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Soil Solution Soil Solution: Soil solution is the liquid phase of soil retained by soil micro pores. Water in exists in the soil partly as combined and bound water (water of constitution and hygroscopic water) and partly as free water in the form of films surrounding the soil particles(capillary water). The soluble products that are liberated as a result of the processes of weathering and soil formation are dissolved in the free water. The free water also contains a part of organic matter that is soluble in water. Some of the soil gases like oxygen and carbon dioxide are also dissolved in this water. In a cultivated soil, some of the products of excretion and secretion of plant roots and of the activities of microorganisms are also dissolved in soil moisture. The free water carrying these various substances and gases in solution is known as soil solution. By definition, ‘Water is the soil containing soluble salts and hence whenever this aspect of soil water is relevant it is usually known as the soil solution’. According to Brady (1984), ‘The aqueous liquid phase of the soil and its solutes consisting of ions dissociated from the surfaces of the soil particles and of other soluble materials are known as soil solution’. Characteristics of soil solution: 1. Soil solution exists in dynamic equilibrium condition with the solid phase in which numerous chemical reaction occur simultaneously. 2. Soil solution can be neutral, acidic or alkaline in nature. 3. In soil solution, various amounts of cations & and anions are found freely moving from one place to another. 4. Soil solution contains dissolved & suspended colloidal particles. 5. The composition of soil solution is not constant. The composition of a particular volume of soil solution depends not only on materials present that might possibly be included in the water but on many other factors such as dissolved gases, solutes, and suspended materials. Infact, it various continuously only from soil to soil but also for one and the same soil.
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varies continuously, from soil to soil, time to time and season to season. 7. It contains some dissolved gases such as O2, CO2, NH3 etc and some other gases that do not interact appreciably with soil-water, such as N2. 8. It contains a part of organic matter that is soluble in water. 9. The soil solution exists in a liquid state in the water films around the soil particles and in soil micropores. 10. Soil solution differs in its characteristics in response to pH, temperature, rainfall etc. Dynamic aspect of soil solution Or, Dynamic equilibrium in soil: The soil solution provides the chemical environment of roots and comprises the soil-water and its dissolved electrolytes plus small quantities of dissolved gases and water soluble compounds. The main function of soil solution is to supply mineral nutrients to the growing plant. There are three possible sources from which plants can extract nutrients: the soil solution, the exchangeable ions, and the readily decomposable minerals. If the soil solution is in equilibrium with the exchangeable cations and adsorbed anions, and if any nutrient (except nitrate whose supply is principally from mineralized organic matter) is removed from the solution, at least a part of this loss will be made good from the nutrient reserves of the solids. I.e. the solid phase begins to dissolve. Again, whenever the activity of a nutrient ion in the soil solution exceeds the equilibrium concentration of a mineral, that mineral begins to precipitate. In this way, the solid materials of the soil keep the solution well buffered both for pH and all nutrients (except nitrate). The manner in which various constituents of the soil interact is depicted diagrammatically in the following figure. The soil solution is the focal point in this diagram and is the liquid phase that completely envelops the solid phase.
Nutrients in plant shoots 1. Function; 2. Retranslocation
Transport in Xylem
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Transport in phloem
Nutrients in plant roots
Adsorption
Efflux, exchange Soil solution Nutrients
Gaseous phase
Mineralization
dissolution
Immobilization
desorption
Soil amendments
upward Movement
Leaching
precipitation adsorption
Nutrients precipitated as inorganic solids
Nutrients held in organic matter
Nutrients adsorbed on surfaces
Leaching & upward movement of ions.
Fig: Diagrammatic representation of the dynamic equilibrium occurring in soil. (Broken line indicates that the process is not neutral).
