nymphs which usually resemble adults in structure and habit eg. Dragon fly, grasshopper, aphids, thrips.

Holometamorphosis :-This term is applied to members of the higher orders which pass through a complex metamorphosis which is accompanied by a pupal instar.  Immature stages of larvae which differ from adult instar and habits. This metamorphosis is often referred to as indirect or complete metamorphosis eg. Butterfly, ant, honeybee etc.