Lecture 4-Nutrient Bioavailability Summer 2024.pdf

Soil Fertility and Fertilizers(0604223) 4- Nutrient Bioavailability Areej AL Khreisat Professor T.M. Abu-Sharar Department of Land, Water and Environment Faculty of Agriculture The University of Jordan ...

Soil Fertility and Fertilizers(0604223) 4- Nutrient Bioavailability Areej AL Khreisat Professor T.M. Abu-Sharar Department of Land, Water and Environment Faculty of Agriculture The University of Jordan  Plant Mineral Nutrients For an element to be regarded as essential, a plant cannot complete its life cycle without the element, no other element can perform the function of the element, and the element is directly involved in plant nutrition. Plants require light, water, and 16 essential elements plus five others of limited essentiality to support all their biochemical needs. For an element to be regarded as essential, three criteria are required: 1. Plant cannot complete its life cycle without that element, 2. No other element can perform the function of that element, 3. The element is directly involved in plant nutrition (direct involvement in plant metabolism). In addition to the above definitions, an element is considered as essential when plants cannot complete vegetative or reproductive stage of life cycle due to its deficiency. This deficiency is corrected only by supplying that element.  What are the Essential Nutrients? Plants need 16 elements for their growth and completion of life cycle. They are: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, zinc, copper, boron, molybdenum and chloride. In addition, five more elements (sodium, cobalt, vanadium, nickel and silicon) are absorbed by some plants for special purposes. According to the former criterion, sodium is considered as nonessential. However, sodium is known to increase yield of several crops such as sugar beet, turnip and celery. Therefore, the term 'functional nutrient’ or limited essentiality is proposed for any mineral element that functions in plant metabolism whether or not its action is specific. With this criterion, the above 5 elements are also considered as functional nutrients in addition to 16 essential elements. Although, Al is not an essential plant nutrient, its concentrations in plant can be high when soil solution contains relatively large amounts of Al as in the case of highly weathered soils of Brazil (oxisols). In fact, plants absorb many non-essential elements and over 60 elements have been identified in plant materials. Among these 21 elements, all carbon atoms and most of the oxygen atoms are derived from CO2, which is assimilated principally in photosynthesis. More specifically, approximately one-third of oxygen atoms in organic material in higher plants are derived from soil water and two-thirds from CO2 of the atmosphere. The elements C, H, O are not minerals. The rest of the elements are absorbed from the soil and these are called mineral elements since they are derived from minerals. These mineral elements are mainly absorbed in ionic form and to some extent in non-ionic form as shown in the Table to follow.  Limited or Functional Essentiality When plant material is burned, the remaining plant ash contains all the essential and non-essential mineral elements as ash oxides except the oxides of C, H, O, N and S which are burned off as gases. The ash can be used as a source of nutrients. For example, wood fires leave the ash on soil surface where it turns to the hydroxides of these elements following rainfall events and help rejuvenate the wood as shown below: CaO + H2O = Ca(OH)2 Ca(OH)2 + CO2 = Ca(HCO3)2 Ca(HCO3)2 2+ + 2HCO3- = Ca The plant content of mineral elements is affected by many factors and their concentration in crops varies considerably. Such a variation doesn’t mean a variation in essentiality. All of them are essential for crops irrespective of their concentrations in plants. Plant nutrient data are valuable to successful fertilizer management programs and can be used to help establish fertilizer recommendations. Forms of mineral elements absorbed by plants. The term EDTA is the abbreviation of ethylene diamine tetra acetic acid. Mineral element Ionic form Non-ionic form Nitrogen (N) NH4 +, NO3-, CO (NH2)2 Phosphorus (P) H2PO4 -. HPO42- Nucleic acid, phytin Potassium (Kalium-K) K+ Calcium (Ca) Ca2+ Magnesium (Mg) Mg2+ Sulphur (S) SO42- SO2 Iron (Fe) Fe2+, Fe3+ FeSO4 with EDTA Manganese (Mn) Mn2+ MnSO4 with EDTA Zinc (Zn) Zn2+ ZnSO4 with EDTA Copper (Cu) Cu2+ CuSO4 with EDTA Boron (B) B4O72-, H2BO3-, HB032- Molybdenum (Mo) MoO42- Chloride (CI) CI- Chemical Composition of Earth Crust  Macronutrients and Micronutrients Depending on the quantity of nutrients present in plants, they can be grouped into three: 1- Basic (Structural) Nutrients:The basic nutrients of carbon, hydrogen and oxygen, constitute 96 per cent of total dry matter of plants. Among them, carbon and oxygen constitute 45 per cent each. The total dry matter produced by rice crop in one season is about 12 t/ha. In this 5.4 t is carbon, 5.4 t is oxygen and 0.7 t is hydrogen. 2- Macronutrients: The nutrients required in large quantities are known as macronutrients. They are N, P, K, Ca, Mg, and S. Among these, N, P and K are called “primary nutrients” and Ca, Mg and S are known as “secondary nutrients”. The later are known as secondary nutrients as they are inadvertently applied to the soils when N, P and K fertilizers, which contain these nutrients, are used. 3- Micronutrients: The nutrients which are required in small quantities are known as micronutrients or trace elements. They are Fe, Zn, Cu, B, Mo and “Cl”. These elements are very efficient and minute quantities produce optimum effects. On the other hand, even a slight deficiency or excess is harmful to the plants.  Nutrients Functions in Plant: Summary Based on the functional role of nutrients, they are grouped into four categories: 1. Elements that provide basic structure to the plant: C, H and O. 2. Elements useful in energy storage, transfer and bonding: N, S and P. These are accessory structural elements which are more active and vital for" living tissues. 