Description: 1. Nutrients in plant roots: “Adsorption & efflux, exchange”. Plant roots absorb nutrients from the soil solution, and the concentration of nutrients in the soil solution is a factor determining their rate of uptake by roots. Adsorption occurs in ionic form through the semi permeable plasma membrane of roots. It may also occur in other forms, but that isn’t common. Since roots are in direct contact with only a small part of the nutrients in solution or of available nutrients absorbed by the soil solids,
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nutrients must move to the root surface. The stages involved in moving a nutrient from a point in the soil into the plant shoot are: First, movement of the nutrient from the bulk to the root surface through absorption; Second, movement form the root surface into the root; and Third, translocation of the nutrients from the root to the shoot. The concentration of free ions in the soil solution is generally low, and many of the cations are adsorbed with varying degrees of firmness on negatively charged clay particles and organic matter in the soil. Nutrients anions such as nitrate and sulfate mostly occur in the soil solution and are relatively mobile except phosphate, which is firmly bound to the soil matrix. Mobile ions such as nitrate and potassium can be utilized from the entire root zone, but immobile ions such as phosphate are available only from soil in the immediate vicinity of the roots. However, mobile ions in the soil solution are presumably available to roots from greater distances since water moves to roots from distances of several centimeters. The nutrient requirements of plants are related to the amount supplied in the soil solution absorbed to replace water lost by transpiration. On the other hand, when plant roots uptake excess amount of any nutrient, or produce some ions in, they may also be released back into the soil solution by exchange reaction or efflux through phloem tissue.
2. Nutrients in plant shoots: “Transport in xylem and transport in phloem” Water uptake in plant occurs through xylem tissue. Ions which reach the xylem sap of the roots are usually carried to the shoots in the transpiration stream. Here in leaves, some function, i.e. some synthesis and retranslocation of some synthesized products out of the leaves occur. In case of excess production, small quantities of plant constituents may also be releases back into the soil solution, through roots by transpiration in phloem. Substances released by roots are synthesized products such as carbohydrate, nucleotide, etc and are designated root exudates. Root exudates are not always same; they depend on the initial chemicals up taken. For legumes having much protein, amino acids will be found as root exudates.
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3. Nutrients held in organic matter: “Mineralization & immobilization” Organic matter and microorganisms also affect the equilibrium relationships in soils. Nutrients are held in organic matter and are released during the decomposition of organic matter or upon the death of organisms. By mineralization the constituents of organic matter are decomposed into ionic or molecular (which also turn into ionic forms) forms. Thus, all these nutrients in ionic form are in soil solution. Similarly liming organisms remove constituents from the soil solution and incorporate then into their body tissues. Thus, from soil solution the mobile inorganic nutrients are converted to organic form by immobilization, a microbial process, in which organic forms are unavailable to plants. But they release nutrients into soil solution after completing their life cycle through decomposition. [There are 17 elements for microbial activity. If those elements are present in soil the process of immobilization occurs. The higher the CN ratio, the more is the growth of microorganisms & the less is the plant growth of microorganisms & less is the plant growth. There also occurs the immobilization process. Organic source can be decreased by carbon neutralization and then immobilization will also be decreased]. Understanding of the upward movement of ions is complicated by the fact that not all the transport of salt occurs as inorganic ions. In many species, most or all of the nitrogen is probably transported as organic compounds such as amides, amino acids, and ureides. There is also limited evidence fro the transport of at least small amounts of sulfur & phosphorus as organic compounds in the xylem. [The activity of micro-organisms results in an increase in the nitrate content of the soil, (as typically, the nitrate and bicarbonate concentrations are dependent on microbial and root activity), with little change in the concentration of the other anions, and in neutral or weakly acid soils the calcium ion concentration increases to balance the nitrate ions produced.]
