Nutrient Mobility in Soil and Plant Point Wise Notes for Competitive exam

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Nutrient Mobility

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.

Fertilizer Acid equivalent
Ammonium chloride 128
Ammonium sulphate 110
Ammonium sulphate nitrate 93
Ammonium phosphate   86
Urea   80

• Fertilizers which leave alkaline residue in the soil are called alkaline -forming fertilizers or basic fertilizers.

Fertilizer Equivalent basicity
 Calcium cyanide  63
 Sodium nitrate  29
Dicalcium phosphate 25
Calcium nitrate   21

• 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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