3. Elements necessary for cell charge balance: K, Ca and Mg. These elements act as regulators and carriers. 4. Elements involved in enzyme activation and electron transport: Fe, Mn, Zn, Cu, B, Mn and Cl. These elements are catalyzers and activators.  Nutrients Mobility in Soil Profile Mobility of nutrients in the soil has considerable influence on their intensity and capacity factors and on method of their fertilizer application. For plants to take up these nutrients, two processes are important: (1) movement of nutrient ions to the absorbing root surface, and (2) roots reaching the soil volume where nutrients are available. In the case of immobile nutrients, the roots have to reach the soil volume where nutrients are availability. The soil forage volume is limited to root surface area or the hyphae of mycorrhizae. For highly mobile nutrients, the entire volume of the root zone is a forage area. Based on the mobility in the soil, the nutrient ions can be grouped as mobile, less mobile and immobile: 1. Mobile nutrients are highly soluble and aren’t or slightly adsorbed on the clay complex; e.g., N03-, S042-, and Cl- 2. Less mobile nutrients are also soluble, but they are adsorbed on clay complex and so their mobility is reduced; e.g., Na+, NH4+, K+, Ca2+, Mg2+, Cu2+, B(0H)4-, and Mn2+. 3. Immobile nutrient ions are highly reactive and get fixed onto the solid complex; e.g. HPO42-, H2PO4-, Fe3+, and Zn2+. Example of soluble ions precipitation: Subsurface calcium carbonate nodules or hard pan.  Nutrient Mobility in Soil Profile Figure. Nutrient gradients and the formation of nutrient patches in soils. Figure. Nutrient gradients and the formation of nutrient patches in soils Many processes are involved in the formation of vertical gradients in soils, such as nutrient uptake, nutrient leaching, biological cycling, and movement of water. An increased nutrient uptake from the superficial soil strata decreases nutrient concentrations in this soil layer. In addition, concentrations of mobile nutrients may decrease as they move downward and become prone to leaching. Biological nutrient cycling acts in an opposite way to leaching, because it recovers nutrients from deeper soil profiles and brings them back to the surface in the form of litter deposits. In addition, the increased topsoil deposition of organic matter (O.M.) increases, for example, the availability of P and organic nitrogen (org. N) in this soil layer. Some nutrients, such as Ca and Mg, do not generally show strong vertical gradients in most soils. At a smaller scale, the intense uptake of immobile (purple) and mobile (green) nutrients can result in the formation of depletion zones for the immobile nutrient at the root-soil interface. Furthermore, localized organic matter depositions associated with intense microbial activity can result in increased availability of the immobile nutrient iron (Fe) within relatively localized patches in soils. Phosphate (PO4 -2 ); sulfate(SO4 -2 ); potassium ion (K+); manganese ion (Mn2+); sodium ion (Na+ ).  Cation Exchange Capacity (CEC) Exchangeable cations on the soil solid complex (organic and inorganic) aren’t leached away from roots.  Nutrients Mobility in Plants Mobility of ions in plant isn’t the same as their mobility in the soil profile. For example, P is highly mobile in plant but isn’t in the soil. Calcium is immobile in plant but mobile in soil as long as partial pressure of carbon dioxide is relatively high. When the pressure is reduced, calcium precipitates as calcium carbonate. Knowledge of the mobility of nutrients in the plant helps in finding what nutrient is deficient. A mobile nutrient in the plant, moves to the growing points in case of deficiency. Deficiency symptoms, therefore, appear on the lower (old) leaves. - N, P and K are highly mobile. - Zn is moderately mobile. - S, Fe, Mn, Cu, Mo and CI are less mobile. - Ca and B are immobile.  Role of Nutrients in Plant Every essential element plays a specific role in plant physiology throughout growth, development and reproduction. These functions may be described as the following: 1- Carbon. It is available in abundance from air. Photosynthetic activities of green plants use CO2 to build plant cells. About 45% or more of the plant tissues is made up of carbon. 2- Hydrogen. It is essential for cell and tissue formation in plants. Hydrogen is obtained from water and is required for energetic reactions. It forms about 6% of the plant tissues. 3- Oxygen. Plants take oxygen from of the CO2 and, partially, from water. Oxygen forms about 43% parts of the plant structure. It is required for photosynthetic and respiratory activities and helps in the formation of plant cells. 4- Nitrogen. It is a major structural part of the cell. Cytoplasm and the particulate fractions of the cell organelles contain nitrogen in varying amounts along with C, H, O, P and S. Primary cells are found to have about 5% of nitrogen. Nitrogen plays a vital role in various metabolic activities of plants and is a constituent part of amino acids, proteins, nucleic acids, porphyrins, flavins, purine and pyrimidine nucleotides, enzymes, co-enzymes and alkaloids. Nitrogen helps capturing solar energy through chlorophyll, in energy transformation through phosphorylated compounds, and in transfer of genetic information through nucleic acids  Nitrogen in Soil Environment Nitrogen in soil is present in different chemical forms: 1- Organic Nitrogen: Here nitrogen is bound to organic substances of different natures. All these form can be released to soil solution as ammonium following organic decomposition (mineralization) by soil microorganisms. Urea [(NH2)2CO] can be included in this group. 