O4. Nutrients precipitated as inorganic solids: “Dissolution and precipitation” Soil contains numerous minerals, some of which are crystalline, others are amorphous. These minerals impose limits on the chemical composition of the soil solution. If the soil solution becomes supersaturated with respect to any mineral (i.e. when water can’t dissolve the
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ions) that mineral can precipitate until equilibrium is attained. Under supersaturated condition nutrient ions in solution can be precipitated. For the precipitation of nutrient ions there should be chemical affinity between the two constituents, and favorable reaction conditions. And, this precipitation is concentration dependent. Certain compound is precipitated at certain condition/ agreement. For example, for the precipitation of Al (PO4), there must be Al³+ & PO4³ˉ ions. Similarly if the soil solution becomes under saturated with respect to any mineral present in the soil, that mineral can dissolve until equilibrium is attained. That is, when plants uptake certain nutrient from the solution, there appears a deficient of that particular nutrient ion in the soil solution and it causes crystalline minerals and other precipitates to dissolve the soil solution and resaturate the depleted soil exchange sites.
05. Nutrients adsorbed on surfaces: “Desorption & adsorption”. Ions in the soil solution are buffered by these adsorbed onto soil surfaces or held by exchange sites. When the concentration of a particular element in the soil solution is increased, soil colloids adsorb some of the ions to maintain equilibrium. Similarly, removal of ions from the soil solution causes partial desorption of similar ions from the exchange complex (until the equilibrium is maintained). By desorption process, mostly cations are released/ plants get mostly cations. 06. Leaching & upward movement of ions:
“Leaching & upward movement” Excess water may drain from the soil profile & carry with its salts and other dissolved constituents. Nutrients that dissolved in soil water go onward by leaching (washing out) even to ground water. The leaching of Clˉ & NO3ˉ ions is much higher than the leaching of PO4³ˉ. On the other hand, nutrient elements may move upward during evaporation, loss of water from the soil. 07. Gaseous phase: Gases in the soil air also tend to attain equilibrium with the soil solution because of partial pressure of individual gases. Gases may either be released to the soil air or dissolved in the soil solution. In soils plants & microorganisms generally utilize O2 as an electron acceptor and give off CO2 from metabolic processes.
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Diffusion gradients are, therefore, established between the gas phases in the finer pore spaces of soil and the atmosphere above. In water-logged soils the exchange of O2(g) CO2(g) is greatly restricted because diffusion rates in water are approximately 10ˉ4 those in air. As O2 in the soil is depleted the soil becomes reduced. Even in unsaturated soils there often zones of fine-textured materials where reducing conditions may prevail. Higher plants synthesize their tissues from simple substances which they take from the atmosphere and the soil. They absorb carbon dioxide and oxygen from the air for photosynthesis and respiration, respectively, and take up water and mineral nutrients from the soil. There are some exception to these general statements; for example, most plants need to have some oxygen supplied to their roots from the soil; second, nitrogen used in the metabolism of leguminous plants (and some others) can be obtained from the atmosphere through biological fixation of dinitrogen gas in their root nodules; and third, plants can absorb mineral nutrients through their leaves, with the absorption of sulfur as sulfur dioxide. 08. Soil amendments: Soil amendments is any substance other than fertilizers, such as lime, sulfur, gypsum, and sawdust, used to alter the chemical or physical properties of a soil, generally to make it more productive. Soil amendments and fertilizers play an important role in the equilibrium of soil solution. Soil amendments contain different nutrient elements. Fertilizers of various kinds are frequently added to soil. These may dissolve, form new reaction products, or be distributed in other ways in soil solution. This reaction in the equilibrium diagram connected with a broken line to indicate that true equilibrium relationship is generally not achieved but is modified by the addition of soil amendments that mediate this reaction.
