plant growth regulators, dormancy and its breakage

types of plant growth regulators, their use , dormancy and its breakage and their types

1. Plant density is an important agronomic factor that manipulates the micro- environment of the field and affects the growth, development and yield formation of crops. Inadequate plant stand is one of the most common yield retardants. Competition has negative effect on plant growth which increases with higher plant population. Optimum plant population provides highest crop yield and profit. Number of factors such as crop factors, input factor and management factor affects the optimum plant population. Careful consideration on those factors right from seeding to harvesting, gap filling, defoliation, thinning, weeding must be done for managing plant population and competition.

2. When the planting population is too low, each individual plant may perform at its maximum capacity, but there are not enough plants as a whole to reach the optimum yield. Therefore, the total yield of crops becomes the limiting factor. Also, when there is too much space left between plants, weed growth is promoted, which could increase weeding cost. If the plant population is too high, plants may compete with each other, known as intraspecific competition. Under those conditions, the performance of individual plant becomes a limiting factor for maximum crop yield. When there is high plant population, we can observe certain alternations in the growth of plant ( Mathiew , 2011). Due to high planting density, the plant height is increased because of the competition for light. The thickness of leaves may be also reduced and leaf geometry is altered due to high population pressure. There is reduction in numbers of ears in indeterminate plants and reduction in size of ear panicles in determinate plants which ultimately leads to decrease in yield.

3. Crop response towards plant density Regarding the crop response towards the plant density (population) two types of crop response has been observed. a. Asymptotic response: In several crops like tobacco, leafy vegetables and fodder crops the entire dry matter is utilized as economic product. Hence, in these types of crop the increase in plant population up to a point fetches higher yields but after the peak the response remains constants and there is no increase in yield. Such a response is asymptotic response. b.Parabolic response Crops, like rice, wheat, maize and several other increase their yield with increase in plant population up to a point however after the peak point there is reduction in yield with the increases in plant population. Such a response is termed as parabolic response.

4. Factors affecting the plant population Genetic/ Crop Factor Seed quality Size of plant Elasticity of Plant Forging area Dry matter Partitioning Tillering etc Input/Environmental Factor : Seed rate,Irrigation Nutrient supply Season, Temperature, Sunlight Managerial Factor : Method of planting,Crop geometry Overall Crop husbandry Early/late planting etc

5. Managing plant population Seed quality, Size of seeds and seed rate Quantity of seed sown/unit area, viability and establishment rate decides the planting density. The best seed rate is that which maximizes grain yield. In practice, grain yield hardly changes with further increase in seed rate once the maximum yield is reached. Seed sown above that needed reach the flat part of curve is money wasted. When the more viable seeds are sown per m2 area proportionately less grow into established plants because adjacent seedlings have to compete more for resources. The number of plants established from a given weight of seed depends on the size of seeds and the percentage of those seeds that are viable and can grow into established plants. The common range of wheat size is 25 to 50 mg and crop establishment varies between 40 and 95 percent of sown seeds depending on soil type, soil moisture, sowing depth, seed quality, disease and insects (FAO, 2005). Size of the plant The volume occupied by the plant at the time of flowering decides the spacing of the crop. Plants of red gram, cotton, sugarcane, etc., occupy larger volume of space in the field compared to rice, wheat, ragi etc. Thus, the highly spaced crop has comparatively lower number of plant population as compared to the narrow spaced one. Elasticity of the plant Variation in size or plant between the minimum size of the plant that can produce some economic yield to the maximum size of the plant that can reach under the limited space and resources is the elasticity of the plant. The optimum plant density range is high in indeterminate plants. The elasticity is less and hence the optimum plant density range is small. Generally narrow leaf angle crops and dwarf varieties can be planted more densely than large angled and tall varieties. Mostly, hybrid cultivars are semi-dwarf with narrow leaf angle are planted closely than local cultivars. Tillering It is one of properties of cereal crops that affect the number of plants to be maintained in the field. Wheat, rice has high tillering capacity as compare to maize and has upto 15-20% compensation. The yield of sugarcane is higher in ring method due to higher number of tillers. The optimum plant population is that which has optimum space after tiller has emerged in those crops. If the density is high, crowding will reduce the number of tillers produced by each individual (Weiner, 1993).

6. Generally, optimum seed rate for broadcast crops can be twice that of drilled crops. Lower yield in Broadcasted is due to rough seedbeds, poor seed covering and poor contact between seed and moist soil which is very good in Drilled.

7. the number of seeds to plant from the depth to achieve the number of established plants for optimum yield is virtually double that from shallow seeding. Optimum depth of Planting and yield

8. Time of sowing The most important factors that influence optimum plant density are day length and temperature. Photosensitive varieties respond to the day length resulting in change in size of the plant. As low temperature retards the growth, higher density is established for quicker ground cover. In agronomy, higher seed rate is used in case of late sown condition to maintain the plant population. 3.6.8 Rainfall/irrigation Plant density has to be less under rain-fed than irrigated condition. Under higher plant densities, more water is lost through transpiration. Under adequate irrigation or under evenly distributed rainfall conditions, higher plant density is recommended. 3.6.9 Fertilizer application Higher plant density is necessary to fully utilize higher level of nutrients in the soil to realize higher yield. Nutrient uptake increases with increase in plant density. Higher density under low fertility conditions leads to development of nutrient deficiency symptoms. For example, rice does not respond to plant density without nitrogen application.

9. Gap Filling/transplanting After germination, gap filling transplanting of seeds/seeding keeps the plant population as desired. Gap transplanting rice seedling maintaining 20 cm after 20 days also found the plant population compensated Thinning and defoliation of overpopulated plant density To decrease the intra crop competition, the thinning is the best method to accompany the double planted crops whereas defoliation is done later stage where there are more canopies in the crop at post vegetative stage. The thinning in mustard is done after 7-15 days of germination but if done 15-30 days results in depletion of yield due to increases competition between plants.

10. Effects of plant population on crop and weed biomass. The weeds and plant population has inverse relationship . Weeds growth and population in field is more means it lesser the plant populations which can be optimized by proper weed management. Sparse density increases the weed population whereas optimum population keeps the weeds population under control

11. Interplant competition When plants are grown in a community, growth is affected by neighboring plant, one plant compete with another for the resources like light, nutrients and water. This competition may modify or changes the growth of crop. For example, plants growing under the high density compete for the light and plant height become taller to receive adequate light interception. The thickness of leaves becomes thinner and vertical to receive more light. Crop like rice, wheat would modify their Tillering behavior and their Tillering number is reduced. Similarly the crop like cotton, pigeon pea may have less number of branches per plant under the high density condition. Intra plant competition Intra plant competition is the competition within the plant. When the flower primordia are formed in large numbers, it leads to the formation of large number of inflorescences. The large load for inflorescences leads to the competition for assimilates among the inflorescences and seeds on the same plant. This leads to the intra plant competition. The intra plant competition may be intense at low densities, resulting in a fewer seed and reduced seed size compared with the denser stand. At the widest spacing, both type of competition are absent, during the early stages of growth but at the reproductive stages the intra plant competition may occur because of the large number of reproductive sink developed due to no competition at early stages.

12. Dormancy Seed dormancy is  the state in which seed is unable to germinate, even under ideal growing conditions A. Innate dormancy / primary dormancy It is the state of the seed itself or dormancy induced in the seeds at the time of dispersal from the mother plant i.e. the dormancy may be induced before maturity, during maturity and after maturity but before seed is dispersed from mother plant.

13. B. Secondary dormancy Secondary dormancy can take place only in a matured and imbibed seed by certain environmental conditions, which are unfavourable to germination. (e.g.) Spring wheat and winter barley, the secondary dormancy could be imposed by Exposure of dry barely seed to temperature between 50 and 90 0 C Storage of winter barely for seven days in high moisture containers at 20 0 C. Storage of spring wheat for one day at high moisture content in airtight containers at 50 0 C. Placement of seed under water and in darkness for 1 to 3 days at 2 0 C. Induction of secondary dormancy was possible one and half months after physiological maturity. Secondary dormancy in Spring wheat could not be broken by two weeks of storage. However, it was completely broken by treatment with 0.1% GA3, 0.5 to 1.0 % Ethanol, low temperature stratifications, removal of pericarp and storage at 20 0 C.

14. Primary dormancy  is further classified into  endogenous and exogenous  . Exogenous dormancy  is due to the seed coat factor either due to presence of inhibitors or hard seed nature.  It is further classified into, Physical  – Dormancy is due to the hard seed coat which prevents the entry of water and sometimes gaseous exchange is also prevented. e.g. Hard seeds of pulses, acacias. Prosopis , sapota etc., Chemical  – Presence of some inhibitors in the seeds coat which prevents the germination Mechanical  – restriction of the growth of protruding radicle due to structure. (e.g.) inadequate space in the seeds of  Terminalia  sp. Endogenous dormancy  – Dormancy due to embryo. May be the presence of inhibitors , immature embryo or combination of both. It is further classified into Morphological  – Due to immature embryo, which is not able to putforth germination even under favourable conditions . (e.g.) Apple Physiological  – Due to arrest of the metabolic activity in the seeds due to presence of some inhibitors like ABA, coumarines ,  phenols etc., Morphophysiological  – Combination of immature embryo with inhibitors.

15. Secondary Dormancy Whose germination is inhibited , fail to recover even when the inhibitory factor is removed. Adoptive mechanism to pass the adverse environmental condition. Types of secondary dormancy Thermo  – Dormancy due to temperature  Skoto  – Light; Photo – Quality of light and Osmotic – stress or high osmotic stress prevents germination According to Harper (1977) dormancy may be classified into following, Nature of origination i . innate  ii. Induced   iii. Enforced Time of origin   i . Primary     ii. Secondary Location of dormancy   i . Exogenous   ii. Endogenous    iii. Combined

16. Advantages of dormancy Storage life of seed is prolonged  Seed can pass through adverse situation  Prevents the  in situ  germination. Disadvantages Long periods of time needed to overcome dormancy (for uniform germination) Contributes to longevity of weed seed. While raising a crop it is very difficult to maintain the population in the field with dormant seed lot

17. Dormancy breaking treatments Physical dormancy: 1. Scarification Any treatments may be physical or chemical that weakens or softens the seed coat is known as scarification. This method is more applicable to Malvaceae and Leguminaceae group of seeds. a) Acid scarification  By using concentrated H2SO4 @ 100 ml/kg of seed for 2-3 minutes treatments dormancy can be overcome in the above group of seeds. The duration of treatment will vary and it depends on type and nature of seed coat.  E.g.  Tree crops 1-3 hours, Rose seeds, treat the seed partially with acid and then given with warm stratification. b) Mechanical scarification  Seeds are rubbed on a sand paper or with a help of mechanical scarifier or by puncturing on seed coat with the help of needle to enhance / increase the moisture absorption by seeds.  E.g.  Bitter gourd for sand scarification, sand and seed 2:1 ratio should be followed. Rub against hard surface of seed for 5 to 10 minutes. 2. Hot water treatments  It is effective in case of leguminous tree crop seeds. The seeds should be soaked in boiled water for 1-5 minutes for 60-80 minutes. Some crops like Bengal gram and Groundnut, hot water treatment for more than 1 minute is found injurious to seed. 3. Stratification treatment When seed dormancy is due to embryo factor, seeds can be subjected to stratification treatments. a) Cold stratification  Incubate the seed at low temperature of 0-5 0 C over a moist substratum for 2-3 days to several months. It depends on the nature of seed and kind of dormancy. (e.g.) Cherry and oil palm seeds. b) Warm stratification  Some seeds require temperature of 40-50 0 C for few days e.g. paddy. In case of oil palm it requires temperature of40-50 0 C for 2 months for breaking dormancy. Care should be taken during the treatment and moisture content of seed should not be more than 15%.

18. 4. Leaching of metabolites (Inhibitors) The seeds can be soaked in water for 3 days. But once in 12 hours fresh water should be changed to avoid fermentation or seeds can be soaked in running water for a day to leach out the inhibitors. (e.g.) Coriander ( Coumarin ), Sunflower (Hydrocyanic acid) 5. Temperature treatments a) Low temperature treatments  Plants which grow in temperate and cooler climates, require a period of chilling for breakage of dormancy.  E.g.  Apple seed dormancy can be released by low temperature treatment by storing the seeds at 5 0 C. b) High temperature treatment  Normally high temperature treatments are exhibited by early flowering "winter " annuals.  E.g.  Blue bell ( Hyacinthoides nonscripta ). Their seeds are shed in early summer and do not germinate until they have been exposed to the heat during high summer. c ) Alternate temperature treatments   Most of the plant species which grow in temperate and cool temperate regions require alternate temperature for breakage of dormancy (e.g . ) Bull rush ( Typha ). d) Fire treatment  Many shrubs and trees of sub tropical and semi-arid regions have extremely hard seeds in which the seed coat is very impervious to water. Dormancy in such seeds is clearly coat imposed, and maybe broken by exposure to extreme heat such as fire.  E.g.  Seeds of  Calluna vulgaris -  dormancy is broken by fire. 6. Light and phytochrome         7. Promoters - inhibitors concept For regulation of germination the promoters and inhibitors present in the seed should be in a balanced manner. • GA helps in translocation of food reserve materials to active site of meristematic activity. GA also helps in cell division. • Cytokinin is a natural endogenous hormone which controls germination through DNA to RNA transcription system. • Abscisic acid is an inhibitor that can prevent germination by affecting RNA synthesis.