2- Inorganic Nitrogen: This group includes ammonium and nitrate which are interrelated by microbially assisted transformation (oxidation) of ammonium (NH4+) to nitrite (NO2) which then can undergo further oxidation to nitrate (NO3-).  Soil Nitrogen: A Budget Model There are three major forms of nitrogen in soil: a) organic nitrogen associated with the soil humus, b) ammonium nitrogen fixed by mica, and c) soluble inorganic ammonium and nitrate compounds. Most of the nitrogen in surface soils is associated with the organic matter. In this form it is protected from rapid microbial release, only 2-3% a year being mineralized under normal conditions. About half the organic nitrogen is in the form of amino compounds. The form of the remainder is uncertain. Much scientific effort has been devoted to study the organic N: how it is stabilized and how it may be released to forms usable by plants. The process of tying up N in organic forms is called immobilization; its slow release from organic to inorganic forms is called mineralization. Climate, natural vegetation, texture, drainage and other soil factors and cropping are the main factors influence organic matter and nitrogen. The inherent capacity of soil to produce crops is closely and directly related to their organic matter and nitrogen content. Second, the satisfactory level of these two constituents is difficult to maintain in the majority of farm soils. Consequently, the methods of organic matter additions and upkeep should receive priority considerations in all soil management programs. Figure. Concept of On farm nutrient management and conservation of soil fertility  Bioavailability and Role of Nutrients in Plant: Phosphorus 5- Phosphorus plays a vital role as a structural component of cell constituents and metabolically active compounds. It is a structural part of the membrane system of the cell, the chloroplasts and mitochondria. It is a part of sugar phosphates-ADP, ATP, etc. nucleic acids, nucleoproteins, purine, pyrimidine nucleotides, flavin nucleotides and many co-enzymes like NADP, pyridoxyl phosphate and thiamine phosphate. The most essential constituents of plant cells like esters, phosphatides and phospholipids are synthesized by phosphorus when it combines with different organic acids. It also plays an important role in energy transformations and various metabolic activities of plants. Being a constituent of adenosine phosphate, phospho-glycerol aldehyde and ribulose phosphate, it helps in basic reactions' of photosynthesis and activates several enzymes participating in dark reactions in photosynthesis. Bioavailability of inorganic P is largely determined by a) soil pH, b) soluble Fe, Al and Mn, c) presence of Fe, Al and Mn containing minerals, d) available Ca and Ca-minerals e) amount and decomposition of organic matter and activities of microorganisms. The first four factors are interrelated since soil pH drastically influences the reaction of P with the different ion and minerals. The availability of phosphorus to plants is determined to no small degree by the ionic form of this element. The ionic form in turn is determined by the pH of the solution in which the ion is found. Thus, in highly acid solutions only H2PO4- ions are present. If the pH is increased, first HPO42- ions and finally PO43- ions dominate. At intermediate pH levels two of the phosphorus ions may be present simultaneously. Thus, in solutions at pH 7.0, both H2PO4- and HPO42- ions are found. The H2PO4- ion is somewhat more available to plants than is the HPO42- ions. In soil, however, this relationship is complicated by the presence or absence of other compounds or ions. For example, the presence of soluble Fe an Al under very acid conditions, or Ca at high pH value, will markedly affect the availability of the P.  Bioavailability and Role of Nutrients in Plants: Phosphorus In addition to pH and related factors, organic matter and microorganisms strikingly affect inorganic P availability. Just as was the case with N, the rapid decomposition of organic matter and consequent high microbial population result in the temporary tying up of inorganic P in microbial tissue. Products of organic decay such as organic acids and humus are thought to be effective in forming complexes with iron and Al compounds. This engagement of Fe and Al reduces inorganic P fixation to a remarkable degree. Apparently the manure was effective in releasing P after it has been fixed as Fe, Al and Ca phosphates. Even though both phytin and nucleic acids can be utilized as source of P, inorganic source of this element are needed for normal production. Plants commonly suffer from a phosphorus deficiency even in the presence of considerable quantities of organic forms of this element. Just as with inorganic P, the problem is one of availability. In acid soils the phytin is rendered insoluble and thus unavailable because of reaction with Fe and Al. Under alkaline conditions Ca phytate precipitates and its P is rendered unavailable. The fixation of nucleic acids involves an entirely different mechanism but the end result would be low P availability. Finally, nucleic acids are strongly adsorbed by clay minerals, especially montmorillonite.  Phosphorus in Environment Phosphorus is common within geological materials. The average continental crust contains 0.27% P2O5. Phosphorus is the primary resources to produce fertilizer and phosphorous-based products. Phosphorus is neither substitutable nor recyclable, therefore, the total demand must be provided through the mining, beneficiation and chemical processing of phosphate ores. The key to understanding the association between environmental pollution and phosphate rocks lies in appreciating the mining and processing effect of phosphate ores. Phosphorus is normally produced by mining and beneficiation of Phosphate ores. Mines produce large amounts of waste including toxic metals and radioactive elements. The mining and beneficiation process results in the majority of these hazardous elements being lost either to waste disposal or to the environment, mainly soil, water, atmosphere and human food chain. Apatite is the dominant mineral in phosphate ores. It may occur as carbonate-fluorapatite [Ca10 (PO4, CO3 )6 (OH, F)2] in sedimentary rocks and as hydroxyl-fluorapatite [Ca10 (PO4)6 (OH)2] in igneous rocks. Apatite is commonly very insoluble in its original state as extracted from the earth and is practically unavailable as a plant phosphorus source. For this reason, drastic chemical processing with strong acids (such as Sulfuric acid, phosphoric or nitric acids) is necessary to produce soluble phosphate products. By virtue of its chemical behavior, apatite is generally associated with fluoride, U, Ra, Rn, Pb and other heavy metals. All pose potential risk for human health and environment.  