Conclusion: The soil solution is affected by all of the reactions depicted in the figure, but its composition is ultimately controlled by the mineral phases of the soil. Often the rates of dissociation and precipitation of soil minerals are so slow that true equilibrium is not attained; consequently both kinetic & thermodynamic factors must be considered. Diffusive and connective gradients are both established in soils, and these gradients must also be considered where transport processes are involved. *Composition & concentration:
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Soil solution contains a number of salts in solution. The salts are present in both ionic and molecular forms. When the solution is dilute as is often the case in cultivated soils, a greater proportion of the salts is present in ionic state. Molecules appear only when the solution gets concentrated, e.g. during dry spell, in dry season, or in arid regions. The common elements and the ionic from in which they are present in soil solution are listed below:
Elements
Ionic form
Sodium Potassium Calcium Magnesium Iron Aluminum Hydrogen Manganese Copper Zinc Nitrogen Chlorine Sulfur Carbon Phosphorus Silica Hydroxyl Boron Molybdenum
Na+ K+ Ca++ Mg++ Fe²+, Fe³+ Al³+ H+ Mn²+, Mn4+ Cu+, Cu²+ Zn²+ NH4+, NO2ˉ,NO3ˉ Clˉ SO3²ˉ, SO4ˉ (HCO3)ˉ, CO3²ˉ PO4³ˉ, HPO4²ˉ, H2PO4²ˉ SiO3²ˉ, SiO4ˉ OHˉ BO3³ˉ, HB4O7ˉ HMoO4ˉ
Of all the cations, calcium, magnesium, sodium and potassium are present to the greatest extent. Ca++ content In mineral soils, calcium forms the predominant cation. Calcium ion is in higher exchange condition than K+ calcium ion varies considerably not only to form soil but also in the same soil at different times. Na+ & K+ content:
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In saline soils sodium constituents the largest quantity. The content of Na+ & K+ ions vary with moisture content with the increase of moisture content the amount of Na+, attached to the surface of colloids, decreases potassium ion vary considerably not only from soil to soil but also in the same soil at different times. H+ Content: In acid soils, the soil solution usually contains a predominance of hydrogen ions. Anions: Among the anions, bicarbonate, sulfate and chloride ions are commonly present in soil solution in large quantities. (HCO3)ˉ content: The bicarbonate ions are usually present in greater quantities than sulfate & chloride ions, except in saline soils. SO4 content: In saline soils, a sulfate ion is in preponderance. SO4²ˉ content varies less than expected. SO4²ˉ content doesn’t change in that level in which NO3ˉ & Clˉ content changes. That is, somehow soil holds/ captures sulfate ions and that’s why, the amount of SO4²ˉion is less in soil solution. Clˉ content: In saline soils, chloride ion is also in preponderance. The total Clˉions remain in soil solution, while the content of PO4³ˉ & SO4²ˉ ions is low. Po4³ˉcontent: Of the major plant nutrients, the phosphate content of the soil solution is the lowest & least variable. P is present in the soil solution as H2PO4ˉ & HPO4²ˉions, and is generally believed to be taken up by plants mainly as H2PO4ˉ. NO3ˉcontent: The amount of nitrates in soil solution is also very variable. Usually it is the highest just after a crop is sown and lowest after it is harvested. (Leaching of NO3ˉ is maximum in soil). OHˉcontent: In the case of alkaline soils, hydroxyl ions are generally present in large quantities.
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## The composition and concentration of soil solution are not constant. They vary continuously, not constant. They vary continuously, not only from soil to soil but also for one and the same soil. The factors that are mainly responsible for this variation are: i. moisture content ii. soil reaction iii. activity of soil microorganisms iv. nature of parent material v. growth stages of plant vi. plant types vii. types of soil viii. adsorption by colloidal complex ix. leaching effect x. season xi. cultivation xii. fertilizer application xiii. irrigation xiv. temperature # Moisture content: The greater the amount of moisture in a soil, the more dilute is the soil solution. The rate of evaporation which governs the amount of soil moisture, therefore, controls the concentration of soil solution. In arid and semi-arid regions where evaporation is high, the soil solution is usually more concentrated than in temperature & cool humid regions. Even in a given soil, soil solution is more dilute soon after rainfall or irrigation than a few days later. Its concentration is gradually increased as the soil moisture content is reduced. #Soil reaction: Soil reaction is the degree of acidity or & alkalinity of a soil. Concentration of ion is differently in high pH & low pH. Metals (e.g. Pb, Co, etc) are soluble in acid. So, in low pH they remain in soil solution. But, if the pH is above7, they are precipitate as well. If pH is above 6, then NO3ˉconcentration is high. When pH is near about 4, then nitrification occurs, thus decreasing the concentration of NO3ˉ #Activity of soil microorganisms: Soil microorganisms help to increase concentration of soil solution. Then activities lead to an increase of certain ions, chiefly the anions, in solution. The anions bring equivalent amount cations in solution.