19. Seed Treatment with Growth Regulators/Chemicals If the endogenous dormancy is due to the presence of inhibitors, we can apply growth regulators at the low level to break dormancy. GA & Cytokinin and kinetin can be used at concentration of 100-1000 ppm to break dormancy. GA is light substituting chemical. KNO3 2% can be used for breaking the dormancy of light requiring seeds (e.g.) Oats, Barley and Tomato. Thiourea can be used for breaking dormancy for both light and chilling treatment requiring seeds (e.g.) lettuce - thiourea @ 10-2 to 10-3 M is used. Ethrel can be used for breaking the dormancy of cotton seed. The dormancy in cotton seed is due to the presence of ABA in pericarp of seed. Nitrogenous compounds like Thiourea , Hydroxylamine, Nitric acid, Nitrtate , can also be used for breaking dormancy. Sulphidral compounds like 2 mercapto ethanol and 2,3 dimercapto ehtanol can also be used. Plant products like strigol (root exudation from striga parasite host plant) can also be used for breaking the seed dormancy. Infra red radiation treatment Infra red rays can be passed on to the seeds and dormancy can be released. Pressure treatment Dormant seeds can be kept in autoclave and required pressure can be employed for breaking dormancy. Magnetic seed treatment Seeds can be kept in the magnetic field for about 1 to 10 days for breaking dormancy

20. PLANT GROWTH AND DEVELOPMENT Growth may be defined as an irreversible permanent increase in size, volume or mass of a cell or organ or whole organism accompanied by an increase in dry weight. Types of growth Primary and secondary growth-. The mitotic division of meristematic cells present at the root and shoot apex increases the length of the plant body. This is called the primary growth. The secondary meristem increases the diameter of the plant body and it is called the secondary growth. Unlimited Growth- The root and the shoot system of plants grow continuously from germination stage to the death or throughout the life span of the plant. It is called ‘Unlimited’ or ‘indeterminate’ type of growth. Limited growth - The leaves, fruits and flowers stop growing after attaining certain size . This is called ‘limited’ or ‘determinate’ type of growth. Vegetative growth- The earlier growth of plant producing leaves, stem and branches before initiation of flowers is called ‘vegetative growth’/ Phase. Reproductive growth - After the vegetative growth, plants produce flowers which is the reproductive part of the plant. This is called reproductive growth/phase.

21. Growth curve – It is an ‘S’ shaped curve obtained when we plot growth against Time. It is also called ‘ sigmoid ‘curve. This curve mainly shows four phases of growth- 1.initial slow growth ( Lag phase), 2.The rapid period of growth (log phase/grand period of growth/exponential phase) where maximum growth is seen in a short period and 3. The diminishing phase where growth will be slow and 4.Stationary / steady phase where finally growth stops.

22. The three phases of cell growth are cell division, cell enlargement and cell differentiation. The first two stages increase the size of the plant cell while the 3 rd stage brings maturity to the cells. Differentiation, is a process during which cells undergoes structural changes in the cell wall and protoplasm . A differentiated cell cannot divide. Development- Development is defined as sum total of growth and differentiation. Development is governed by both environmental and internal factors. One of the internal factors that regulate growth and development is ‘plant hormones’.

23. Plant hormones/ phytohormones / Growthregulators Plant hormones are chemically diverse substances produced in minute quantity and they involve in most of the plant cell activities. They regulate the growth process both by promoting and inhibiting growth. They are produced in certain parts of the plant and transported to other tissues where their action is shown. Based on their promotory and inhibitory activity they have been classified into growth promoters and growth inhibitors.

24. Plant growth regulators (PGRs) The plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition. They could be indole compounds (indole-3-acetic acid, IAA); adenine derivatives (N6-furfurylamino purine, kinetin),derivatives of carotenoids ( abscisic acid, ABA); terpenes (gibberellic acid,GA3) or gases (ethylene, C2H4). Plant growth regulators are variously described as plant growth substances, plant hormones or phytohormones in literature. The PGRs can be broadly divided into two groups based on their functions in a living plant body. One group of PGRs are involved in growth promoting activities, such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation. These are also called plant growth promoters, e.g., auxins, gibberellins and cytokinins . The PGRs of the other group play an important role in plant responses to wounds and stresses of biotic and abiotic origin. They are also involved in various growth inhibiting activities such as dormancy and abscission. The PGR abscisic acid belongs to this group The gaseous PGR, ethylene, could fit either of the groups, but it is largely an inhibitorof growth activities.

25. Growth promoters Auxins , Gibberellins, cytokinins Cell division. cell enlargement Tropic movement Fruiting and flowering Seed germination Root formation Growth inhibitors Ethylene, Dormin or abscissic acid Induces senescence. Induces dormancy of seeds

26. Auxins Auxins (from Greek ‘ auxein ’ : to grow) was first isolated from human urine. The term ‘auxin’ is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth regulating properties. They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. Auxins like IAA and indole butyric acid (IBA) have been isolated from plants. NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) are synthetic auxins. All these auxins have been used extensively in agricultural and horticultural practices. They help to initiate rooting in stem cuttings, an application widely used for plant propagation. Auxins promote flowering e.g. in pineapples. They help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits. In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance. Removal of shoot tips (decapitation) usually results in the growth of lateral buds It is widely applied in tea plantations, hedge-making. Auxins also induce parthenocarpy , e.g., in tomatoes. They are widely used as herbicides. 2, 4-D, widely used to kill dicotyledonous weeds, does not affect mature monocotyledonous plants. It is used to prepare weed-free lawns by gardeners. Auxin also controls xylem differentiation and helps in cell division.

27. Gibberellins Gibberellins are another kind of promotory PGR. There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as GA1, GA2, GA3 and so on. However, Gibberellic acid (GA3) was one of the first gibberellins to be discovered and remains the most intensively studied form. All GAs are acidic. They produce a wide range of physiological responses in the plants. Their ability to cause an increase in length of axis is used to increase the length of grapes stalks. Gibberellins, cause fruits like apple to elongate and improve its shape. They also delay senescence. Thus, the fruits can be left on the tree longer so as to extend the market period. GA3 is used to speed up the malting process in brewing industry. Sugarcane stores carbohydrate as sugar in their stems. Spraying sugarcane crop with gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre. Spraying juvenile conifers with GAs hastens the maturity period, thus leading to early seed production. Gibberellins also promotes bolting (internode elongation just prior to flowering) in beet, cabbages and many plants with rosette habit.

28. Cytokinins Cytokinins have specific effects on cytokinesis, and were discovered as kinetin (a modified form of adenine, a purine). Kinetin does not occur naturally in plants. Search for natural substances with cytokinin -like activities led to the isolation of zeatin from corn-kernels and coconut milk. Since the discovery of zeatin , several naturally occurring cytokinins , and some synthetic compounds with cell division promoting activity, have been identified. Natural cytokinins are synthesised in regions where rapid cell division occurs, for example, root apices, developing shoot buds, young fruits etc. It helps to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation. Cytokinins help to overcome the apical dominance. They promote nutrient mobilisation which delays leaf senescence.

29. Ethylene Ethylene is a simple gaseous PGR. It is synthesised in large amounts by tissues undergoing senescence and ripening fruits. Influences of ethylene on plants include horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings. Ethylene promotes senescence and abscission of plant organs especially of leaves and flowers. Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of the fruits. This rise in rate of respiration is called respiratory climactic . Ethylene breaks seed and bud dormancy, initiates germination in peanut seeds, sprouting of potato tubers. Ethylene promotes rapid internode/petiole elongation in deep water rice plants. It helps leaves/ upper parts of the shoot to remain above water. Ethylene also promotes root growth and root hair formation, thus helping the plants to increase their absorption surface. Ethylene is used to initiate flowering and for synchronising fruit-set in pineapples. It also induces flowering in mango. Since ethylene regulates so many physiological processes, it is one of the most widely used PGR in agriculture. The most widely used compound as source of ethylene is ethephon . Ethephon in an aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly. Ethephon hastens fruit ripening in tomatoes and apples and accelerates abscission in flowers and fruits (thinning of cotton, cherry, walnut). It promotes female flowers in cucumbers thereby increasing the yield.

30. Abscisic acid As mentioned earlier, abscisic acid (ABA) was discovered for its role in regulating abscission and dormancy. But like other PGRs, it also has other wide ranging effects on plant growth and development. It acts as a general plant growth inhibitor and an inhibitor of plant metabolism. ABA inhibits seed germination. ABA stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses. Therefore, it is also called the stress hormone. ABA plays an important role in seed development, maturation and dormancy. By inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth. In most situations, ABA acts as an antagonist to GAs. We may summarise that for any and every phase of growth, differentiation and development of plants, one or the other PGR has some role to play. Such roles could be complimentary or antagonistic. These could be individualistic or synergistic.

31. Similarly, there are a number of events in the life of a plant where more than one PGR interact to affect that event, e.g., dormancy in seeds/ buds , abscission, senescence , apical dominance, etc.

32. Commercial application of Plant growth regulators in Agriculture and Horticulture A) Auxins a) IBA (@250 ppm) and NAA were found to increase root development in the propagation of stem cuttings. b) 2,4-dichlorophenoxy acetic acid (2,4-D) stimulates excessive uncontrolled growth in broad leaf plants for which it is used as a herbicide. c) Application of NAA ( Napthalene Acetic Acid) reduces flower and fruit drop in Mango. d) NAA application brings uniform flowering and fruit set by inducing ethylene formation in Pineapple. e) NAA application @ 10-100 ppm during fruit setting period controls boll shedding in cotton crop.

33. B) Gibberellins: a) GA is used extensively on seedless grape varieties to increase the size and quality of the fruit. Pre- bloom spray of 20 ppm induces rachis of the fruit cluster to elongate. This creates looser clusters that are less susceptible to disease during the growing season. b) GA is used to increase the yield of barley malt and to decrease the time required for this process to occur. Application of GA to germinating barley supplements the endogenous content of this hormone and accelerates the production and release of hydrolytic enzymes. They can easily degrade the stored carbohydrates. c) Foliar spray of GA3, at 100 ppm during panicle initiation stage enhances the panicle exertion and increases seed weight and yield in hybrid rice. d) GA has also has been used to control flower sex expression in cucumbers and squash. GA application tends to promote maleness in these plants. e) Gibberellic acid is also applied to citrus crops, though the actual use depends on the particular crop. For example GA3 is sprayed onto oranges and tangerines to delay or prevent rind-aging, so that fruit can be harvested later without adverse effects on rind quality and appearance. For lemons and limes, GA3 synchronizes ripening and enhances fruit size. f) Gibberellic acid is used extensively to increase the sucrose yield of sugarcane. Sugarcane, a normally fast-growing C4 member of the Poaceae (grass) family, is sensitive to cooler winter temperatures, which reduce internode elongation and subsequent sucrose yield. The adverse effects of cooler temperatures can be counteracted by the application of GA3.

34. C) Ethylene: a) Because ethylene regulates so many physiological processes in the plant development, it is the most widely used plant hormones in agriculture. Auxins and ACC can trigger the natural biosynthesis of ethylene and in several cases are used in agricultural practice. b) Because of its high diffusion rate, ethylene is very difficult to apply in the field as a gas, but this limitation can be overcome if an ethylene releasing compound is used. The most widely used such compound is ethephon or 2chloro ethyl phosphonic acid (CEPA) (trade name: ethrel ). c) Ethrel @ 100-250 ppm sprayed at 2-3 leaf stage induce femaleness in cucumber and melons. d) It helps in degreening of citrus and banana which increases its market acceptability. e) Storage facilities developed to inhibit the ethylene production and promote preservation of fruits have a controlled atmosphere of low 02 concentration and low temperature that inhibits ethylene biosynthesis. A relatively concentration of CO2 (3-5%) prevents ethylene action as a ripening promoter.

35. D) Other growth regulators: AMO 1618 (a quaternary ammonium salt) is used in the cultivation of ornamental plants and causes a bushy shape and a sturdy growth of the treated plants. paclobutrazol : Reduces the problem of biennial bearing in Mango. Mepiquat chloride, Chlormequat chloride ( Cycocel ) : used in ornamental plants for shorter internodes and thicker stems (used in poinsettias), it also prevents lodging and increases tillering in cereals. Malichydrazide (MH): prevents premature sprouting of onion and potato. 2,3,5-T or Triiodo benzoic acid (TIBA): Increases flowering in chrysanthemum

36. Essential Nutrients There are 18 essential nutrients for plant growth: 3 structural, 6 macronutrients, and 9 micronutrients Macronutrients Plants need large amounts of   macronutrient s  relative to other essential nutrients. Three structural are: Carbon (C), Hydrogen (H), Oxygen (O) Six macronutrients are from soil: Nitrogen (N), Phosphorus (P), Potassium (K), Magnesium (Mg), Calcium (Ca), Sulfur (S) Micronutrients Compared with macronutrients, the concentrations of the nine   micronutrients  in plants may be very small. This does not mean they are not important. Deficiencies or excesses of the micronutrients can cause yield loss just as macronutrient deficiencies or toxicities. Micronutrients are often considered enzyme nutrients: All micronutrients are from soil Six micronutrients are cations: Copper (Cu), Manganese ( Mn ), Iron (Fe), Zinc (Zn), Nickel (Ni), Cobalt (Co) Three micronutrients are anions: Chloride (Cl), Boron (B), Molybdenum (Mo).

37. Criteria of Essentiality The term essential mineral element (or mineral nutrient) was proposed by Arnon and Stout (1939). They concluded three criteria must be met for an element to be considered essential.  Three Criteria for Essentiality of Nutrients: A plant cannot complete its life-cycle in the absence of the element The action of the element must be specific, with no other element being able to completely substitute for it The element must be shown to be directly involved in the nutrition of the plant. It must be a constituent of a metabolic pathway or at least be required for the activity of an essential enzyme.

