Nutrient Mobility in Soil and Plant Pont Wise Notes : Agrilearner

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 http://premier-pharmacy.com 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)