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The anions so produced are nitrates, sulphates and bicarbonates like calcium and magnesium. The microorganisms utilize some of the salts from soil solution for their own nitrification some of which are liberated again later on and brought into solution when the microorganisms die and decompose. Water content, pH, moisture, O2 supply, temperature, supply of organic matter or the important factors for the activity of microorganisms. 1-5% organic-nitrogen is mineralized in soil. If there are thiobacillus bacteria in soil, they oxidize sulfur and then SO4²ˉ can be found in that soil, and H2SO4 may also form. We can know about the composition & concentration of certain soil solution by knowing which microorganisms are active in soil detecting by the environmental factors. # Nature of plant material: The minerals primarily present in certain parent material, release their components into soil solution by the weathering of that parent material. So, if orthoclase is present in a parent material by the weathering of it, more K+ will be found in soil solution. In this case, pH of the medium will be increased. KAlSi3O8 + H-OH HAlSi3O8 + K+ + OHˉ Likewise, in case of albite, Na+ will be high in soil solution. There are acidic, basic, organic parent materials. Natural parent material detects composition & concentration of soil solution. Growth stages of plant: The growing plant which absorbs nutrients from the soil changes the concentration as well as the composition of soil solution. The nutrient requirements in vegetative & reproductive stage are not the same. Plant needs more nutrients in its vegetative stage of growth. For example, in a crop field nitrogen fertilizer is given in three steps, because excess nitrogen is removed by leaching. Fertilizer is added to soil at the beginning, because more nutrients needed for tiller formation. An analysis showed that, a green rice plant contains 2% N, whereas there is less than 1% N in straw. It means that, the plant utilizes N, by storing in grains. After flowering, nutrients accumulate in soil, as plants do not need them in that stage. Plants exert considerable selection in the adsorption of nutrients. While some ions like nitrates are adsorbed almost completely in
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preference to others, some ions are not adsorbed at all or to a much lesser extent. This selective absorption of ions changes the composition of soil solution considerably. On the other hand, the action of carbon dioxide and other acidic substances exerted by plant roots brings some of the insoluble soil constituents into solution thereby changing its concentration & composition. Even plant transpiration affects its concentration. On all these accounts, the concentration of the soil solution is always lower in cropped soil than in the same soil that is not cropped. It is usually higher in the beginning of the growing season than at its end. #Plant types: There are wide differences among plants of different species in their ability to absorb various ions from the same soil or culture solution. For instance, sugarcane absorbs more K+ & Ca++ ions, whereas calcifillus plants growing in high pH soil like to absorb Ca++ ion. Because roots are selective in their uptake of some ions the relative ionic composition of the shoot system is often very different from that of the soil solution. Plants typically contain more phosphate, potassium and nitrogen relative to calcium than does the soil solution. The degree of selectivity depends on the plant species. The ionic composition of the growing leaves of a crop differs between species and depends also on the ionic composition of the soil solution. Thus, some species such as Lucerne typically have higher calcium: potassium ratio in their leaves than do grasses. These differences between plant species become very noticeable if an ion is in excess of its normal value in the soil solution, for then plants can often be classified into accumulators or rejecters of that ion. Some plants (for example, tea) growing in acid soils high in aluminum will accumulate high concentration of aluminum in their leaves without their growth being affected. Some plants growing in salt marshes will accumulate very high concentrations of sodium in their leaves; yet other plant species, growing in these same soils, will have compositions little affected by these high concentrations. A classic example is the study made by Colander (1941) of cation adsorption by 21 species of plants from various families and habitats when grown in the same nutrient medium. The genus Astragalus is well known as an accumulator of selenium, but A. mussouriensis accumulated by 3.1 ppm from a soil containing 2.1 ppm, while A. bislactus accumulated 1250 ppm.