38. Classification of plant nutrients on the basis of their availability.

39. Macronutrients Nitrogen  is a major component of proteins, hormones, chlorophyll, vitamins and enzymes essential for plant life. Nitrogen metabolism is a major factor in stem and leaf growth (vegetative growth). Too much can delay flowering and fruiting. Deficiencies can reduce yields, cause yellowing of the leaves and stunted growth. Phosphorus  is necessary for seed germination, photosynthesis, protein formation and almost all aspects of growth and metabolism in plants. It is essential for flower and fruit formation. Low pH (<4) results in phosphate being chemically locked up in organic soils. Deficiency symptoms are purple stems and leaves; maturity and growth are retarded. Yields of fruit and flowers are poor. Premature drop of fruits and flowers may often occur. Phosphorus must be applied close to the plant's roots in order for the plant to utilize it. Large applications of phosphorus without adequate levels of zinc can cause a zinc deficiency. Potassium  is necessary for formation of sugars, starches, carbohydrates, protein synthesis and cell division in roots and other parts of the plant. It helps to adjust water balance, improves stem rigidity and cold hardiness, enhances flavor and color on fruit and vegetable crops, increases the oil content of fruits and is important for leafy crops. Deficiencies result in low yields, mottled, spotted or curled leaves, scorched or burned look to leaves. Sulfur  is a structural component of amino acids, proteins, vitamins and enzymes and is essential to produce chlorophyll. It imparts flavor to many vegetables. Deficiencies show as light green leaves. Sulfur is readily lost by leaching from soils and should be applied with a nutrient formula. Some water supplies may contain Sulfur. Magnesium  is a critical structural component of the chlorophyll molecule and is necessary for functioning of plant enzymes to produce carbohydrates, sugars and fats. It is used for fruit and nut formation and essential for germination of seeds. Deficient plants appear chlorotic, show yellowing between veins of older leaves; leaves may droop. Magnesium is leached by watering and must be supplied when feeding. It can be applied as a foliar spray to correct deficiencies. Calcium  activates enzymes, is a structural component of cell walls, influences water movement in cells and is necessary for cell growth and division. Some plants must have calcium to take up nitrogen and other minerals. Calcium is easily leached. Calcium, once deposited in plant tissue, is immobile (non- translocatable ) so there must be a constant supply for growth. Deficiency causes stunting of new growth in stems, flowers and roots. Symptoms range from distorted new growth to black spots on leaves and fruit. Yellow leaf margins may also appear.

40. Micronutrients Iron  is necessary for many enzyme functions and as a catalyst for the synthesis of chlorophyll. It is essential for the young growing parts of plants. Deficiencies are pale leaf color of young leaves followed by yellowing of leaves and large veins. Iron is lost by leaching and is held in the lower portions of the soil structure. Under conditions of high pH (alkaline) iron is rendered unavailable to plants. When soils are alkaline, iron may be abundant but unavailable. Applications of an acid nutrient formula containing iron chelates, held in soluble form, should correct the problem. Manganese  is involved in enzyme activity for photosynthesis, respiration, and nitrogen metabolism. Deficiency in young leaves may show a network of green veins on a light green background similar to an iron deficiency. In the advanced stages the light green parts become white, and leaves are shed. Brownish, black, or grayish spots may appear next to the veins. In neutral or alkaline soils plants often show deficiency symptoms. In highly acid soils, manganese may be available to the extent that it results in toxicity. Boron  is necessary for cell wall formation, membrane integrity, calcium uptake and may aid in the translocation of sugars. Boron affects at least 16 functions in plants. These functions include flowering, pollen germination, fruiting, cell division, water relationships and the movement of hormones. Boron must be available throughout the life of the plant. It is not translocated and is easily leached from soils. Deficiencies kill terminal buds leaving a rosette effect on the plant. Leaves are thick, curled and brittle. Fruits, tubers and roots are discolored, cracked and flecked with brown spots. Zinc  is a component of enzymes or a functional cofactor of a large number of enzymes including auxins (plant growth hormones). It is essential to carbohydrate metabolism, protein synthesis and internodal elongation (stem growth). Deficient plants have mottled leaves with irregular chlorotic areas. Zinc deficiency leads to iron deficiency causing similar symptoms. Deficiency occurs on eroded soils and is least available at a pH range of 5.5 - 7.0. Lowering the pH can render zinc more available to the point of toxicity.

41. Copper  is concentrated in roots of plants and plays a part in nitrogen metabolism. It is a component of several enzymes and may be part of the enzyme systems that use carbohydrates and proteins. Deficiencies cause die back of the shoot tips, and terminal leaves develop brown spots. Copper is bound tightly in organic matter and may be deficient in highly organic soils. It is not readily lost from soil but may often be unavailable. Too much copper can cause toxicity. Molybdenum  is a structural component of the enzyme that reduces nitrates to ammonia. Without it, the synthesis of proteins is blocked and plant growth ceases. Root nodule (nitrogen fixing) bacteria also require it. Seeds may not form completely, and nitrogen deficiency may occur if plants are lacking molybdenum. Deficiency signs are pale green leaves with rolled or cupped margins. Chlorine  is involved in osmosis (movement of water or solutes in cells), the ionic balance necessary for plants to take up mineral elements and in photosynthesis. Deficiency symptoms include wilting, stubby roots, chlorosis (yellowing) and bronzing. Odors in some plants may be decreased. Chloride, the ionic form of chlorine used by plants, is usually found in soluble forms and is lost by leaching. Some plants may show signs of toxicity if levels are too high.

42. Nickel  has just recently won the status as an essential trace element for plants according to the Agricultural Research Service Plant, Soil and Nutrition Laboratory in Ithaca, NY. It is required for the enzyme urease to break down urea to liberate the nitrogen into a usable form for plants. Nickel is required for iron absorption. Seeds need nickel in order to germinate. Plants grown without additional nickel will gradually reach a deficient level at about the time they mature and begin reproductive growth. If nickel is deficient plants may fail to produce viable seeds. Sodium  is involved in osmotic (water movement) and ionic balance in plants. Cobalt  is required for nitrogen fixation in legumes and in root nodules of nonlegumes . The demand for cobalt is much higher for nitrogen fixation than for ammonium nutrition. Deficient levels could result in nitrogen deficiency symptoms. Silicon  is found as a component of cell walls. Plants with supplies of soluble silicon produce stronger, tougher cell walls making them a mechanical barrier to piercing and sucking insects. This significantly enhances plant heat and drought tolerance. Foliar sprays of silicon have also shown benefits reducing populations of aphids on field crops. Tests have also found that silicon can be deposited by the plants at the site of infection by fungus to combat the penetration of the cell walls by the attacking fungus. Improved leaf erectness, stem strength and prevention or depression of iron and manganese toxicity have all been noted as effects from silicon. Silicon has not been determined essential for all plants but may be beneficial for many.

43. Visual symptoms of deficiency and toxicity Element/status Visual symptoms Nitrogen (N)  Deficiency Light green leaf and plant color with the older leaves turning yellow, leaves that will eventually turn brown and die. Plant growth is slow, plants will be stunted, and will mature early. Excess Plants will be dark green in color and new growth will be succu­lent; susceptible if subjected to disease and insect infestation; and subjected to drought stress, plants will easily lodge. Blos­som abortion and lack of fruit set will occur. Ammonium toxicity Plants fertilized with ammonium-nitrogen (NH4- N) may exhibit ammonium-toxicity symptoms, with carbohydrate depletion and reduced plant growth. Lesions may occur on plant stems, there may be a downward cupping of the leaves, and a decay of the conductive tissue at the base of the stem with wilting of the plants under moisture stress. Blossom-end rot of fruit will occur and Mg deficiency symptoms may also occur. Phosphorus (P) Deficiency Plant growth will be slow and stunted, and the older leaves will have a purple coloration, particularly on the underside. Excess Phosphorus excess will not have a direct effect on the plant but may show visual deficiencies of Zn, Fe, and Mn . High P may also interfere with the normal Ca nutrition, with typical Ca deficiency symptoms occurring. Potassium (K) Deficiency On the older leaves, the edges will look burned, a symptom known as scorch. Plants will easily lodge and be sensitive to disease infestation. Fruit and seed production will be impaired and of poor quality. Excess Plants will exhibit typical Mg, and possibly Ca deficiency symp­toms due to a cation imbalance

44. Calcium (Ca) Deficiency The growing tips of roots and leaves will turn brown and die. The edges of the leaves will look ragged as the edges of emerging leaves stick together. Fruit quality will be affected with the occur­rence of blossom-end rot on fruits. Excess Plants may exhibit typical Mg deficiency symptoms, and when in high excess, K deficiency may also occur. Magnesium (Mg) Deficiency Older leaves will be yellow in color with interveinal chlorosis (yellowing between the veins) symptoms. Plant growth will be slow and some plants may be easily infested by disease. Excess Results in a cation imbalance showing signs of either a Ca or K deficiency. Sulfur (S) Deficiency A general overall light green color of the entire plant with the older leaves being light green to yellow in color as the deficiency intensifies. Excess A premature senescence of leaves may occur.

45. Boron (B) Deficiency Abnormal development of the growing points ( meristematic tis­sue) with the apical growing points eventually becoming stunted and dying. Rowers and fruits will abort. For some grain and fruit crops, yield and quality is significantly reduced. Excess Leaf tips and margins will turn brown and die. Chlorine ( Cl ) Deficiency Younger leaves will be chlorotic and plants will easily wilt. For wheat, a plant disease will infest the plant when Cl is deficient. Excess Premature yellowing of the lower leaves with burning of the leaf margins and tips. Leaf abscission will occur and plants will easily wilt. Copper (Cu) Deficiency Plant growth will be slow and plants stunted with distortion of the young leaves and death of the growing point. Excess An Fe deficiency may be induced with very slow growth. Roots may be stunted. Iron (Fe) Deficiency Interveinal chlorosis will occur on the emerging and young leaves with eventual bleaching of the new growth. When severe, the entire plant may be light green in color. Excess A bronzing of leaves with tiny brown spots on the leaves, a typical symptom frequently occurring with rice.

46. Manganese ( Mn ) Deficiency Interveinal chlorosis of young leaves while the leaves and plants remain generally green in color. When severe, the plants will be stunted. Excess Older leaves will show brown spots surrounded by a chlorotic zone and circle. Molybdenum (Mo) Deficiency Symptoms will frequently appear similar to N deficiency. Older and middle leaves become chlorotic first, and In some instances, leaf margins are rolled and growth and flower formation are restricted. Excess Not of common occurrence. Zinc (Zn) Deficiency Upper leaves will show inter veinal chlorosis with an eventual whitening of the affected leaves. Leaves may be small and dis­torted with a rosette form. Excess An Fe deficiency will develop.

47. Hidden Hunger Hidden hunger  refers to a situation in which a crop needs more of a given nutrient yet has shown no deficiency symptoms. The nutrient content is above the deficiency symptom zone but still considerably needed for optimum crop production. With most nutrients on most crops, significant responses can be obtained even though no recognizable symptoms have appeared.

48. Crop growth rate (CGR) Crop growth rate (CGR) is the gain in dry matter production on a unit of land in a unit of time. Formula:- 𝐶𝐺𝑅= 𝑊2−𝑊1 𝑡2−𝑡1 Where, W1 = dry weight per unit area at t1, W2 = dry weight per unit area at t2 t1 = first sampling, t1 = second sampling. Unit: - g m-2 day-1 Example: - Calculate the CGR from following data: Dry weight of groundnut at t1= 200 g m-2 (W1), Dry weight of groundnut at t2 = 300 g m-2 (W2), Time interval of sampling (t2-t1) = 10 days Solution:- 𝐶𝐺𝑅= 300−200 = 100 = 10 g m-2 day-1 10 10 CGR of 20 g m-2 day-1 (200 kg ha-1 day-1) is considered respectable for more crops; particularly C3 types.CGR of 30 g m-2 day-1 (300 kg ha-1 day-1) is obtainable from C4 types such as maize and sorghum (Gardener et al 1988). Crop growth rate is affected by a range of factors including temperature, levels of solar radiation, water and nutrient supply, crop, cultivar and its age. These factors influence the size and efficiency of leaf canopy and hence the ability of crop to convert solar energy into economic growth.

49. Relative growth rate (RGR) Since CGR is an absolute measure of growth; similar values could be expected for crops of different initial weights. The RGR expresses the dry weight increase in time interval in relation to the initial weight. In practical situations, the mean RGR is calculated from measurements at t1 and t2 Formula:- 𝑅𝐺𝑅 = log𝑒𝑊2−log𝑒𝑊1 𝑡2−𝑡1 Where, W1 = dry weight per unit area at t1, W2 = dry weight per unit area at t2 t1= first sampling, t2 =second sampling. Unit: - g g -1 day-1 Example: - Calculate the RGR from following data: Dry weight of groundnut at t1= 5 g (W1), Dry weight of groundnut at t2 = 10 g (W2), Time interval of sampling (t2-t1) = 7 days Solution:- 𝑅𝐺𝑅=log𝑒10−log𝑒57=2.3026−1.60947=0.69327 = 0.099 g g -1 day-1 The RGR generally, begins slowly just after germination, reaches high values soon after and then falls off (Hunt 1978). Calculation of RGR is only really useful for short harvest intervals where growth is assumed to be linear and for comparisons under similar environmental conditions (between treatments within a dial).

50. Net assimilation rate (NAR):- Net assimilation rate (NAR) or unit leaf rate is the net gain of assimilate per unit of leaf area and time. Formula:- 𝑁𝐴𝑅=(𝑊2−𝑊1)(log𝑒𝐿𝐴2−log𝑒𝐿𝐴1)/(𝑡2−𝑡1)(𝐿𝐴2−𝐿𝐴1) Where, W1 = dry weight per unit area at t1, W2 = dry weight per unit area at t2 LA1 = leaf area at t1, LA2 = leaf area at t2 t1= first sampling, t1 =second sampling, Unit: - g m-2 week-1 or g m-2 day-1 Example: - Calculate the NAR from following data: W1 = 450 g, W2 = 570 g, LA1 = 3.5 m2, LA2 = 4.0 m2, t2-t1= 2 week 6 Solution:- 𝑁𝐴𝑅=(570−450)(1.3863−1.2528)/2 (4.0−3.5)=1202×0.13350.5 = 16.02 g m-2 week-1 NAR expresses plant's capacity to increase dry weight in terms of the area of its assimilatory surface. The term, therefore, represents photosynthetic efficiency in the overall sence and in connection with LAR and RGR it can be used lo analyse the response of plant growth to environmental conditions. As Watson (1956) explains, NAR does not measure real photosynthesis, since it represents the net result of photosynthetic gain over respiratory loss and may, therefore, vary according to the magnitude of respiration.