Effect of pH on Predominance of Phosphorus Ionic Species in Soil Solution Orthophosphoric acid (H3PO4) is a weak acid and dissociates in three-steps as the following: H3PO4 = H2PO4- + H+ K1 = 10-2.15 H2PO4- = HPO4-2 + H+ K2 = 10-7.21 HPO4-2 = PO4-3 + H+ K3 = 10-19.56 Predominance of any species depends on soil solution pH. For example: K1 = 10-2.15 = (H2PO4-) (H+)/(H3PO4) or (H2PO4-)/(H3PO4) = (10-2.15)/( H+) and Log[(H2PO4-)/(H3PO4)] = -2.12+pH. At pH 7.5, the relation becomes: = 5.38 i.e. the ionic form (H2PO4-) becomes almost 220,000 times the molecular form (H3PO4). Effect of Mycorrhizal Root Infection on P Uptake by Cereal Crops  Bioavailability and Role of Nutrients in Plant: Potassium 6- Potassium helps in the maintenance of cellular organization by regulating the permeability of cellular membranes and keeping the protoplasm in a proper degree of hydration by stabilizing the emulsions of colloidal particles. Its salts stabilize various enzyme systems. It plays a catalytic role in activating several enzymes as incorporation of amino-acids in proteins, synthesis of peptide bonds etc. Presence of potassium is essential for optimal activation of aldehyde dehydrogenase, phosphate acetyl transferase etc. in vitro. Potassium increases resistance in plants against drought (the following slide shows the mechanism by which guard cells of the stomata open and close in response to moisture stress), heat, frost and various diseases caused by fungi, nematode and other micro- organisms. It helps in formation of mechanical tissues in cereals resulting into resistance to lodging. In fruit crops it improves color, flavor and increases the size and weight of the fruits. The various forms of K in soils can be classified on the basis of availability in three general groups: 1- unavailable (K-feldspars like orthoclase, KAlSi3O8), 2- readily available (exchangeable and soluble in soil solution), 3- slowly available (non-exchangeable=fixed like K-mica). In arid and semi-arid regions, source of available-K largely comes from non-exchangeable forms. Several soil and weather conditions markedly influence the relation between fixed-K and available-K of which are clay mineralogy of the soil, cycles of wetting and drying, freezing and thawing, and the presence of excess lime.  Role of potassium in stomata opening-closure mechanism Stomata closure-opening mechanism is driven by a plant hormone called abscisic acid. When the acid is present inside the guard cells, the Ca2+ gets in and Cl-, K+, and H2O move out (cell vacuole water potential gets high) then the water moves out of the guard cells which become flaccid causing stomata closure. When K+ and H+ get into the guard cells, water potential drops driving the outside water to flow to the guard cells which become turgid causing stomata to open.  Bioavailability and Role of Potassium in Plant Ability of the various clay minerals to fix K varies widely. For example, 1:1 type clay such as kaolinite fix little potassium. Clays of the 2:1 type, such as vermiculite, smectite and fine grained mica (illite) fix K very readily and in large amounts. The mechanism for K fixation is probably the same as that for fixation of the ammonium ion. These two ions do not have as high an affinity for water of hydration as do other cations such as Na+ and Ca2+. As a consequence they can easily be dehydrated and then their small size is such as to permit them to fit snugly in the hexagonal cavity of the silica sheets of adjoining platelets of the 2:1 type clay minerals. Once in place, these ions become trapped as a part of the rigid crystal structure, thereby preventing normal crystal lattice expansion and reducing the CEC of the clay. The larger hydrated ions of cations such as Na+, Ca2+ and Mg2+ are not able to fit between these layers and consequently escape fixation. Alternate wetting and drying, and freezing and thawing have been shown to result in the fixation of K in nonexchangeable forms as well as its ultimate release to the soil solution. In humid regions (not in Jordan), applications of lime sometimes result in an increase in K fixation of soils. Under normal liming conditions this may be more beneficial than detrimental because of the conservation of K so affected. Thus, K in well drained soils is not as likely be leached out as drastically as is that in acid soils. These are conditions however, under which the effects of lime on the availability of K are undesirable. For example, in soils where the negative charge is pH-dependent liming can greatly reduce the level of K in the soil solution. Furthermore, high Ca levels in the soil solution may reduce K uptake by the plant. Finally, K deficiency has been noted in soils with excess calcium carbonate. K fixation as well as cation ratios may be responsible for these adverse effects. Example: Phenotypic Nitrogen Uptake by Crops  Bioavailability and Role of Nutrients in Plant: Calcium In soils not containing CaCO3, Ca-Mg (CO3)2, or CaSO4, the amount of soil solution Ca2+ depends on the amount of exchangeable Ca2+. Soil factors of the greatest importance in determining the Ca2+ availability to plants are the following: 1. Total Ca supply 2. Soil pH 3. CEC 