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In another study, the ash of Andropogan contained 65.4% of silica, while the ash of Prunus pumila growing on the same soil contained only 1.5%; but Prunus accumulated three or four times as much as Andropogan. In some substances, the differences in uptake of specific ions by different plants growing in the same environment are known to be controlled genetically. The control may be either on the uptake mechanism directly or on the roots and vascular system. # Type of soil: No two soil types have the same composition and concentration of soil solution. Saline soils differ markedly from non-saline soils in both composition & concentration of their soil solution. Soils formed in arid and semi-arid regions, or under restricted drainage usually have a more concentrated soil solution than those formed in temperate and moist tropical regions or those having free drainage. The soil solution of the former soil types is usually more rich in monovalent cations while that of the latter in divalent cations. The former is also richer in sulfate and chloride anions than the latter. # Adsorption by colloidal complexes: The rate at which the colloidal complex adsorbs ions from soil solution changes its concentration as well as composition. Most of the ions present in soil solution, except those which are adsorbed by colloids, e.g. nitrate & chloride, are in equilibrium with those present in the adsorbed form, i.e. the exchangeable cations and anions. Anything that disturbs this equilibrium brings about a change in the character of soil solution. # Leaching effect: The continuous removal, through leaching, of certain ions that are not adsorbed by colloids or those that are in excess of what can be adsorbed and retained in soil, also affects the nature of soil solution. For instance, nitrate & chloride ions, if they are not utilized by the growing crop, are continuously lost in drainage water. So, also calcium, magnesium and sodium ions which are present in excess of the adsorbing capacity of the colloidal complex or the requirement of the growing crop are removed through leaching. #Season: Seasonal variations, especially temperature & rainfall, also bring about a change in the concentration of soil solution.
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The rate of chemical reactions doubles with every rise of 10ºC in temperature. Soil temperature has a pronounced effect upon the decomposition of organic & mineral components of the soil. Thus, a rise in temperature brings some of the soil constituents into solution, thereby increasing its concentration. Rainfall, on the other hand, tries to dilute it. The composition of soil solution is also changed by seasonal variations due to the adjustments in equilibrium taking place. #Cultivation: Cultural operations, i.e. cultivation helps to increase the solubility of soil constituents, thereby increasing the concentration of soil solution. At the time tillage, the underlying soils appear in the contact of the atmosphere and then weathering of those soils increases, thus releasing the mineral constituents to the soil solution. #Fertilizer application: The addition of manures, fertilizers and soil amendments changes the concentration & composition of soil solution. Nutrient composition of soil solution depends on the nutrient elements present in the added fertilizer (multi nutrient fertilizer, mixed fertilizer, urea for only N, gypsum for Ca++ & So4²ˉ, etc.). Infact, the concentration and composition of soil solution are influenced by the solubility & composition of the ingredients. #Irrigation: Irrigation may also affect the concentration & composition of soil solution. The water used for irrigation dilutes the soil solution and if salt is present in that water; it influences the concentration as well. The salts present in irrigated water deposited in the soil after several years and problems, e.g. arsenic problem occur. So, irrigated water should be free from salt and there should be certain level for a nutrient in the irrigated water. Moreover, irrigation must be followed by drainage. Because, irrigated water accumulates below the soil, thus increasing the water layer upto root zone and then plant growth stunted. This problem can be solved by drainage. Conclusion: These considerations go to slow that soil solution as it exists in the soil is a highly reactive and dynamic medium, always undergoing changes which add further to the dynamic character of the soil.