51. Leaf area index (LAI) Leaf area index (LAI) is the ratio of leaf area to the area of ground. It is the leaf area (one surface only) divided by the land occupied by the plants. It is a unit less figure. Formula:- 𝐿𝐴𝐼=LA/𝐺𝐴 Where, LA= Leaf area, GA=Ground area Example: - Calculate the leaf area index from given data: Leaf area of 80 plants in 7500 cm2 was 2,494.58 cm2 Solution:- 𝐿𝐴𝐼=2494.58/7500 = 0.33 For maximum production of dry matter of most crops, LAI of 3 to 5 is usually necessary. Forage crops, such as grasses, with erectophile (upright) leaf orientation may require 8-10 LAI under favorable conditions to maximize light interception. Higher LAI is also required where total biomass, not the economic yield, is the objective (forage crops). LAI with 95 per cent solar radiation interception has been adopted as the critical LAI by most crop physiologists. The LAI at maximum CGR is called optimum LAI, because the CGR decreases as the LAI increase beyond the optimum.

52. Leaf area duration (LAD) Leaf area duration (LAD) expresses the magnitude and persistence of leaf area or leafiness during the period of crop growth. It reflects the extent or seasonal integral of light interception. Formula:- 𝐿𝐴𝐷=(𝐿𝐴2+𝐿𝐴1)(𝑡2−𝑡1)2 Where, LA1 and LA2 are leaf areas at times t1 and t2 respectively. Unit: - cm2 d-1 If the LAI declines rapidly it may restrict growth

53. Leaf area ratio (LAR) Leaf area ratio (LAR) is the ratio of the total leaf area to the whole plant dry weight and is a further measure of the efficiency of leaf surface in producing dry matter. Formula:- 𝐿𝐴𝑅= (𝐿𝐴1/𝑊1)+(𝐿𝐴2/𝑊2) 2 Unit: - m2 g-1 Example: - Calculate the LAR from following data: W1 = 450 g, W2 = 570 g, LA1 = 3.5 m2, LA2 = 4.0 m2 Solution:- 𝐿𝐴𝑅={(3.5/450)+(4.0/570)}/2={(0.008)+(0.007)}/2=0.0152 = 0.0075 m2 g-1

54. Growth curves sigmoid, polynomial and asymptotic Sigmoid growth curve: - Typical growth pattern of an annual plant is represented in a ‘S’ shaped curve.This can be divided into three phases. I. Lag period of growth: During this period the growth rate is quite slow because it is the initial stage of growth. II. Log period of Growth: During this period, the growth rate is maximum and reaches the top because at this stage the cell division and physiological processes are quite fast. III. Senescence period or steady state period: During this period the growth is almost complete and become static. Thus the growth rate becomes zero.

55. A typical sigmoid growth curve

56. Polynomial growth curve: - The parabolic response curve is typically a flat-topped one with decrease in grain yield on both sides of an optimum (Figure 2.2). The curve could be fitted by a quadratic equation y= a+ bx +cx2 Where, y= Yield per unit area, x= Plant population, and 10 a ,b and c = regression constants

57. Asymptotic growth curve: - When yield is the product of vegetative crop growth, the density-yield relationship is asymptotic. In an asymptotic relationship, with increase in density, yield rises to a maximum and then relatively constant at high densities. Further increase in plant density above this maximum does not increase the yield. (Figure 2.2)

58. Parabolic and asymptotic relation between plant population and yield

59. Phytohormones : - Phytohormones play an important role in plant growth, source-sink relations and root-shoot relationships. The production and action of these substances is strongly influenced by environmental factors such as water stress and nitrogen deficiency. There is a characteristic shoot: root ratio for each species at each growth stage. Shoot: root ratios tend to increase with plant size (decrease for root crops), reflecting increasingly preferential assimilate partitioning above ground (below ground for root crops). Thus, shoot: root ratio comparisons should be made at equal dry weight, or at equal plant developmental stage, not at equal time. Shoot: root ratios are influenced by changes in environmental conditions, such as light, nutrient availability, temperature and water supply. These changes usually reflect an adaptive advantage for the plant in acquiring the limiting resource.

60. Photosynthesis: Light and Dark reactions, C3, C4 and CAM plants Photosynthesis : Photosynthesis is the absorption of light energy and its conversion into chemical energy. During photosynthesis, CO2 and water transformed into simple carbohydrates and O2 is liberated into the atmosphere. ♦ The simple CH2O3 produced during photosynthesis are converted by additional metabolic process, into lipids, nucleic acids, proteins and other organic molecules. ♦ These organic molecules in turn, are elaborated into leaves, stems, roots, tubers, fruits, seeds and other tissues and organ system. Thus, the overall reaction of oxygenic photosynthesis can be represented as. LIGHT 6CO2+12H2O-------------------→C6H12O6+6H2O+6O2 CHLOROPHYLL This equation is frequently represented by the simplified form: CO2 +2 H2O (CH2O) + H2O + O2

61. photosynthetic process The photosynthetic process is carried out by three steps: i . The absorption of light and retention of light energy. ii. The conversion of light energy into chemical potential. iii. The stabilization and storage of chemical potential. Based on the three steps, the yield of a crop can be expressed by an equation Y = Q x I x E x H Q = Quantity of solar radiation received by the leaf or striking the leaf. I = Fraction of Q utilized by plants. E = Overall photosynthetic efficiency of the canopy (i.e. efficiency of the conversion of solar energy to chemical energy) in terms of total dry matter produced by the plants. H = Fraction of dry matter allocated to the harvested parts (Harvest index).

62. Light phase of photosynthesis The absorption of radiant energy of green leaves is due to the presence of several pigments: 1. Chlorophyll a 2. Chlorophyll b 3. Carotenoids Chlorophyll a and b account for the absorption of red light (600-700nm) and blue light (400 to 500 nm). chlorophyll ‘a’ only participate directly in the conversion of light energy into chemical energy. The carotenoids also absorb in the blue region of the spectrum. In the absence of light chlorophyll ‘a’ synthesis is impaired. That is why plants grown in dark are usually lack chlorophyll ‘a’. So they are usually yellow in colour and possess elongated growth habit. Their leaf development is strongly reduced. Plants displaying the characters are said to be etiolated plant.

63. ATP formation : Regarding ATP formation, it is generated during 1.) The oxidation of glucose to CO2 and H2O in mitochondria. This process is known as oxidative phosphorylation 2.) The formation of ATP by the absorption of radiant energy by the chlorophyll pigments is known as photophosphorylation. ADP + inorganic phosphate (pi) Radiant energy → ATP Chloroplast NADPH formation : NADPH is formed by accepting electrons from water molecules and releasing O2. 4H2O Radiant Energy → O2 + 4 (H+ + e-) + 2H2O (Photolysis of water) 4H 2O + 2 NADP RE → 2NADPH + O2 + 2H+ + 2H2O Chloroplast Therefore, the illuminated chloroplasts are capable of generating both ATP and NADPH through the following reaction. RE 2 ADP + 4 H2O + 2Pi + 2NADP------------- → 2 ATP+2NADPH+2H2O+O2+2H Chloroplast

64. Reaction scheme for ATP and NADPH formation Two pigment systems are involved: 1. Photo system I (PS I) (Photo system I contains chlorophyll ‘a’, ‘b’ and carotenes) 2. Photo system II (PS II) (Photo system II contains chlorophyll ‘a’, ‘b’ and xanthophylls ) The two photo systems are connected by several intermediates: 1. Plastoquinone 2. Cytochrome 3. Plastocyanin

65. Dark reaction (CO2 fixation) All photosynthetic eukaryotes, from most primitive algae to the most advanced angiosperms, reduce CO2 to carbohydrates via the basic mechanism, the C3 photosynthetic carbon reduction (PCR) cycle. The PCR cycle is sometimes referred to as Calvin cycle in honour of its discovered, the American biochemist, Melvin Calvin. In PCR cycle, CO2 from atmosphere and water are enzymatically combined with a five-carbon acceptor molecule( RuBP ) to generate 2 molecules of a three carbon intermediate. These intermediates are reduced to carbohydrate using the photochemically generated ATP and NADPH. The cycle is complete by regeneration of 5 carbon acceptor.

66. Respiration Plant Respiration : “Plant respiration is the chemical reaction by which plants cells stay alive.” The process of respiration is expressed as: Glucose + Oxygen → Carbon Dioxide + Water (+ Energy) C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy The complete combustion of glucose produces H2O and CO2 as end products and release energy in the form of heat. in the absence of oxygen partially oxidize glucose in the absence of oxygen. This process is also called as Glycolysis which includes breaking down of glucose to Pyruvic Acid .

67. Respiratory Quotient ( RQ ) : This is another aspect of respiration. “Respiratory quotient is the ratio of CO2 produced to O2 consumed while food is being metabolized.” RQ depends on the type of respiratory substrate used in respiration. When carbohydrate is used as substrate and is completely oxidized, RQ becomes 1. It implies equal amount of O2 and CO2 are consumed and evolved.

68. Factors affecting Respiration in Plants There are eight environmental factors that has significant impact on respiration in plants – • Oxygen content of the atmosphere • Effect of water content • Effect of temperature • Effect of availability of light • Impact of respirable material • Effect of concentration of carbon dioxide in atmosphere • Protoplasmic conditions, i.e. younger tissues have greater protoplasm as compared to older tissues. • Other factors, i.e. fluorides, cyanides, azides , etc.

69. Translocation Translocation is  a biological mechanism involving the transfer of water and other soluble nutrients from one part of the plant to another through the xylem and phloem , which occurs in all plants. Xylem is responsible for the transport of water as well as the necessary mineral salts, from the roots to various other parts of the plant, and Phloem transports the sucrose and amino acids that have been developed in the leaves to the other parts of the plant.

70. The products of photosynthesis (mainly the sugar sucrose) are a major component of the substance found in the phloem, called assimilate. Ions, amino acids, certain hormones, and other molecules are also found in assimilate.  The movement of assimilate  is called translocation, or assimilate transport

71. . Over small distances substances move by diffusion and by cytoplasmic streaming supplemented by active transport. Transport over longer distances proceeds through the vascular system (the xylem and the phloem) and is called translocation.

72. The phloem conduits distribute the sugars made in the leaves to growing tissues and organs that cannot carry out photosynthesis. These ‘sinks’ include shoot and root apices, flower buds, and developing fruit and seed Xylem conduits are responsible for delivery of water, inorganic nutrients and organic forms of nitrogen to transpiring leaves

73. Plants transport various substances like gases, minerals, water, hormone and organic solutes to short distance (one cell to another) or long distance as water from roots to tips of stem. Long distance transport occurs through vascular system, xylem and phloem called  translocation  through mass flow.

74. Passive Transport Simple Diffusion- Movement by diffusion is passive and slow along the concentration gradient through permeable membrane. No energy expenditure takes place. It occurs in liquid and gases. Rate of diffusion are affected by gradient of concentration, permeability of membrane, temperature and pressure.

75. Facilitated Diffusion Lipid soluble particles easily pass through cell membrane but the hydrophilic solutes movement is facilitated. For facilitated diffusion, membrane possesses aquaporins or water channels.  Aquaporins  are membrane proteins for passive transport of water soluble substances without utilization of energy. The protein forms channels in membrane for molecules to pass through. The  porins   are proteins that forms huge pores in the outer membrane of the plastids, mitochondria etc. Water channels are made up of eight different types of aquaporins .

76. Symport , Antiport and Uniport - In  Symport , both molecules cross the membrane in the same direction. In  Antiport , both molecule moves in opposite direction. When a molecule moves across a membrane independent of other molecules, the process is called  uniport .

77. Active Transport Active transport uses energy to transport and pump molecules against a concentration gradient. Active transport is carried out by specific membrane-proteins. Hence different proteins in the membrane play a major role in both active as well as passive transport. Pumps are proteins that use energy to carry substances across the cell membrane. These pumps can transport substances from a low concentration to a high concentration (‘uphill’ transport). Transport rate reaches a maximum when all the protein transporters are being used or are saturated. Like enzymes the carrier protein is very specific in what it carries across the membrane. These proteins are sensitive to inhibitors that react with protein side chains.

78. Active Transport Uses energy to pump molecules against the concentration gradient. It is carried out by membrane proteins. In active transport movable carrier proteins are called  pumps. The pumps can transport substance from low concentration to high concentration. The carrier proteins are very specific in what it carries across the membrane.

79. Comparison between Transport mechanisms Simple diffusion Facilitated diffusion Active transport Special membrane protein is not required. Special membrane protein is required. Special membrane protein is required. Not selective Highly selective Highly selective Transport do not saturate Transport saturate Transport saturate No uphill transport Uphill transport Uphill transport No ATP energy is required. No ATP energy is required. ATP energy is requi

80. Osmosis The plant cell is surrounded by a cell membrane and a cell wall. The cell wall is freely permeable to water and substances in solution hence is not a barrier to movement. In plants the cells usually contain a large central vacuole, whose contents, the vacuolar sap, contribute to the solute potential of the cell. In plant cells, the cell membrane and the membrane of the vacuole, the tonoplast together are important determinants of movement of molecules in or out of the cell. Osmosis is the term used to refer specifically to the diffusion of water across a differentially- or selectively permeable membrane. Osmosis occurs spontaneously in response to a driving force. The net direction and rate of osmosis depends on both the pressure gradient and concentration gradient. Water will move from its region of higher chemical potential (or concentration) to its region of lower chemical potential until equilibrium is reached. At equilibrium the two chambers should have nearly the same water potential.