4. Percent of exchangeable Ca2+ (exchangeable calcium divided by CEC) 5. Type of soil colloid 6. Ratio of Ca2+ to other cations in solution. Total Ca in sandy or acid soils with low CEC can be too low to provide sufficient Ca2+ to crops. On such soils, supplemental Ca may be needed to supply Ca2+. High (H+) activity will impede Ca2+ uptake. For example, much higher Ca2+ concentrations are required for soybean growth as the pH is lowered from 5.6 to 4.0. In some instances, calcium deficiency may appear on crops growing even in calcareous soils when the need for Ca exceeds the potential soil supply of that nutrient. This had been the case in apple orchards in Shubak mountains some time ago. In acid soils, Ca is not readily available to plants at low saturation. For example, a low CEC soil having only 1000 ppm exchangeable Ca2+ but representing a high %Ca saturation might well supply plants with more Ca2+ compared to 2000 ppm exchangeable Ca2+ with a low %Ca saturation on a high CEC soil. In other words, as the %Ca2+ saturation decreases in proportion to the total CEC, the amount of Ca2+ absorbed by plants decreases.  Bioavailability and Role of Nutrients in Plant: Calcium and Magnesium 7- Calcium regulates permeability of cellular membrane. Calcium is a also a structural part of the chromosomes in which it binds the DNA with protein. It is required by a number of enzymes for their proper functioning like lipase, phosphatase and amylase. Calcium makes the stems stiff and thereby reduces lodging in cereals. It also neutralizes organic acids formed within the plant body and eliminates their toxic effects. Calcium accelerates nitrogen fixation in legumes and helps in boosting nitrogen uptake by plants. 8- Magnesium. Being constituent part of polyribosomes, it helps in protein synthesis in the plants. Mg is also a constituent part of chromosomes and chlorophyll. It plays a catalytic role of numerous enzymes concerning carbohydrate metabolism, phosphate transfer and decarboxylation. It is involved in photosynthesis and organic acid metabolism. Mg helps in synthesis of fat and increases oil content in oilseed crops when it combines with sulfur. Calcium in humid region soils occurs largely in the exchangeable form and as primary minerals. In most of these soils, Ca2+, Al3+ and H+ ions dominate the exchange complex. As with any other cation, the exchangeable and solution forms are in dynamic equilibrium. If the activity of solution Ca2+ is decreased by leaching or plant removal, Ca2+ will desorb to resupply solution Ca2+. Other cations like H+ and or Al3+, will occupy the exchange sites left by the desorbed Ca2+. Conversely, if solution Ca2+ is increased, the equilibrium shifts in the opposite direction, with subsequent adsorption of some of the Ca2+ by the exchange complex.  Bioavailability and Role of Nutrients in Plant: Calcium High Ca2+ saturation indicates a favorable pH for plant growth and microbial activity. Also, a prominence of Ca will usually mean low concentration of undesirable exchangeable cations such as Al3+ in acidic soils and Na+ in sodic soils. Many crops will respond to Ca application when percentage of Ca-saturation falls below 25%. Ca2+ saturation 40 to 60% have lowered cotton yields. In humid areas, soybeans are reported to suffer Ca deficiency at 65% Al3+ saturation. However, normal growth of sugarcane in Hawaii is possible with 12% Ca2+ saturation in volcanic soils. Type of clay minerals influences Ca2+ availability i.e. 2:1 clays require higher Ca2+ saturation than 1:1 clays. Montmorillonite requires >70% Ca2+ saturation for adequate Ca availability, whereas kaolinitic clay (with very less CEC) are able to supply sufficient Ca2+ at 40 to 50% Ca2+ saturation. Increasing Al3+ concentration in the soil solution reduces Ca2+ uptake by corn, cotton, soybeans and wheat. Ca availability and uptake by plants are also influenced by the ratios of Ca2+ to other cations in the soil solution. A Ca/total cation ratio of 0.10 to 0.15 is desirable for an adequate Ca2+ supply to most crops. Uptake of Ca2+ is depressed by NH+, K+, Mg2+, Mn2+ and Al3+; its absorption is increased when plants are supplied with NO3--nitrogen. A high level of NO3- nutrition stimulates organic anion synthesis and the resultant accumulation of cations, particularly Ca2+.  Bioavailability and Role of Nutrients in Plant: Sulfur 9- Sulphur helps in synthesis of protein and amino acids like cysteine, methionine, vitamins (thiamine and biotin), lipoic acid, acetyl coenzyme, ferredoxin and glutathione. Sulfur forms active sulphate phospho- adenosine, phospho-sulphate which synthesizes glucosides in mustard oil, pungency in onion, radish etc. It is required in conversion of nitrogen into protein in symbiotic nitrogen fixing legumes. It is involved in activating enzymes participating in the dark reactions of photosynthesis and carbohydrate metabolism of plants. It increases oil content in soybean, groundnut and linseed. Sulfur is present in soil in both organic and inorganic forms, although nearly 90% of the total S in most non-calcareous surface soils exists in organic forms. The inorganic forms are SO42-, adsorbed SO42-, insoluble SO42- and reduced inorganic S compounds. Solution plus adsorbed SO42- represents the readily available fraction of S utilized by plants. There are the similarities between the N and S cycles in that both have gaseous components and their occurrence in soils is associated with soil redox potential, organic matter content and concentration of soluble SO42-. Under anaerobic conditions in waterlogged soils, there may be accumulations of H2S formed by the anaerobic decay of OM. Here, SO42- serves as an electron acceptor for the reducing bacteria and, subsequently, is usually reduced to H2S. Little or no S2- accumulates in oxidized soil (> -150 mv) or with a pH outside the range 6.5 to 8.5. In normal submerged soils well supplied with Fe, the H2S liberated from OM and from SO42- is almost completely removed from solution by reaction with Fe2+ to form amorphous FeS, which undergoes conversion to pyrite (FeS2), a substance of longer persistence. In contrast, oxidation of amorphous FeS precipitates may be completed after only few hours of exposure to the atmosphere to form again the SO42- ion.  