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Growing plants have, therefore, to adjust themselves to frequent fluctuations and variations in the nature of soil solution. Importance of soil solution: 1. Soil solution is the medium in which most of the chemical & biochemical reactions occur. 2. Soil solution serves as a source of available nutrients (except O2 & C) essential for plant growth. 3. Soil solution also serves as a nutrient source for soil microorganisms. 4. Dissolved oxygen present in soil water is vital for the growth of many organisms such as aerobic bacteria. 5. Carbon dioxide present in soil solution may react with water to form carbonic acid which may (conditionally) affect soil pH. 6. Soil solution acts as a transporting agent for nutrient elements & waste products in soil. 7. Concentration of soil solution affects the water availability to plants. 8. Soil solution makes mineral elements soluble through its solvent energy. 9. Concentration of soil solution controls water absorption by plants. 10. A more concentrated soil solution makes the soil more fertile and they increase the crop production capacity of soil. 11.The nature of proportion of the various ions constituting the salts present in the soil solution also influences crop production. 12. The soil solution in most fertile soils is and adequate source of supply for the phosphate (PO4³ˉ) requirement of plant. 13. The soil solution increases the osmotic pressure due to the presence of high soluble salts and helps in diffusion of nutrients. 14.Soil solution affects on the viscosity and swelling of soil colloids. 15.Soil solution controls the soil air and soil temperature. 16.Soil solution controls the leaching effect of the plant nutrients. The movement of salt in soil:
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Movement of ions in soil solution to the surface of roots is an important factor to satisfy the nutrient requirements of plants. Salt movement related to temperature and moisture content. There are three ways in which nutrient ions in soil may rich to the root surface: 1. Diffusion 2. Mass flow 3. Root interception. [If we do not consider nutrient uptake by plants then only diffusion & mass flow are of concern]. These are described below: Diffusion: Diffusion can be defined as the movement of substance from an area of its own high chemical potential to another area of its lower chemical potential, which is due to the straight, random (no particular direction) transitional kinetic motion of molecules, ions or atoms of a gas or of a liquid. Diffusion occurs when ions move along a concentration gradient established between the root surface and the body of the soil; ions diffuse towards the root if they are taken up faster than they are carried to the surface by mass flow and away from the root if the converse proteins. Net movement of gases or solutes by diffusion occurs when the partial pressures of individual gases or solutes in two neighboring systems are different, but the total presence is the same in both. The tendency of diffusion flow is greater between field capacity water content & wilting point. There are three principle factors responsible for the movement of salt by diffusion: 1. Diffusion coefficient: It exerts the most important role. Each ion has a specific rate of diffusion and it moves certain distance per second. The diffusion rate of nitrogen is much lower. For diffusion in the soil’s liquid phase, the effective diffusion coefficient is generally less than the diffusion coefficient in bulk water. Volumetric water content & tortuosity are the factors affecting the diffusion coefficient. 2. Concentration of the nutrient in the soil solution: It is diffusion related. More is the concentration, less is the movement. 3. Buffer capacity of the solid phase of the soil for the nutrient in the soil solution phase:
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Buffer capacity is the capacity to hold nutrients. The higher the exchange capacity of a soil, the greater is the buffer capacity. A soil having high contents of clay & organic matter is of more buffer capacity. The diffusion process can be described by Fick’s law: De = Dw θ f 1/b Where, Effective diffusion coefficient, De= for the diffusion of an ion soil it is influenced by three principal factorsa. Volumetric water percentage, (θ) b. Tortuosity factor which expresses the irregular and indirect pathway of the diffusion in the pores of the soil, (f) c. Buffer capacity, (b) & The relationship of parameter and diffusion coefficient, Dw= is the diffusion coefficient for the particular nutrient in the water. For plants growing in soil, the concentration of a nutrient at the root surface depends on the relative rates of uptake by the root and of transport to the root surface from the bulk soil solution. In control to flowing nutrient solutions, this introduces the requirement for diffusion of the nutrient, (or mass flow of the soil solution), to carry the nutrient to the root surface. An example: typical average