NUTRIENT MOBILITY IN SOILS
• In case of immobile nutrients, roots have to reach the area of nutrient availability and forage volume is limited to root surface.
• For highly mobile nutrients, entire volume of the root zone is forage area.
• Mobile nutrients are highly soluble and they are not adsorbed on clay complex. e.g., NO3-, SO42-, BO32-, Cl-, Mn2+
• Less mobile nutrients are also soluble, but they are adsorbed on clay complex and so their mobility is reduced e.g., NH4+, K+, Ca+, Mg2+, Cu2+
• Immobile nutrient ions are highly reactive and get fixed in the soil e.g., H2PO4-, HPO42-, Zn2+
• Nutrients are transported to plant roots by mass flow and diffusion.(Nutrient Mobility)
• Mass flow is movement of nutrient ions and salts along with moving water.
• In mass flow movement of nutrients reaching the root is dependent on the rate of water flow.
• Diffusion occurs when there is concentration gradient of nutrients between the root surface and surrounding soil solution.
• In contact exchange theory a close contact between root surfaces and soil colloids allows a direct exchange of H+ released from the plant roots with cations from soil colloids.
• Importance of contact exchange in nutrient transport is less than with soil solution movement.(Nutrient Mobility)
• Labile pool of nutrients in the soil represents quantity factor.
• Nutrient concentration of the soil solution represents intensity factor.
• Nutrient absorption by plant roots directly depends on the concentration of the soil solution (intensity factor) which in turn is regulated by the labile pool (quantity factor).
• Many elements in their most oxidized state are favored in absorption.
• Fe and Mn are more available in their reduced form.
MOBILITY IN PLANTS
• N, P and K – Highly mobile
• Zinc is moderately mobile in plants and deficiency symptoms appear in middle leaves.
• S, Fe, Mn, Cu, Mo and Cl – less mobile
• Ca and B – Immobile
• A mobile nutrient in plant moves to the growing points in case of deficiency, so deficiency symptoms appear on lower leaves.
• Cell wall is differentially permeable and selectively absorbs particular cations and anions.(Nutrient Mobility)
• Among the cations and anions, cations have competitive advantage.
• But three anions NO3-, H2PO4- and SO42- are taken in large quantities.
• Entry of inorganic materials into xylem occurs at the root tips.
• Absorption of nutrients is primarily by root hair cells.
• Each root hair may be effective for absorption just for a few days.
• In passive absorption nutrients enter the plants along with transpiration stream without the use of energy.
• Active absorption is the absorption of nutrients from soil solution containing low concentration of nutrients compared to plant sap, by expending energy.
• A portion of absorbed nitrate nitrogen (NO3-N) is reduced to ammonical nitrogen (NH4-N) and glutamine in roots.These compounds along with remaining portion of NO3-N passes through the symplast (living connection between cells) and enters the xylem.
• Nitrate reduction takes place in leaves.(Nutrient Mobility)
• Reduced compounds enter phloem vessels and are translocated to growing points like young leaves, roots, fruits etc.
• Metabolic transformation of inorganic plant nutrients into organic plant constituents is known as assimilation.
• A fraction of absorbed nutrients may be stored in vacuoles without being assimilated.
• Plants absorb nitrogen mostly as NO3- and NH4+.
• Nitrate assimilation is mainly carried out in leaves and a small fraction in roots.
• Nitrate in plants is transformed into ammonia in two steps. First, nitrate is reduced to nitrite by nitrate reductase enzyme and the reducing power is supplied by NADH.
• Molybdenum is a constituent of nitrate reductase.
• Deficiency of molybdenum blocks nitrate assimilation resulting in nitrate accumulation.(Nutrient Mobility)
• Deficiency of molybdenum causes nitrogen deficiency symptoms in addition to molybdenum deficiency symptoms.
• Nitrite is reduced to ammonia in presence of nitrite reductase enzyme. Nitrite reduction occurs in chloroplasts in leaves and reducing power is supplied by respiration. A portion of energy released during light reaction of photosynthesis is used for the reduction of nitrite to ammonia.
• Nitrite reduction occurs in dark also both in root and leaf and the reducing power is supplied by respiration.
• C4 plants assimilate nitrate more efficiently than C3 plants.(Nutrient Mobility)
• Presence of ammonia in plants is due to reduction of nitrates, absorption of ammonical nitrogen, breakdown of proteins, break down of urea absorbed by plants and due to nitrogen fixation.
• Ammonia is assimilated rapidly in plants.
• Glutamic acid combines with ammonia to form glutamine.
• Glutamine combines with alpha ketoglutaric acid to form two molecules of glutamic acid which is an amino acid.
• Glutamic acid is used as base material for the synthesis of other amino acids and this process is known as transamination.
• Most of the ammonia absorbed by plants is transformed into glutamine in root cells and transported to leaves where it combines with alpha ketoglutaric acid to form glutamic acid.
• Assimilation of sulphates takes place in chloroplasts.
• Sulphate ion reacts with ATP and is activated and is reduced to sulphite.
• Sulphite is reduced to sulphide with the help of the enzyme ferredoxin which in turn is incorporated into cysteine, a sulphur containing amino acid.
• Straight fertilizers are those which supply only one primary plant nutrient, namely nitrogen or phosphorus or potassium. e.g., urea, ammonium sulphate, potassium chloride, potassium sulphate.(Nutrient Mobility)
• Complex fertilizers contain two or three primary plant nutrients of which two primary nutrients are in chemical composition. e.g., DAP, nitro-phosphates, ammonium phosphate.
• Mixed fertilizers are physical mixtures of straight fertilizers. They contain two or three primary plant nutrients.
• Low analysis fertilizers contain less than 25 percent of primary nutrients. e.g., SSP (16%P2O5), Chilean nitrate or sodium nitrate (16% N)
• In high analysis fertilizers total content of primary nutrients is above 25 percent. e.g., urea (46%N), anhydrous ammonia (82.2% N), ammonium phosphate (20% N + 20% P2O5) and DAP (18% N + 46% P2O5)
• SSP is in powder form.
• Ammoniumsulphate is in crystal form.(Nutrient Mobility)
• Urea, DAP and superphosphate occurs as prills.
• When the nitrogenous, phosphatic, potassic and other fertilizer materials are completely dissolved in water, these are called clear liquid fertilizers.
• Suspension liquid fertilizers are those in which some of the fertilizer materials are suspended as fine particles.
• Fertilizers which leave an acid residue in the soil are called acid forming fertilizers.
• Amount of calcium carbonate required to neutralize the acid residue of a fertilizer is called as its equivalent acidity.
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• Fertilizers which leave alkaline residue in the soil are called alkaline -forming fertilizers or basic fertilizers.
• Fertilizer grade refers to the guaranteed minimum percentage of N, P2O5 and K2O contained in fertilizer material. e.g., 28-28-0 indicates 100 kg of fertilizer material contains 28 kg N, 28 kg P2O5 and no potash.(Nutrient Mobility)
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