81. Mass flow Diffusion is a slow process. It can account for only short distance movement of molecules. Special long distance transport systems become necessary so as to move substances across long distances and at a much faster rate. Water and minerals, and food are generally moved by a mass or bulk flow system. Mass flow is the movement of substances in bulk from one point to another as a result of pressure differences between the two points

82. mechanism of translocation Basically, there are three theories to explain the mechanism of translocation of solutes. They are: Munch's mass flow or pressure flow hypothesis Diffusion hypothesis Protoplasmic streaming hypothesis

83. Munch's mass flow or pressure flow hypothesis Sugar and then water enter sieve tubes at a source, in this case the leaves. This creates a positive pressure, which causes phloem contents to flow. Sieve tubes (composed of sieve tube elements) form a continuous pipeline from a source to a sink, in this case it is the root (sink), where sugar and then water exit sieve tube According to this theory translocation is a kind of blood circulation within the plant body and the mesophyll cells of the leaves acting as a heart. The pumping force is provided by the Osmotic concentration of the solutes to be translocated in phloem which is supported by the hydrostatic pressure developed due to entry of water from xylem. The carbohydrates like sucrose is produced in the mesophyll cells of leaves during photosynthesis which causes an increase in the osmotic concentration of these cells. Hence, these cells absorb water from the neighboring xylem cells which in turn brings about an increased hydrostatic (turgor) pressure in these cells. This pressure forces the solution of mesophyll cells into the sieve tubes of phloem tissue through the sieve tubes which form a continuous net work within the plant body, food materials are transported from the leaf to root through the stem. In the leaves, food materials are continuously produced by  photosynthesis  and thus the concentration of food materials is always kept high. At the stem and roots, the food materials are continuously utilized for various metabolic activities and thus the concentration of solutes in stem and root are always kept at low level. Thus a gradient of hydrostatic pressure is established between the leaf and the root. Because of the gradients of hydrostatic pressure there will be a bulk or mass flow of solution and dissolved solutes from the leaves to the stem through the phloem. Hence, a mass flow of solutes occurs continuously from the leaf to the root through the stem. The tissue concerned in translocation is phloem tissue of root, stem and leaf.

84. Diffusion Hypothesis Diffusion is a simple process by which substances will move from its region of higher concentration to the region of lower concentration. Diffusion  hypothesis believes that translocation is fundamentally simple diffusion  and translocation will take place only if there is a concentration gradient between the supply end and consumption end. If there is no concentration on gradient there is no translocation. The rate of translocation will be greater when the concentration of the solutes in the supply end is also greater. The only merit of the theory is that it explains the simultaneous flow of organic compounds in opposite directions. But this theory is also not accepted because; translocation of solute is a rapid process where as  diffusion  is a very slow process. So it cannot account for the rapid movement of food materials. Activated Diffusion  Hypothesis In 1937, Mason and Phillis proposed that the transport of food materials takes place through stationary cytoplasm by activated  diffusion . According to them, diffusion  is hastened by activating the diffusing molecules or by decreasing the resistance to  diffusion  through the protoplasmic medium. This theory is not accepted because no such activation has been demonstrated experimentally.

85. Protoplasmic Streaming Hypothesis This theory was proposed first by de Vries in 1885 and elaborated by Curtis and her associates in 1935. According to them, moving protoplasm carries the solutes within the sieve elements and the protoplasmic fluid moves from cell to cell through large protoplasmic connections across the sieve plates. A circular movement or Cyclosis of living protoplasm has been observed in many different plant cells. (e.g.) Chara , Nitella , Staminal hairs of Tradescantia .

86. Ascent of saps  (Translocation of water) The upward movement of water from roots towards the tips of stem, branches and their leaves is called ascent of sap. Vital force theory  was forwarded by rJ.C.Bose in 1923. This theory believes that the innermost cortical cells of the root absorb water from the outer side and pump the same into xylem channels. Root  pressure theory  was forwarded by Priestley in 1916. Root pressure is positive pressure that develops in the xylem sap of the root of plants. It can be responsible for pushing up water to small heights in plants. Loss of water in liquid phase by herbaceous plants from the tips of leaf blades is known as  guttation. Water rises in tubes of small diameters, kept in vessels having water due to force of surface tension. Similarly water rises up in the walls of xylem channels due to adhesion and cohesion. This theory is called  Theory of Capillarity . Cohesion  Tension theory  was put forwaded by Dixon and Joly in 1894. According to this theory water is mostly pulled due to driving force of transpiration from the leaves. The water molecules remain attached with one another by cohesion force. The water molecule does not breaks in vessels and tracheid due to adhesive force between their walls and water molecules. On account of tension created by transpiration, the water column of plant is pulled up passively from roots to great heights. Transpiration is the loss of water in the form of water vapour from aerial parts of plants. The following purpose is fulfilled by transpiration-

87. Phloem transport: Flow from Source to Sink Food (sucrose) is transported by phloem from source to sink. The part of plant that synthesize the food is called source and part where food is used or stored is called sink. The source and sink can be reversed by the plants depending upon the season or plant’s need. So, the direction of movement in the phloem is bi-directional. Phloem sap is mainly water and sucrose but other sugars, hormones and amino acids are also translocated through it. Pressure flow or Mass flow hypothesis It is the most accepted theory for the translocation of sugar from source to sink. Glucose is prepared at source by photosynthesis which is converted into disaccharides (sucrose). Sucrose moves into companion cells and then into sieve tube cells by active transport. Loading of phloem at source creates a water potential gradient that facilitates the mass movement in the phloem. Sieve tube cells of phloem forms a long column with holes in their wall called sieve plates. Cytoplasmic strands pass through the hole in the sieve plates to form continuous filament. Hydrostatic pressure developed in sieve tube cells moves the sap in the phloem. At sink, incoming sugar is actively moved out of the phloem as complex carbohydrates. The loss of solute produces a high water potential in the phloem and water passes out and returning into xylem.

88. Plant- Water Relations   Water provides the medium for the plants in which most substances are dissolved. The cells’ protoplasm is water, in which many molecules dissolve and many are suspended. Terrestrial plants consume a large quantity of water on a daily basis, yet the majority of it is lost to the atmosphere by transpiration. A mature corn plant absorbs about three liters of water per day, but a mustard plant absorbs water equal to its own weight in approximately five hours.

89. Water Potential The difference between the free energy of water molecules in a pure solvent and the free energy of water molecules in a solution is referred to as water potential. The two primary components that define water potential are solute potential ( Ψs ) and pressure potential ( Ψp ). The higher the concentration of water in a system, the higher its kinetic energy or “water potential.” Water potential is represented by the Greek letter  Psi or Ψ  and is measured in pressure units such as pascals ( Pa ). At normal temperatures, pure water’s water potential is assumed to be zero. All solutions have a lower water potential than pure water; the amount of this decrease owing to solute dissolution is referred to as solute potential. For an atmospheric pressure solution, (water potential)  Ψw = (solute potential) Ψs . The pressure exerted by protoplasts owing to water ingress against the stiff walls is referred to as pressure potential ( Ψp ). When water enters a plant cell by diffusion, causing pressure to build up against the cell wall, the cell becomes turgid, increasing the pressure potential. A cell’s water potential is influenced by both solute and pressure potential:  Ψw = Ψs + Ψp .

90. ransport of Water in Plants Plants need active transport of two main components namely, water from the roots and the food prepared from the leaves to the other parts.  Water is transported through Xylem in plants. Xylem is a vascular tissue. It contains more than one type of cells to facilitate the process of transport within the plant system. Xylem carries the dissolved minerals and water to the leaves where, with the help of the chlorophyll, sunlight, and carbon dioxide absorbed from the atmosphere, the leaves can render their function of preparing sucrose which gives energy to the plant for survival. Transport in plants of water starts from the roots to the stem by way of a potential gradient. The roots of the plant absorb water through apoplast or symplast , and the same is then carried to the stem for absorption by the leaves. The plants need to transport water to the leaves as water is an integral component of the process of photosynthesis. If the plants do not receive water, the process of photosynthesis cannot be completed.

91. Osmosis Osmosis is the flow of molecules over a semi-permeable membrane from an area of greater concentration to a region of lower concentration until an equilibrium is established. The plant cell wall is permeable to solution and water-soluble compounds. There are two kinds of osmosis: Endosmosis : It is the migration of water molecules into the cell when it is immersed in a hypotonic solution. Exosmosis : The migration of water molecules out of a cell when it is immersed in a hypertonic solution.

92. Plasmolysis Plasmolysis occurs when a plant cell is immersed in a hypertonic solution and loses water. It is determined by three sorts of solutions: Isotonic : Two solutions that have the same osmotic pressure across the semi-permeable membrane. Hypotonic : This is a solution with a lower osmotic pressure than another. Hypertonic : This solution has greater osmotic pressure than another. When cells are put in a hypotonic solution, they expand or get deplasmolyzed . When cells are put in a hypertonic solution, they shrink or become plasmolyzed .

93. Imbibition The process of adsorption of water molecules by hydrophilic substances is known as imbibition. Water absorption by seeds (raisins) and dry wood are two examples. Imbibition requires a water potential gradient between the absorbent and the liquid imbibed, as well as the affinity between the adsorbant and the liquid.

94. Food is transported in plants by Phloem, which is again avascular tissue like the Xylem. The process of transporting food or photosynthates is called translocation in Biology. Phloem contains living cells. Phloem cells are active because they need ATP for the movement of food actively. The living tendency of the Phloem cell is what is responsible for the upward movement of water by the Xylem, as it creates the potential gradient that is needed for the upward movement of water.

95. 96 PROBLEMATIC SOILS The soils which are unfavorable for cultivation of field crops because of one or more unfavorable soil properties/characteristics (viz. Soluble salts, soil reaction, ESP, water logging, aeration etc. ) are adversely affect the optimum soil productivity is called problematic soils. The problematic soils need to be classified in to various groups for developing special systems of management for specific types of problems and constraints in the production of crops. The major problematic soils of India are as follows.

96. 97 Sr. No. Problematic Soils Key Diagnosis Major constraints 1. Clay soils Dominated by clay particles Water logging, compaction, poor aeration, difficult to cultivate 2. Sandy soils Dominated by Coarse sand particles poor fertility, low SOM, low water holding capacity, erosion 3. Acid soils Soil pH is less than 6.5 Fe, Al toxicity (Strong acid soil) 4. Salt affected soils Saline soils ECe is greater than 4.0 dS/m High osmotic potential, nutrient imbalance Sodic soils ESP is greater than 15 Deteriorated physical condition, Na toxicity, nutrient imbalance Saline sodic soils ECe is greater than 4.0 dS/m and ESP is greater than 15 High osmotic potential, deteriorated physical condition, nutrient  imbalance 7. Calcareous soils CaCO 3 is greater than 5.0 % P, Fe deficiency 8. Water logged soils Water Stagnation, Low infiltration rate, Poor aeration 9. Degraded soils Based on soil analysis - 10. Compacted soils High bulk density Poor aeration, poor root penetration, water logging 11. Impermeable soils Low hydraulic conductivity (HC) and infiltration rate Poor aeration, water logging Major problematic soils of India

97. 98 Obtaining the maximum production potential of a particular crop depends on the growing season environment and the skill of the producer to identify and eliminate or minimize factors that reduce yield potential. More than 50 factors affect crop growth and yield potential. ( i.e. climatic, Adaphic and genetic). Although, the producer cannot control many of the climate factors, most of the soil and crop factors can and must be managed to maximize productivity. Therefore, more emphasis is given to soil related constraints because majority of soil related problems can be altered in to favourable condition to maximize the soil productivity.    

98. 99 Salt affected soils: Soils, in which concentration of salts is so high as to adversely affect plant growth and crop productivity, are called salt affected soils. Some amounts of salts are always present in the soil. When the concentration of these salts is low, they are not harmful for the growth of plants. But with the increase in salt content of the soil to high levels, the plant growth adversely affected which, in turn, decreases the productivity of agricultural crops. The extent of reduction in growth and decrement in productivity, however, depend upon many factors such as kind and content of salt constituents, soil texture, distribution of salts in the soil profile, the species of plant grown, level of soil - water - crop management and climatic condition.

99. 100 Classification of salt affected soils: The salt affected soils needs to classified in to various groups for developing special systems of management for specific types of problems and constraints in the production of crops. In 1954, the US Salinity Laboratory Staff grouped salt affected soils in to three distinct classes based on the behavior of salts in the soils viz. (i) Saline soils, (ii) Alkali soils and (iii) Saline- alkali soils

100. 101 Saline Soils (Synonymous: Solonchak (Russian term), Saline non sodic , White alkali ) Saline soils contains sufficient concentration of soluble salts in the root zone soil which are adversely affects the crop productivity OR simply, the accumulation of water soluble salts in the soil which restrict the crop production is called saline soil. The amount of soluble salts present in the soil is determined by the electrical conductivity or individual analysis of salts present in the soil. Among the salts present in the soil, Ca, Mg, Na and K are the dominant cations whereas CO 2 , CO 3 , Cl , SO 4 are the dominant anions in arid and semi arid region of the world. The process of accumulation of soluble salts in the soils is known as salinization . Soluble salts: Salts which dissolved in soil water and are free to move with the soil water Soil solution :The liquid phase of soil, consisting of soil water also contains dissolved salts and thus it is called soil solution

101. 102 Causes of Salinization : Salinization or the accumulation of the salts occurs in the following ways: 1. Primary minerals: It is the original and important direct source of all the salt constituents. During the process of weathering, which involves hydrolysis, hydration, solution, oxidation and carbonation various constituents like Ca, Mg and Na are gradually released and made soluble. Eg . Halite ( NaCl ), Calcite (CaCO 3 ), Dolomite [Ca Mg (CO 3 ) 2 ]. 2. Arid and semi-arid climate: Salt affected soils are mostly formed in arid and semi-arid climate where low rainfall and high evaporation prevails. The low rainfall in these regions is not sufficient to leach out the soluble weathered products and hence the salt accumulates in the soils. Further, high evaporation in these areas, lead to accumulate salt in the root zone due to capillary rise of salt with evaporating water from the lower zone. The intensity of salinization increases with increases in dryness of the climate. Salt affected soils in humid region exists only in areas subjected to sea water intrusions in deltanic regions and other low lying areas along the sea cost which get inundated by the sea water.                    Question: Climate is a major source of soil salinity. Justify

102. 103 3. Sea as a source of salts : The ocean may be the source of salts as in soils where the parent materials consists of marine deposits that were laid down during earlier geological periods and have since been uplifted. The ocean is also the source of the salts in low-lying area along the margin of seacoasts. Sometimes salts is moved inland through the transportation of spray by winds be called “cyclic salts”. In Gujarat salts affected soils observed in Bha l region is due to marine deposit of receding of the sea while seawater inundation is the cause in several coastal regions in south Gujarat.   4. Restricted leaching and transportation: In arid regions, the leaching and transportation of soluble salts to ocean is not as complete as in humid regions. In arid regions the leaching is usually localized in nature and soluble salts may not be transported far. This occur not only because there is less rainfall available to leach and transport the salt but also because of the high evaporation rates which tend to concentrate the salt in the soils and in surface water.