distances for diffusion to the roots are: Nitrogen 1 cm Phosphorus 0.02 cm Potassium 0.2 cm Here, the movement of ‘P’ is difficult, because of low content of ‘P’ in soil; special affinity of Fe & Al to ‘P’ in acid soil and of Ca to ‘P’ in alkaline soil; and unavailability of ‘P’ (not in available form). Mass flow: It is a massive process by which ions and all other substances dissolved in soil-water move together with the flow of water in soil. Mass flow occurs when the soil has high moisture content, above field capacity which is called super flows. Mass flow depends on two things: i. Rate of flow & ii. Rate of uptake by plant. Besides these, it also depends on the gravitational pull of upward & downward movement due to evaporation. Mass flow occurs through three processes: 1. Leaching or downward movement:
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The downward movement of water is due to gravitational pull, and as water moves downward the dissolved substances present in soil solution also move with it. The downward movement depends on water transpiration through soil which is regulated by the soil properties. If there are capillary pores more in soil, then water translocation will be low. But, in coarse soil having large pores, downward movement is high. Downward and upward movement of the ions that are independently remaining in soil solution (e.g. NO3ˉ) will be high. 2. Upward movement: By evaporation water is released from the surface of the soil and water within the soil moves upward to establish an equilibrium state and the mass or ions present in the soil-water also move upward with the water. 3. Transpirational pull: Plants uptake water by roots, utilizes 1% of it for biochemical reactions, and removes the rest 99% (to balance heat) by transpiration. For this uptake, water has to reach to the roots. By transpirational pull water near the root surface moves upward into plant and then the neighboring water molecules also move toward the roots to replenish that space, of course with the salts & nutrient ions present in it. Thus, transpirational pull causes mass flow. Mass flow supplies an over abundance of calcium & magnesium in many soils, and most of the mobile nutrients such as nitrogen and sulfur, if concentrations is in the soil are sufficient. The rate of mass flow can be calculated with the help of the following equation: MF= C × WU. Where, MF= the concentration to ion uptake by mass flow. C = is the solution concentration of any given ion. WU= is the total water uptake, which is the water content in the plant + the water transpired. Root interception: In case of salt movement, if any ion comes into direct contact with the roots, that is called root interception. Factors affecting salt movement: As roots are in direct contact with only a small part of the nutrients in solution or of available nutrients absorbed by the soil solids, so nutrients must move to the root surface from bulk of soil solution. Salt accumulation in root cells and transport to shoots are affected by four (4) factors. This are-
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1. Climatic condition 2. Soil properties 3. Solubility of nutrients & 4. Plant species. 1. Climatic condition: Climatic conditions such as rainfall & temperature controls the salt movement in soil solution. • Rainfall Water uptake by plants depends on moisture content. Because of rainfall moisture content in soil increases. When water content in soil is high above field capacity which is called superflows, plant uptake more water and mass flow of water including salts in it increases. It also controls the gravitational pull. Moreover rate of reaction increases with the increase of rainfall. • Temperature: When temperature is high, the rate of evaporation increases and then the water in soil moves upward into the soil surface, and thus movement of water increases. 2. Soil properties: • Soil texture: Buffer capacity in sandy soil is low whereas in clayey soil is high. • Organic matter content: Buffer capacity increases with the increase of organic matter in a soil. 3. Solubility of nutrients 4. Plant species
Nutrient
N
Amount of nutrient for 150 bu/A of corn(lb/A) 170
Percentage supplied by Root interception 01
Percentage supplied by Mass flow
Percentage supplied by Diffusion
99
0
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P K Ca Mg S Cu Zn B Fe Mn Mo
35 175 35 40 20 0.1 0.3 0.2 1.9 0.3 0.01
03 02 171 38 05 10 33 10 11 33 10
06 20 429 250 95 400 33 350 53 133 200
94 78 0 0 0 0 33 0 37 0 0
Labile nutrients: For plants growing in soil, the concentration of a nutrient at the root surface depends on the relative rates of uptake by the root and of transport to the root surface from the bulk soil solution. This introduces a requirement to replenish the soil solution because the concentrations of many of the nutrients in the solution are low. Replacement occurs by desorption from the surfaces of the mineral & organic components of soil and by mineralization of soil organic matter. The amount of nutrient