Bioavailability and Role of Nutrients in Plant: Sulfur Elemental S is not a direct product of SO42- reduction in reduced soils, but is an intermediate formed during chemical oxidation of S2-. Accumulation of So may occur, however, in soils where oxidation of reduced forms of S is interrupted by periodic flooding. Factors affecting So oxidation in soils. Elemental So, S2- and other inorganic S compounds can be oxidized in the soil by purely chemical means, but these are usually much slower and therefore of less importance than microbial oxidation. The rate of biological So oxidation depends on the interaction of three factors: 1- type of soil microbial population, 2- characteristics of the S source, and 3- soil environmental conditions. Most of the S in surface horizons of well drained agricultural soils is present in organic forms, which account for over 90% of the total S in most non-calcareous surface soils. The proportion of total S existing in organic forms varies considerably according to soil type and depth in the soil profile. There is close relationship between organic C, total N and total S in soils. The C/N/S ratio in most well drained, non- calcareous soils is approximately 120/10/1.4. Generally more variability exists in C/S ratio in soils than in N/S ratio. Differences in the C/N/S ratio among and within types of soils are related to variations in parent material and other soil forming factors such as climate, vegetation, leaching intensity and drainage. The N/S ratio in most soils falls within the narrow range 6 to 8:1. The nature and properties of the organic S fraction in soils are important since they govern the release of plant available S. While much of the organic S in soils remains uncharacterized, three broad groups of S compounds are recognized. These are HI reducible S, C bonded S and residual or inert S.  Bioavailability and Role of Nutrients in Plant: Sulfur Sulfur is absorbed by roots as SO42- ions that reach roots by diffusion and mass flow. Because of anionic nature and solubility of SO42- salts, SO42- like NO3- can readily leached from surface soil. Another factor influencing the loss of SO42- is the nature of the cation in the soil solution. Leaching losses of SO42- are greatest when monovalent ions such as K+ and Na+ predominate, next in the order are Ca2+ and Mg2+ ions; least in acid soils with appreciable amount of exchangeable Al3+. Adsorbed SO42-. Adsorbed SO42- is an important fraction in soils containing large amount of Al and Fe oxides. Adsorbed in highly weathered soils can contribute significantly to the S needs of plants because it is usually readily available but not as rapidly as solution SO42-. Sulphate adsorption (non specific) is readily reversible and is influenced by the following soil properties 1. Clay content and type of clay mineral 2. Hydrous oxides 3. Soil horizon or depth 4. Effect of pH 5. SO42- concentration 6. Effect of time 7. Presence of other anions 8. Effect of cations 9. Organic matter content.  Role of Micronutrients in Plant: Fe, Mn and Cu 10- Iron forms cytochromes, haeme and metalloproteins like ferredoxin and hemoglobin in plants. These cytochromes play a vital role in oxidative and photophosphorylation's during respiratory electron transport and photosynthesis, respectively. The ferredoxin helps in reduction of carbon dioxide, sulphate and of atmospheric nitrogen. It synthesizes chlorophyll precursor (protoporphyrin- 9), which forms chlorophyll in green plants. Its specific requirement has been identified in synthesis of enzymes like oxide-reductase, sulphate oxidase, catalase, peroxidase and aconitase etc. Being a constituent part of metabolically active compounds, iron is responsible for all major metabolic processes in plants. 11- Manganese being a part of nitrite reductase and hydroxylamine reductase, it helps in the nitrogen assimilation. It activates several enzymes related to oxidation-reductions (oxidoreductase), hydrolysis (hydrolases), breakdown of phosphates bonds in ATP or ligases. It activates photosynthesis and nitrogen metabolism. It also accelerates enzyme participating in Calvin cycle, helps in chlorophyll and chloroplast synthesis for boosting photosynthetic rates. 12- Copper helps in oxidation-reduction process in plants. The compounds containing copper like plastoquinone's and plastocyanin's help in electron transport from chlorophyll to NADP and from water to chlorophyll during photosynthesis.  Role of Micronutrients in Plant: Zinc and Molybdenum 13- Zinc regulates the auxin concentration in plants and helps in synthesis of protein, carotin and chlorophyll etc. 14- Molybdenum helps in protein and amino acid synthesis. It accelerates nitrogen-fixing efficiency of aerobic (Azotobacter), anaerobic (Clostridium), blue-green algae, Azolla and symbiotic bacteria. It regulates the carbohydrate metabolism in plants. Soil pH is the most important factor influencing the availability and plant uptake of molybdenum. At low pH the relatively unavailable H2MoO4 and HMoO4- forms are prevalent, whereas the more readily available MoO42- anions is dominant at pH above 5 or 6. The MoO42- ion is subject to adsorption by oxides of iron and possibly aluminum just as is phosphate, but calcium molybdate is much more soluble than its phosphorus counterpart. The liming of acid soils will usually increase the availability of Mo. The utilization of P by plant seems to favor that of Mo and vice versa. For this reason, Mo salts are often applied along with superphosphate to Mo deficient soils. The practice apparently encourages the uptake of both elements and is a convenient way to add the extremely small quantities of Mo required. A second common anion, the sulphate, seed to have the opposite effect on plant uptake of Mo. Sulphate reduces Mo uptake, although the specific mechanisms for this antagonism is not yet known.  