103. 104 5. Low permeability of the soil: This causes poor drainage by impeding the downward movement of water. Low permeability may be results of an unfavorable soil texture or structure or hard pan/clay pan, because of the low permeability the ground water table may raise or because of continuous deposition of soluble salts in the soils. 6. Ground water: Ground water contains large amounts of water soluble salts which depend upon the nature and properties of the geological material with which water remains in contact where water table and evapotranspiration rate is high, salts along with water move upward through capillary activity and salts accumulation on the soil surface. 7. Irrigation water: The application of irrigation water without proper management (i.e. lack of drainage and leaching facilities) increases the water table and surface salt content in the soils.

104. 105 8. Poor drainage of soil: During the periods of high rainfall, the salts are leached from the upper layer and if the drainage is impeded, they accumulate in the lower layer. When the water evaporates the salts are left in the soils. Such soils are generally developed in low-lying areas. 9. High water table: The ground waters of arid regions usually contain considerable quantities of soluble salts. If the water table is high, large amount of water moves to the surface by capillary action and evaporated, leaving soluble salts on the surface. 10. Canal as a source of salinization : Although, canal water practically contain very little amount of soluble salts, but during earlier stages, the excess use of canal water hastens the rise of ground water table. When, the water table rises within 5 or 6 feet from the soil surface, the ground water move upward into the root zone and to the soil surface. Under such condition, ground water as well as irrigation water contributes to salinization of soils.

105. 106 B. Alkali Soils ( Synonymous : Solonetz (Russian term), Non saline sodic , sodic , Black alkali ) Alkali soils have sufficient sodium saturation on the exchange complex and alkalinity to adversely affect plant growth and crop productivity. Carbonates (CO 3 -- + HCO 3 - ) of sodium are dominant salts. The concentration of natural salts ( Cl - and SO -- 4 ) is much lower. Alkalinity or Alkaline : It indicate the reaction of soil, means soils contains excess alkalinity (pH more than 7.00). Alkali: It indicates condition of soil where alkali ion (sodium) is dominant on exchange complex of the soil. Alkalization:    It is the process of accumulation of sodium ion on soil exchange complex is known as alkalization.

106. 107 Causes of Alkalinity : Process where exchangeable Na content in soil increased due to precipitation of Ca and Mg as carbonate (Na 2 CO 3 or NaHCO 3 ) by low of mass action, Ca and Mg replaced by Na on exchange complex. Hydrolysis of sodium silicate or weathering of minerals. Na X     +   H 2 O  — hydrolysis  →    NaOH   +   H X NaOH   +   CO 2   — hydrolysis →   NaHCO 3  → 2NaHCO 3   — decomposition    →   Na 2 CO 3   +   CO 2   +   H 2 O Na 2 SiO 3     → Sodium silicate   → highly sodic NaOH → also highly sodic 2. Replace Na 2 SO 4 or NaCl by CaCO 3 Na 2 SO 4   +   CaCO 3   →   Na 2 CO 3   +   CaSO 4 2NaCl     +   CaCO 3   →   Na 2 CO 3   +   CaCl 2

107. 108 3. Hydrolysis of exchangeable Na Na X Na   +   H 2 O   +   CO 2   →   H X H   +   Na 2 CO 3 Na X Na   +   CaCO 3   →   Ca X Ca   +   Na 2 CO 3 Na X Na   +   Ca(HCO 3 )   →   Ca X Ca   +   NaHCO 3 4. Reduction of Na 2 SO 4 through microorganisms Na 2 SO 4   +   2C     +   CO 2   →   Na 2 S   +   2CO 2 Na 2 S       +   CO 2   +   H 2 O   →   H 2 S     +   Na 2 CO 3 5. Decomposition of organic matter Na 2 SO 4   →   Na 2 S   +   2O 2 Na 2 S   +   2H 2 O   →   H 2 S   +   2NaOH NaOH   +   CO 2   →   NaHCO 3

108. 109 6. Use of alkali or sodic water for irrigation 7. Excessive use of basic fertilizers: Use of basic fertilizers like Na 2 NO 3 , basic slag, etc. may develop alkalinity in the soils. 8. Humid and semi-humid regions : Alkaline soils develop in other area also e.g. in semi-humid and temperate regions especially in depressions where drainage is defective and where the underground water table is high or close to the surface.

109. 110 Degraded alkali or sodic soil:              If the extensive leaching of a saline- sodic soil occurs in the absence of any source of calcium or magnesium, part of the exchangeable sodium is gradually replaced by hydrogen. The resulting soil may be slightly acid with unstable structure. Such a soil is called degraded alkali or sodic soil.  clay Na     +     H 2 O     ↔     H clay    +      NaOH ↓     (Acid soil on the leaching surface horizon)      2NaOH      +     CO 2     =     Na 2 CO 3     +     H 2 O (from soil) (Alkali soil in the sub-surface horizon)

110. 111 Sodium carbonate (Na 2 CO 3 ) dissolves humus and is deposited in the lower layer. The lower layer thus acquires a black colour . At the same time H-clay formed in this way does not remain stable. The process of break down of H-clay under alkaline condition is known as solodisation . Distribution of Salt Affected Soils: World - 952 million hectares India    - 7.02 million hectares Gujarat - 1.12 million hectares

111. 112 States Area (Million ha) States Area (Million ha) Uttar Pradesh 1.295 Orissa 0.404 Gujarat 1.214 Maharashtra 0.534 West Bengal 0.850 Karnataka 0.404 Rajasthan 0.728 Madhya Pradesh 0.224 Punjab 0.688 Andhra Pradesh 0.042 Haryana 0.526 Other states 0.040 In India the state wise estimated area having salt affected soils (C.S.S.R.I., Karnal ) is as follows It is seen from the above data that Gujarat has second largest salinized area.

112. 113 Characteristics Saline soil Alkali soils Saline-alkali soils Degraded alkali soil Content in soil Excess soluble salts Presence of exchangeable sodium on the soil complex. Soil contains Na-clay as well as soluble salts. Hydrogen (H + ) ions in the upper layer and sodium (Na + ) in the lower layer. ECe (dS/m) > 4 < 4 > 4 - Soil pH Less than 8.5 8.5-10 More than 8.5 pH about 6 in the surface soil and pH 8.5 in the lower layer ESP Less than 15 More than 15 More than 15 More than 15 Sodium adsorption ratio (SAR) Less than 13 More than 13 More than 13 Less than 13 in the surface and greater than 13 in the lower horizon Characteristics Saline soil Alkali soils Saline-alkali soils Degraded alkali soil Total soluble salt content More than 0.1 %. Less than 0.1 % More than 0.1 % Less than 0.1 % Dominant salts Sulphate (SO 4 2- ), chloride (Cl - ) and nitrates (NO 3 - ) Sodium carbonate (Na 2 CO 3 ) - Sodium carbonate (Na 2 CO 3 ) in lower layer Organic matter content Slightly less than normal soils Very low due to the presence of sodium carbonate (Na 2 CO 3 ) barren (Usar) Variable Low Colour White Black - Black in lower layer Physical condition of the soil Flocculated condition, permeable to water and air. Soil structure optimum. Deflocculated condition, permeability to water and air is poor. Very poor soil structure. Flocculated or deflocculated depending upon the presence of sodium salts and Na-clay Compact low infiltration and permeability. It develops columnar structure Other name White alkali (Solonchak) Black alkali (Solonetz) Usar Solod , Soloth Characterization of salt affected soils

113. 114 Reason: Saline soil is also known as white alkali soils because these soils are characterized by saline efflorescence or white encrustation of salt at the surface. ( Simply, Saline soil has a surface crust of white salts due to capillary rise of salts with evaporating water during summer.) Alkali soil is also known as black alkali soil. Due to presence of high amount of exchangeable sodium and high pH, the soil colloids get dispersed and organic matter present in the soil are dispersed and dissolved. When these dispersed and dissolved organic matter is deposited in the surface, alkali soils give dark brown - black appearance.

114. 115 Appraisal/evaluation of Saline and Sodic Soils: Saline Soils: Different criteria are employed for characterizing soil salinity and those are given below: Soluble salt concentration in the soil solution: In saline soils, the water soluble salts concentration in the soil solution is very high and as a result the osmotic pressure of the soil solution is also very high. As a result of which the plant growth is affected due to wilting and nutrient deficiency. Salt content more than 0.1 % is injurious to plant growth. Osmotic Pressure (O.P.): It should be assessed at field capacity soil moisture regimes. Besides the relation between OP and electrical conductivity (EC) for salt mixtures found in saline soils, is given below: OP (in atmospheres or bars) = 0.36 X EC          Where; EC expressed as dS /m

115. 116 Class Boron concentration (ppm) I Safe < 0.7 II Marginal 0.7 – 1.5 III Unsafe > 1.5 Electrical conductivity (EC) of the soil saturation extract: Measurement of EC of the soil saturation extract ( ECe ) is also essential for the assessment of the saline soil for the plant growth and is expressed as dS /m (formerly mmhos /cm). Concentration of water-soluble boron: The determination of water-soluble boron concentration is also another criterion for characterization of saline soils. The critical limit of boron concentration for the plant growth is given below:

116. 117 ESP = Exchangeable Na (me/100 g soil) x 100 CEC (me/100 g soil) Alkali soils: There are various methods employed for its approval that are as follows: Exchangeable sodium percentage :              Sometimes soil pH also gives an indication of soil alkalinity indirectly. It is generally found that the higher the ESP, the higher is the soil pH. Sodium adsorption ratio : The U.S. Salinity Laboratory developed the concept of Sodium Adsorption Ratio (SAR) to define the equilibrium between soluble and exchangeable cations as follows :   (Where, the concentration of Na + , Ca 2+ , and Mg 2+ of Saturation extract are expressed in me/l)

117. 118 ESP = 100 (-0.0126 + 0.01475 SAR) 1 + (-0.0126 + 0.01475 SAR) The value of SAR can be also used for the determination of Exchangeable Sodium Percentage (ESP) of the saturation extract by using the following formula:              Sometimes the following regression equation is used for the appraisal of alkali soil by determining the value of ESP from the value of SAR. Y = 0.0673 + 0.035 X     Where; Y indicates ESP and X indicates SAR Soils having SAR value greater than 13 are considered as alkali or sodic

118. 119 Causes of poor growth on saline soils: The crop growth on salt affected soils is poor due to one or another reason. The various reasons given for poor crop growth under such conditions are discussed below: (A) Water availability theory: Due to high salt concentration plants have to spent more energy to absorb water and to exclude salt from metabolically active sites. At the same time various nutrient elements become unavailable to plants. (B) Osmotic inhibition theory: According to osmotic inhibition theory, plant growth is inhibited by the excess of solute taken up from saline media. The osmotic inhibition theory thus postulates that the salts act inside the plants, but it does not specify how the inhibition of growth is effected. The inhibition could even result in even part from water deficiency in a sense different from that envisioned by the water availability theory. The presence of excess solutes in the plant decreases the free energy of unit mass of water even though the absolute mass of water in the excess of salts present externally

119. 120 (C) Specific toxicity theory : According to the specific toxicity theory, soil salinity exerts a detrimental effect on plants through the toxicity of one or more specific ions (cations as well as anions) in the salts present in excess. Accordingly, there may be toxicity of chlorides, bicarbonates and boron and to a lesser concentration of magnesium or its salts may also induce calcium deficiency.

120. 121 Causes of poor crop growth on alkali soils: The reasons of low crop production on such soils are as follows: (A) Adverse physical conditions : The alkali soils have poor physical conditions. The permeability of air and water and the hydraulic conductivity are at a lower most state due to breakdown of aggregates and dispersion of individual clay-colloids. The breakdown of aggregates is due to dissolution of organic matter, which acts as a cementing agent for binding individual clay particles, due to formation of alkali solution. The dispersed clay plugs all the macro and micro capillaries thereby hampering the movement of air and water. Such dispersed clay swells considerably due to high hydration capacity of Na ions and remain unflocculated in presence of water. The downward movement of water is practically zero and hence they remain waterlogged when irrigation is given or water is added through rain. On drying, such soils form very large clods, which are very hard in nature. The hard crust formation on the surface of the soil is most common characteristics. The tillage operations are very difficult to carry due to increase in bulk density, which is due to deflocculation of clay. Because of very adverse physical conditions, the germination as well as the root growth is considerably reduced. Because of this reduction, the overall crop growth is not at all satisfactory.

121. 122 (B) High sodium on exchange complex: Excess amount of sodium reduces the crop growth considerably i.e., there arises sodium toxicity because of excess concentration of Na. The plants exhibit the deficiency of Ca and to some extent Mg due to well-known principle complementary ion effect i.e. higher concentration of Na would reduce the uptake of Ca ++ and Mg ++ . The relative concentrations of Ca and Mg in such soils are low due to precipitation of these ions during process of alkalization. The relationship such as Ca/Na, Mg/Na, (Ca + Mg)/Na or K/Na are well-known and antagonistic relationships.

122. 123 (C) Effect of high pH : The solution soils have high pH and the values range from 8.5 to as high as 10 or 11. The high pH reduces the availability of P, Zn, Cu, Mn and Fe (the availability of P increases at a very high pH value, 12 and above, due to formation of soluble sodium phosphate). The microbial activity is also at standstill due to unfavourable pH and the processes of mineralization, ammonification or nitrification are practically negligible. Apart from nutritional deficiency due to high pH, the higher concentration of OH ion itself is not favorable to crop growth. In case of saline-alkali soils excess sodium may reduce the crop growth. However, the physical conditions of such soils are not disturbed due to excess amount of salts.

123. 124 METHODS OF RECLAMATION I. Mechanical Construction of embankment to prevent tidal see water Land leveling and contour bunding Establishment of drainage network Breaking of hardpan in the subsurface layer through boring auger hole II. Hydrological Flushing Leaching Drainage III. Chemical Use of amendments IV. Physical Scrapping of salt crust Deep tillage, sub soiling, profile inversion Use of soil conditioners e.g. sand, tanch , ash, manures and synthetic polymers like PVAC, PAM, and PVPC V. Biological Agroforestry system Use of manures Green manure Selection of salt tolerant crops after afforestation

124. 125 Hydrological method 1. Flushing: The salts can be removed by flushing which is the surface washing out of salts with the runoff water, which is collected at the sloppy end of the field. One serious drawback with flushing, as a means of removing soluble salts is its inability to flush through the soil and the salts have to come up to the surface for being removed through flushes. The flushing method is employed where moisture transmission characteristics into the profile are extremely poor. The USSSL does not advocate flushing as an effective means for washing out the salts. 2. Leaching: Leaching is the process of dissolving and transporting soluble salts by the downward movement of water through the soil. The leaching may be done by two methods viz., (i) flooding and (ii) sprinkler methods. The flooding method requires more amount of water and removes the salt at a greater depth. The sprinkler method has been found efficient in removing the salts from top layer (60 cm) with less amount of water. The method to be adopted for reclamation depends on (i) the crop to be grown, (ii) topography, (iii) soil characteristics, (iv) availability of water, (v) depth of under ground water table and (vi) magnitude of salinity/sodicity in the soil.

125. 126 3. Drainage: Drainage in agriculture is the process of removal of excess water from soil. Excess water discharged by flow over the soil surface is referred to as surface drainage, and flow through the soil is termed internal or subsurface drainage. The term “artificial drainage” and “natural drainage” indicate whether or not man has changed or influenced the drainage process. The design of drainage systems is influenced by many factors which taken into consideration e.g. drainage requirement, water transmission properties of soil and boundary conditions, water application efficiency, physiography of land, etc. The types of relief drains are pumped wells, tile or open drains may serve of these purposes. The drain should be placed below 2 m depth with a distance of 25 to 75 m according to soil texture orienting perpendicular to the direction of ground water flow.

126. 127 CHEMICAL PROCEDURE – USE OF AMENDMENTS In case of saline sodic and sodic soils, the exchange complex is saturated to varying degree with Na. The reclamation procedure in such cases also involves the use of amendments for replacing exchangeable Na. A. Different types of amendments The chemical amendments used are as under : 1. Soluble calcium salts e.g. (i) Calcium chloride (CaCl 2 .2H 2 O) (ii) Gypsum (CaSO 4 .2H 2 O) (iii) Calcium sulphate (CaSO 4 ) 2. Acid or acid formers e.g. (i) Sulphur (S) (ii) Sulphuric acid (H 2 SO 4 ) (iii) Iron sulphate (FeSO 4 .7H 2 O) (iv) Aluminiumsulphate (Al 2 (SO 4 ) 3 .18H 2 O) (v) Lime sulphur (calcium poly sulphide ) (CaS 5 ) (vi) Pyrites (FeS 2 ) 3. Calcium salt of low solubility (i) Ground lime stone (CaCO 3 )

127. 128 (ii) By-product lime from sugar factories e.g. pressmud The kind and amount of chemical amendment to be used for the replacement of exchangeable Na in soils depend upon the soil characteristics, the desired rate of replacement and economic considerations. Soluble calcium salts are preferred when soil does not contain alkaline earth carbonates or calcium carbonate. Acid or acid formers are preferred when soil contains alkaline earth carbonates or CaCO 3 . Acid or acid formers are also used along with calcium salt of low solubility but the rate of reaction is very low. B. Chemical reactions of amendments in soil : The following chemical reactions illustrate the manner in which various amendments react in the different classes of alkali soils. In these equations the letter X represents the soil exchange complex.

128. 129 Class 1: Soils Containing Alkaline-Earth Carbonates GYPSUM: 2Na X + CaSO 4 ↔ Ca X 2 + Na 2 SO 4 SULPHUR: (1)2S + 3O 2 ↔ 2SO 3 (microbiological oxidation) (2) SO 3 + H 2 O ↔ H 2 SO 4 (3) H 2 SO 4 + CaCO 3 ↔ CaSO 4 + CO 2 + H 2 O* (4) 2Na X + CaSO 4 ↔ Ca X 2 + Na 2 SO 4

129. 130 C. The organic amendments: The organic amendments as such do not help in replacing the exchangeable Na as against the gypsum or other amendments. Primarily, they improve the physical condition of the soil by improving the aggregation in the soil. The most common organic amendment is the FYM which is added in the first year of reclamation @ 50 tonnes /ha and is reduced to half in succeeding years. The efficiency of gypsum has been found to increase when it is applied along with FYM. Molasses and pressmud , which are sugar factory waste, have also been used. Molasses contain 60 – 70 % carbohydrates, 4 – 5 % potash, 2 % lime and 0.5 % each of N, P 2 O 5 , H 2 SO 4 and iron and aluminium oxide.

130. 131 Green manuring with Dhaincha ( Sesbaniaaculeata ) has been found most successful. The juice of green plants can neutralize high alkalinity, its initial pH being 4.01, with only slight rise even within a month. In black cotton soil, it thrives well under moderately saline conditions and can with stand high alkalinity, water logging or drought so that it is remarkably suited in that region to alkali soils, characterized by such adverse conditions. Sulphurated hydrogen is generated by the decomposition of Dhaincha . Paddy straw or rice husk have also been used at a rate varying between 15 to 30 tonnes /ha. Weeds like Argemonemexicana has been found very suitable for alkali soils. It contains (on dry weight basis) 1.8 % KNO 3 , 0.3 % CaHPO 4 , 0.4 % CaSO 4 , 4.2 % organic acid and 0.8 % sugar. When powder of argemone was added to the soil @ 2.5 tonnes /ha, it lowered the soil pH from 10.0 to 7.8 which slowly leveled to 8.5 in 30 days. The other weeds found suitable for the purpose of green manuring are Ipomea grandiflora and Pongamiaglabra . The Russian workers have suggested the addition of cellulose with sufficient addition of nitrogen for easy decomposition.

131. 132 MANAGEMENT PRACTICES During the process of reclamation of salt affected soils the field is not kept fallow. Growing of crop is always practiced as the roots of the growing crop exert a marked beneficial effect on the process of leaching by improving the state of aggregation of soil. A crop can be grown by adopting following practices A. Selection of crop The selection of crop is based on tolerance of a crop to either salinity or sodicity. The list of salt tolerant as well as sodium tolerant crops is given below:

132. 133 Crop EC x 10 3 of saturation extract causing yield decreases 10 % 25 % 50 % Field crops Barley for grain 12 16 18 Sugar beet 10 13 16 Cotton 10 12 16 Safflower 8 11 12 Wheat 7 10 14 Sorghum 6 9 12 Soybean 5.5 7 9 Sugarcane 3 7 5.5 Rice 5 6 8 Corn 5 6 7 Bean 1.5 2 2.5 Vegetable crops Beet (garden) 8 10 12 Tomato 4 6.5 8 Cabbage 2.6 4 7 Potato 2.5 4 6 Sweet potato 2.5 3.5 6 Onion 2 2.5 4 Carrot 1.5 2.5 4 Table 1: Yield decrease of certain crops due to variable salt levels in soil solutions

133. 134 ESP Class Crop 2 – 10 Very sensitive Deciduous fruits, Nuts, Citrus, Avocado 10 – 20 Sensitive Beans 20 – 40 Moderately tolerant Clover, Oats, Tall fescue, Rice, Dallis grass 40 – 60 Tolerant Wheat, Cotton, Alfalfa, Barley, Tomatoes, Beet More than 60 Highly tolerant Crested wheat grass, Fairway wheat grass Tall wheat grass, Rhodes grass The salt tolerance limits given in Table 1 are general. The salt tolerance of a crop would vary according to age of the crop or the growth stage. It will also vary according to variety of a given crop. Hence considerable work is needed for establishing these limits for local crops and their varieties. The tolerance of various crops to ESP is given in Table 2.   Table 2: Tolerance of various crops to ESP

134. 135 Crop Varieties Wheat : Kharachia, J-24, Popatia, Arnej-206, Sonalika, KalyanSona Bajra : B.K.-560, GHB-235, MH-169, MH-179, GHB-227 for fodder Cotton : G.Cot.D.H.-7, Dhumal , Kalyan Sugarcane : Co.8338, Co.791 Groundnut : JL-24, J-11, Robert, Punjab-1 Castor : GCH-4, SKF-73, GAUCH-1, VP-1 Jowar : Gundari, C-10-2, CSH-5, S.R.F.-204 for fodder Paddy : T.N.-1, Jaya Mustard : Varuna, A.S.-10 Sunflower : EC-68414, EC-68415 Pigeon pea : GT-100, GT-1 Chick pea : ICCC-4, JCP-29 Screening of varieties of different crops for saline condition

135. 136 B. Tillage: Deep tillage should be practiced as it increases the permeability of the soil thereby facilitating leaching of salts. It also makes the root zone more friable. If the subsoil layer has accumulation of salts or sodium, the deep tillage is not advisable. C. Layout: Crop sown in furrow show better performance than those in flat layout. Salt accumulate on ridges leaving the furrow relatively free of salts. However in alternate furrow irrigation system, sowing at ridges can be advocated. D. Seed rate and spacing: Higher seed rate and closer spacing have been found satisfactory as high plant populations insures against the failure of germination due to salt stress.

136. 137 E. Irrigation and drainage: Maintenance of low moisture stress by frequent irrigation and applying water in excess enables the plant to grow better. Among the methods of irrigation, the minimum salt accumulation takes place in the order of drip > sprinkler > flood basin > ridge furrow. F. Fertilizers: A higher dose of N, P 2 O 5 and K 2 O than recommended dose has given higher yields of many crops such as rice, barley, wheat, etc. Addition of micronutrients, particularly Zn, Fe and Mn has helped in increasing the yield. In saline conditions urea, single super phosphate and calcium ammonium nitrate, while in sodic conditions ammonium sulphate and diammonium phosphate found more effective. In highly sodic condition, foliar application of urea is the only effective and economical method of fertilization. In highly saline condition phosphatic fertilizer did not found economical.

137. 138 Volume of water for 5 cm hectare water = 100 x 100 x 5 = 500 cu.m . 100 kg salt added due to 5 cm ha water = Cu.m water. x 1000 x % salt 100 ESP = Na x 100 Total cations IMPORTANT FORMULA USED IN CALCULATING EXAMPLES (A) Water sample % soluble salt = EC x 640 , where EC in dS /m ppm salt = % salt x 10,000 % soluble salt = ppm salt/10,000 RSC = (CO 3 + HCO 3 ) – (Ca + Mg) in me/l Total cation concentration in me/l = EC  x 10   where EC in dS /m Na me/l = EC ( dS /m) x 10 – (Ca + Mg)                     1 Cu.m. Water = 1000 kg or liter

138. 139 ACID SOILS: Soil acidity is more common in humid regions with rainfall high enough to leach appreciable quantities of exchangeable base forming cations ( Ca++, Mg++, K+, Na+ ) from surface soil layers. In Gujarat, it is found in soils of some part of Waghai and Saputara region, where rainfall is very high. The pH of acidic soil is less than 5.5 Characteristics of Acid Soils Acid soils have low pH and high proportion of exchangeable H + and Al 3+ . Kaolinite and illite types of clay minerals are dominant in these soils. These soils have low CEC and low base saturation. These soils have high toxic concentration of Al, Fe and Mn and deficiency of Ca and Mg. These soils have nutrients and microbial imbalances. These soils are generally low in available phosphorus. Soil acidity inhibits biological N-fixation.

139. 140 CAUSES OF SOIL ACIDITY 1 . Excessive rainfall : In soils of dry region, a large supply of bases is usually present because little water passes through the soil. With an increase in rainfall, the content of soluble salts is reduced to a low level and gypsum and CaCO 3 are removed in the order named. With further increase in rainfall, a point is reached at which the rate of removal of bases exceeds the rate of liberation from non-exchangeable forms. The considerable loss of bases due to intensive rainfall and leaching reduces the pH of the soil as well as increase the concentration of H + on exchange complex.

140. 141 2. Ionization of water : The water may ionize and contribute H + on exchange complex as follows: H 2 O →HOH → H + OH - → H + [ X] + Bases + OH - 3. Contact exchange : The contact exchange between exchangeable H on root surface and the bases in exchangeable form on soil particle may take as follows :

141. 142 4. Soluble acid production : The decomposition of organic matter in the soil produces many organic as well as inorganic acids. These acids may contribute H on exchange complex. 5. Use of nitrogenous fertilizers : Continuous use of nitrogenous fertilizers containing NH 4 -N or giving NH 4 -N on hydrolysis (i.e. urea) produce various acids in soils e.g. 1 mole of NH 4 in NH 4 NO 3 gives 2 moles of HNO 3 ; 1 mole of (NH 4 ) 2 SO 4 gives 2 moles of HNO 3 + 1 mole of H 2 SO 4 ; 1 mole of NH 4 OH gives 1 mole of HNO 3 . Thus, continuous use of such fertilizers will produce acidity in soil. 6. Oxidation of FeS : FeS or iron poly sulphide accumulates under anaerobic conditions as a result of reduction of Fe 3+ and SO 4 . Under aerobic conditions, they will be oxidized and will produce H 2 SO 4. Under such conditions, soil pH values of 2 to 4 are frequently observed. 4FeS 2 + 15O 2 + 2H 2 O → 2Fe 2 (SO 4 ) 3 + 2H 2 SO 4

142. 143 7. Hydrolysis of Fe 3+ and Al 3+ : The Fe 3+ and Al 3+ ions may combine with water and release H + as follows : Al + H 2 O → Al(OH) 2- + H + Al(OH) + H 2 O → Al(OH) 2 - + H + Al(OH) 2 + H 2 O → Al(OH) 3 + H + The hydrogen produced may enter on exchange complex. 8. Acidic parent material : Some soils have developed from parent materials which are acid, such as granite and that may contribute to some extent soil acidity.