Role of Micronutrients in Plant: Boron 15- Boron regulates development and differentiation of vascular tissues formation and lignification of cell-wall. It is associated with reproductive phase in plants and under imbalanced nutrition it causes sterility and malformation in reproductive organs. It is involved in carbohydrate metabolism, particularly in translocation of photosynthates. It boosts nodulation in legumes, regulates water absorption and is essential for synthesis of ATP, DNA, RNA and pectin. Boron is most soluble under acid conditions. It apparently occurs in acid soils in part as boric acid (H3BO3), which is readily available to plants. In quite acid sandy soils, soluble B fertilizers may be leached downward with comparable ease. In heavier soils, especially if they are not too acid, this rapid leaching does not occur. At higher pH B is less easily utilized by plants. This may be due to lime induced fixation of this element by clay and other minerals, since the Ca and Na borates are reasonably soluble. In any case over liming can and often does result in a deficiency of B. B is held in organic combinations from which it may be released for crop use. The content of this nutrient in the top soil is generally higher than that in the subsoil. This may in part account for the noticeably greater B deficiency in periods of dry weather. Apparently, during drought periods plant roots are forced to exploit only the lower soil horizons, where the B content is quite low. When the rains come, plant roots again can absorb B from the topsoil, when its concentration is highest.  Role of Micronutrients in Plant 16- Chloride. During photosynthesis Cl helps in evolution of oxygen. Chloride is a part of anthocyanin and affects protein synthesis. It increases turgor pressure. Most of the chloride in soils is in the form of simple, soluble chloride salts such as potassium chloride. The chloride anions isn’t tightly adsorbed by positively charged clay sites and is subject to movement both upward and downward with the water in the soil profile. In humid regions, one would expect little chloride to remain in the soil since it would be leached out. In semiarid and arid regions, a higher concentration might be expected, the amount reaching the point of salt toxicity in some of the poorly drained saline soils. In most well drained areas, however, one would not expect high chloride content in the surface of arid region soils. Accretions of Cl from the atmosphere are believed to be sufficient to meet crop needs. Salt spray alongside ocean beaches evaporates, leaving sodium chloride dust, which move into the atmosphere, to be returned later dissolved in snow and rain. The amount added to the soil in this way varies (20 kg/ha/year) according to the distance from the salty boy of water and other factors. In any case, this form of accretion plus that commonly added as an incidental component of commercial fertilizers, should largely prevent a field deficiency of chlorine. 17- Cobalt is required for symbiotic and non-symbiotic nitrogen fixation. It is a constituent of vitamin B-12. 18- Sodium controls the cell vacuole osmotic potential. It also regulates water uptake by plants. Plants take sodium as a substitute for potash under deficient potash supply.  Bioavailability and Role of Micronutrients in Plant Micronutrients are most apt to limit crop growth under the following conditions: a) highly leached acid sandy soils, b) muck (organic) soils, c) soils very high in pH and d) soils that have been very intensively cropped and heavily fertilized with macronutrients only. Strongly leached acid sandy soils are low in micronutrients for the same reasons they are deficient in most of the macronutrients. Their parent materials were originally deficient in the elements and acid leaching has removed much of the small quantity of micronutrients originally present. In the case of molybdenum, acid soil conditions also have a markedly depressing effect on availability. The micronutrient contents of organic soils are dependent upon the extent of the washing or leaching of these elements into the bog area as the deposits were formed. In most cases, this rate of movement was too slow to give deposits as high in micronutrients as are the surrounding mineral soils. Intensive cropping of muck soils and their ability to bind certain elements, notably copper, also accentuate trace element deficiencies. Much of the harvested crops, especially vegetables are removed from the land. Eventually the micronutrients as well as macronutrients must be supplied in the form of fertilizers if good crop yields are to be maintained. Intensive cropping of heavily fertilize mineral soils can also hasten the onset of micronutrient shortage especially if the soils are coarse in texture.  Factors Affecting Bioavailability of Micronutrients in Soil The soil pH has a decided influence on the availability of all the micronutrients except chlorine especially in well aerated soils. Under very acid conditions, molybdenum is rendered unavailable; at high pH values all the cations are unfavorably affected. Over liming or a naturally high pH is associated with deficiencies of Fe, Mn, Zn, Cu and even B. Such conditions occur in nature in many of the calcareous soils of the west. Each of the four micronutrient cations (Fe, Mn, Zn, Cu) and Co are influenced in a characteristic way by the soil environment. However certain soil factors have the same general effects on the availability of all of them. 1- Soil pH. The micronutrient cations are most soluble and available under acid conditions. In very acid soils there is abundance of the ions of Fe, Mn, Zn, and Cu. In fact, under these conditions the concentrations of one or more of these elements is often sufficiently high to be toxic to common plants. As the pH is increased, the ionic forms of the micronutrient cations are changed first to the hydroxyl ions of the elements and finally to the insoluble hydroxides. All of the hydroxides of the trace element cations are insoluble, some more so than others. The exact pH at which precipitation occurs varies from element to element an even between oxidation states of a given element.  