143. 144 9. Acidification from the air : Industrial exhausts, if contain appreciable amount of SO 2 may cause acidity in soil in course of time due to dissolution of SO 2 in water (rain) as follows : SO 2 + H 2 O → H 2 SO 3 (Rain water) 2H 2 SO 3 + O 2 → 2H 2 SO 4 (Sulphuric acid)

144. 145 FORMS OF ACIDITY: Because of the increase in H ion concentration in soil solution some of them occupy the position on exchange complex because of its very high replacing ability. The situation gives high H ion concentration in soil solution and on exchange complex, or the acidity in solution and on exchange complex. The acidity in soil solution is known as Active Acidity and is measured by pH. The acidity on exchange complex is known as Passive Acidity or Reserve Acidity and is measured by determining the exchangeable H by BaCl 2 + Triethanol amine reagents. The total acidity or the titratable acidity is summation of H ion concentration present in solution as well as on exchange complex which can be measured by titration. All the forms of acidity are in equilibrium.

145. 146 PROBLEMS IN ACIDIC SOILS Problems of soil acidity may be divided into three groups: 1. Toxic effects (a) Acid Toxicity : The higher hydrogen ion concentration is toxic to plants under strong acid conditions of soil. The acid toxicity includes possible toxicities of acid anions as well as H + ions. (b) Toxicity of elements (i) Iron, Manganese and Aluminium : The concentration of these ions (Fe 2+ , Mn 2+ and Al 3+ ) in soil increased in acidic condition to a very high and toxicity of these elements develop.

146. 147 2. Nutrient Availability ( i ) Exchangeable Bases : There are two aspects of availability of exchangeable bases i.e., ion uptake process and the release of bases from the exchangeable form may be adversely affected due to soil acidity. Deficiency of bases like Ca 2+ and Mg 2+ are found in acid soils.

147. 148 (ii) Nutrient Imbalances : It is evident that soluble iron, aluminium and manganese are usually present in their higher concentrations under moderate to strong acid soils. Phosphorus reacts with these ions and produces insoluble phosphatic compounds rendering phosphorus unavailable to plants. Besides these, fixation of phosphorus by hydrous oxides of iron and aluminium or by adsorption, the availability of phosphorus is decreased. In acid soils, iron, manganese, copper and zinc are abundant, but molybdenum is very limited and unavailable to plants. In acid soils having very low pH, the availability of boron may also be decreased due to adsorption on sesquioxides , iron and aluminiumhydroxy compounds. Nitrogen, potassium and sulphur become less available in an acid soil having pH less than 5.5.

148. 149 3. Microbial Activity : It is well-known that soil organisms are influenced by fluctuations in the soil reaction. Bacteria and actinomycetes function better in soils having moderate to high pH values. They can not show their activity when the soil pH drops below 5.5. Nitrogen fixation in acid soils is greatly affected by lowering the activity of Azotobacter sp. Besides these, soil acidity also inhibits the symbiotic nitrogen fixation by affecting the activity of Rhizobium sp. Fungi can grow well under very acid soils and caused various diseases like root rot of tobacco, blights of potato, etc.

149. 150 Reclamation of Acidic Soils Principles of Liming Reactions: The reclamation of acidic soils is done by addition of liming material which may be calcitic limestone (CaCO 3 ) or dolomitic limestone [ CaMg (CO 3 ) 2 ]. The rate of lime requirement is determined in the laboratory by method of Shoemaker (1961). The particle size of liming material affects the rate of neutralization reaction. Both these limestones are sparingly soluble in pure water but do become soluble in water containing CO 2 . The greater the partial pressure of CO 2 in the system, the more soluble the limestone becomes. Dolomite is somewhat less soluble than calcite. The reaction of limestone (CaCO 3 ) can be written as: CaCO 3 + H 2 O + CO 2 → Ca(HCO 3 ) 2 Ca(HCO 3 ) 2 → Ca 2+ ↓ + 2HCO 3 - (Takes part in cation exchange reactions) H + + CO 3 2- → H 2 CO 3 - ↔ H 2 O + CO 2 (From soil solution) (from lime)

150. 151 In this way hydrogen ions (H + ) in the soil solution react to form weakly dissociated water, and the calcium (Ca 2+ ) ion from limestones is left to undergo cation exchange reactions. The acidity of the soil is, therefore, neutralized and the per cent base saturation of the colloidal material is increased. Why Gypsum is not considered as a Liming Material? Gypsum is not considered as liming materials because on its application to an acid it dissociates into (Ca 2+ ) and sulphate (SO 4 2- ) ions: CaSO 4 ↔ Ca 2+ + SO 4 2- The accompanying anion is sulphate and it reacts with soil moisture produces mineral acid (H 2 SO 4 ) which also increases soil acidity instead of reducing soil acidity.

151. 152 Beneficial effect of lime Lime makes P 2 O 5 more available. Lime increase availability of N, increase nitrification and nitrogen fixation. Increase soil pH favours the microbial activity and increase organic matter decomposition and nutrient transformation for root growth. Mo an essential element to rhizobium in N fixation process increases with increase in soil pH following lime. Reduce toxicity of Al, Fe and Mn . Lime is essential source of essential Ca as well as Mg if dolomitic lime stone has been applied as liming material. It causes an increase in CEC, which reduces the leaching of base cations, particularly K.

152. 153 Reclamation and Management of Alkali (Saline alkali and non-saline alkali) Soils. Alkali soils cannot be reclaimed by mere flooding the land. In the case of saline0alkali soils, flooding is likely to do more harm. Leaching (Flooding) down of soluble salts make the soil alkaline (Only Na-Clay remain in the soil). Soil get dispersed and become compact (impervious). In alkali (non-saline-alkali) soils, exchangeable sodium Na-Clay-is so great as to make the soil almost impervious to water. But even if water could move downward freely in alkali soils, the water alone would not leach out the excess exchangeable sodium. The sodium- cation must be replaced by calcium- cation and then leached downward. Following chemical methods are use for reclaiming the alkali soils :

153. 154 Na 2 CO 3 + CaSO4 Na + + CaSO 4 Ca ++ Clay Complex Clay Complex + Na2SO 4 ↓ Leachable Gypsum Alkali soil Na + (A) Chemical Methods . (i) Application of gypsum. By cationic exchange, calcium is often used to replace sodium in alkali soil. If the soil has no reserve of calcium carbonate, the addition of gypsum (calcium sulphate ) is necessary. When gypsum is used as a reclaiming agent, calcium replace the exchangeable sodium and converts the clay back into calcium-clay (Ca-clay). Normal soil Leachable CaCO 3 + Na 2 SO 4 ↓

154. 155 Sodium sulphate goes into solution and is then remove by washing it out with water or leaching down with water with the help of artificial drains. Addition of gypsum improves physical conditions of soil. Soils become flocculated and drainage improves. pH is lowered down to a desirable level. Gypsum requirement in alkaline soil. For reasonable crop production on a sodic soil, the lowering of the ESP to the level of 10 is considered sufficient. The amount of gypsum required to be added to a sodic soil to lower the ESP to a desired value is known as gypsum requirement. It is expressed in milliequivalents of Ca ++ per 100 gm of soil. Gypsum requirement can be calculated from the data on CEC and ESP of the soil. For a sodic soil, suppose, CEC=30 and ESP=60, gypsum requirement to lower the ESP to 10, will be or = 10 m.e . of Ca ++ per 100 gm soil.

155. 156 Amendments Gypsum equivalent 1. Gypsum (CaSO 4 .2H 2 O) 1.00 2. Sulphur (S) 0.19 3. Sulphuric acid (H 2 SO 4 ) 0.57 4. Iron sulphate (FeSO 4 .7H 2 O) 1.62 5. Iron pyrite (FeS 2 ) 0.63 Besides gypsum that is best soil amendement for sodic soil, several other materials may be used for reclaiming alkaline soils. Gypsum equivalents of some such materials are given below:

156. 157 (ii) Use of sulphur. In the case of alkali soil that contain free calcium carbonate, addition of sulphur. sulphuric acid, iron and aluminiumsulphate , green manure (produce acidity) etc. reclaim the soil very effectively. The acidity developed during the course of their decomposition in soil, naturalises alkalinity. At the same time brings calcium carbonate into solution which then reacts with the sodium clay and converts it into calcium clay. When sulphur is spread on the soil, it is oxidised to sulphuric acid, which converts sodium carbonate into sodium sulphate if calcium carbonate is not present in the soil, it should be added artificially when sulphur is used for reclamation. Reactions are as follows: 2S + 2H 2 O + 3O 2 biological 2H 2 SO 4 Sulphur Oxidation Sulphuric acid Na 2 CO 3 + H 2 SO 4 CO 2 + H 2 O + Na 2 SO 4 ↓ Sodium Carbonate Leachable sodium sulphate 2CaCO 3 + H 2 SO 4 ---->CaSO 4 + Ca (HCO 2 ) 2 Calcium carbonate Calcium Sulphate In above mentioned both cases, it is necessary to leach out the sodium salts, formed as a result of bases exchanges with the help of artificial drains.

157. 158 (iii) Addition of organic matter. The addition of organic matter increases acidity, thus helps in lowering the pH. Organic matter is especially helpful where sulphur is added to correct the alkalinity. The organic matter sulpplies food for the bacteria that stimulates the oxidation of sulphur to the sulphate form. The combination of sulphur, organic matter and gypsum has also been used with success. (iv) Use of sulphuricacid. Sulphuric acid change the sodium carbonate to the less harmful sulphate and also tends to reduce the intense alkalinity. It should be used in the presence of calcium carbonate. Na 2 CO 3 + H 2 SO 4 CO 2 + H 2 O + NaSO 4 ↓ Leachable

158. 159 (v) Addition of molasses. Addition of molasses in the soil provide the source of energy for microorganism which on fermentation produce organic acids. The organic acids reduce alkalinity.

159. 160 CALCAREOUS SOILS: In the context of agriculturalproblem soils, calcareous soils are soils in which a high amount of calcium carbonate dominates the problems relatedtoagricultural land use.  They are characterized by the presence of calcium carbonate in the parentmaterial and by a calcic horizon , a layer of secondary accumulation of carbonates (usually Ca or Mg) in excess of 15% calcium carbonate equivalent and at least 5% more carbonate than an underlying layer. Calcareous soils cover more than 30% of the earth surface, and their CaCO 3 content varies from a few percentto 95%. Hagin and Tucker (1982) define calcareous soil as a soil that its extractable Ca and Mg levels exceed the cation exchangecapacity .

160. 161 Origin of Calcareous Soils Calcareous soils occur naturally in arid and semi-arid regions because of relatively little leaching They also occur in humid and semiarid zones if their parentmaterial is rich in CaCO 3 , such as limestone, shells or calcareous glacial tills, and the parent material is relatively young and has undergone little weathering. Some soils that develop from calcareous parent materials canbe calcareous throughout their profile. This will generally occur in the arid regions where precipitation is scarce. In other soils, CaCO 3 has been leached from the upper horizons, and accumulated in B or C horizons. These lower CaCO 3 layers can be brought to the surface after deep soil cultivation.

161. 162 In some soils, the CaCO 3 deposits are concentrated into layers that may be very hard and impermeable to water. These caliche layers are formed by rainfall leaching the salts to a particular depth in the soil at which water content is so low that carbonates precipitate. Soils can also become calcareous through long period of irrigationwith water containing dissolved CaCO 3

162. 163 Main production constraints Calcareous soils develop in regions of low rainfall and must be irrigated to be productive.  Therefore one of the main production constraints is the availability of water for irrigation. The quality of the irrigation water is of crucial importance for sustainable agricultural production on calcareous soils.  Frequently, the irrigation water is the cause of many management problems.  Almost all waters used for irrigation contain inorganic salts in solution.  These salts may accumulate within the soil profile to such concentrations that they modify the soil structure, decrease the soil permeability to water, and seriously injure plant growth. Crusting of the surface may affect not only infiltration and soil aeration but also the emergence of seedlings. Cemented conditions of the subsoil layers may hamper root development and water movement characteristics.

163. 164 Calcareous soils tend to be low in organic matter and available nitrogen. The high pH level results in unavailability of phosphate (formation of unavailable calcium phosphates as apatite) and sometimes reduced micronutrient availability e.g. zinc and iron (lime induced chlorosis ). There may be also problems of potassium and magnesium nutrition as a result of the nutritional imbalancebetween these elements and calcium

164. 165 Role of CaCO 3 in plant nutrition: The carbonates are characterized by a relatively high solubility, reactivity and alkaline nature; their dissolution resulting in a high solution HCO 3 - concentration which buffers the soil in the pH range of 7.5 to 8.5: CaCO 3 + H 2 O Ca 2+ + HCO 3 - + OH - Symptoms of impaired nutrition in calcareous soils are chlorosis and stunted growth. This is attributed to the high pH and reduced nutrient availability, as direct toxicity of bicarbonate ions (HCO 3 - ) to physiological and biochemical systems are much less likely. The availability of P and Mo is reduced by the high levels of Ca and Mg that are associated with carbonates. In addition, Fe, B, Zn, and Mn deficiencies are common in soils that have a high CaCO 3 due to reduced solubility at alkaline pH values