Factors Affecting Bioavailability of Micronutrients in Soil 2- Oxidation state and pH level. Three of the trace elements (Fe, Mn and Cu) are found in soils in more than one valency. The lower valency states are encouraged by conditions of low oxygen supply and relatively higher moisture level. They are responsible for the subdued subsoil colors, grays and blues in poorly drained soils in contrast to the bright reds, browns, and yellows of well drained mineral soils. The changes from one valent state to another are in most cases brought about by microorganisms an organic matter. In some cases the organisms may obtain their energy directly from the inorganic reactions. In other cases, organic compounds formed by the microbes may be responsible for the oxidation or reduction. In general, high pH favors oxidation, whereas acid conditions are more conducive to reduction. At pH values common in soils, the oxidized states of Fe, Mn and Cu are generally much less soluble than are the reduced states. The hydroxides of these high valency forms precipitate at lower pH values and are extremely insoluble. For example, the hydroxide of trivalent ferric cations precipitate at pH 3.0, whereas ferrous hydroxides does not precipitate until a pH of 6.0 or higher is reached.  Factors Affecting Bioavailability of Micronutrients in Soil 3- Other inorganic reactions. Micronutrient cations interact with silicate clay in two ways. First they may be involved in cation exchange reactions much like those of Ca an H. Second they may be more tightly bound or fixed to certain silicate clays, especially of the 2:1 type. Zinc, manganese, Co and Fe are found as integral elements in these clays. Depending on the conditions, they may be released from the clays or fixed by them. The fixation may be serious in the case of Co and sometimes Zn since these two elements are present in soil in such small amounts. The application of large quantities of phosphate fertilizers can adversely affect the supply of some of the micronutrients. The uptake of both iron and Zn may be reduced in the presence of excess phosphates. From a practical standpoint, phosphate fertilizers should be used in only those quantities that are required for good plant growth. Lime induced chlorosis (Fe deficiency) in fruit trees is encouraged by the presence of the bicarbonate ion. The chlorosis apparently results from iron deficiency in soils with high pH level. In some way, the bicarbonate ion interferes with iron metabolism.  Factors Affecting Bioavailability of Micronutrients in Soil 4- Organic Complexations. Each of the four micronutrients is a cation that may be complexed with organic ligands to form a metal-ligand combination. Such complexes (chelates) may protect the micronutrients from certain harmful reactions, such as the precipitation of Fe by phosphates and vice versa. On the other hand complex formation may reduce micronutrient availability below that necessary for normal plant needs. Microorganisms also assimilate micronutrients since they are apparently required for many microbial bioreactions. The organic compounds in which these trace elements are combined undoubtedly vary considerably, but they include proteins, amino acids and constituents of humus, including the humic, fulvic, and organic acids such as citric and tartaric.  Factors Affecting Bioavailability of Boron in Soil Boron is most soluble under acid conditions. It apparently occurs in acid soils in part as boric acid (H3BO3), which is readily available to plants. In quite acid sandy soils, soluble B fertilizers may be leached downward with comparable ease. In heavier soils, especially if they are not too acid, this rapid leaching does not occur. At higher pH, B is less easily utilized by plants. This may be due to lime induced fixation of this element by clay and other minerals, since the Ca and Na borates are reasonably soluble. In any case over liming can and often does result in a deficiency of B. B is held in organic combinations from which it may be released for crop use. The content of this nutrient in the top soil is generally higher than that in the subsoil. This may in part account for the noticeably greater B deficiency in periods of dry weather. Apparently, during drought periods plant roots are forced to exploit only the lower soil horizons, where the B content is quite low. When the rains come, plant roots again can absorb B from the topsoil, when its concentration is highest.  Factors Affecting Bioavailability of Molybdenum Soil pH is the most important factor influencing the availability and plant uptake of molybdenum. At low pH the relatively unavailable H2MoO4 and HMoO4- forms are prevalent, whereas the more readily available MoO42- anions is dominant at pH above 5 or 6. The MoO42- ion is subject to adsorption by oxides of iron and possibly aluminum just as is phosphate, but calcium molybdate is much more soluble than its phosphorus counterpart. The liming of acid soils will usually increase the availability of Mo. The utilization of P by plant seems to favor that of Mo and vice versa. For this reason, Mo salts are often applied along with superphosphate to Mo deficient soils. The practice apparently encourages the uptake of both elements and is a convenient way to add the extremely small quantities of Mo required. A second common anion, the sulphate, seed to have the opposite effect on plant uptake of Mo. Sulphate reduces Mo uptake, although the specific mechanisms for this antagonism is not yet known.