Mechanism of nutrient absorption

Institute of Agriculture and Animal Science (IAAS)
Prithu Technical College
Deukhuri, Dang
SSC-121 Mechanism of Nutrient Absorption
Mechanism of Nutrient Absorption
Krishna Aryal
Asst. Professor (Soil Science)
Krish.aryal2014@gmail.com
1

Deukhuri, Dang
SSC-121 Mechanism of Nutrient Absorption 2
Movement of nutrient from soil to root
• Roots do not intentionally grow towards a nutrient source.
• For nutrient uptake to occur, the individual nutrient ion must be in position
adjacent to the root.
• There are three basic methods in which nutrients comes in contact with
the root surface for plant uptake; they are;
i. Root interception
ii. Mass flow
iii. Diffusion

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i. Root interception:
• Root interception occurs when a nutrient comes into physical contact with
the root surface.
• As a general rule, the occurrence of root interception increases as the root
surface area & mass increases, thus enabling the plant to explore a greater
amount of soil.
• Root interception may be enhanced by mycorrhizal fungi, which colonize
roots & increases root exploration into the soil.
• Root interception is responsible for an appreciable amount of Ca and P
uptake, & some amounts of Mg, Zn & Mn.

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ii. Mass flow:
• Mass flow occurs when nutrients are transported to the surface of roots
by the movement of water in the soil (i.e. percolation, evaporation etc.).
• The rate of water flow governs the amount of nutrients that are
transported to the root surface.
• Therefore, mass flow decreases as soil water decreases.
• Most of the nitrogen, calcium, magnesium, sulphur, copper, boron,
manganese & molybdenum move to the root by mass flow.

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iii. Diffusion:
• Diffusion is the movement of a particular nutrient along a concentration
gradient.
• When there is a difference in concentration of a particular nutrient within
the soil solution, the nutrient will move from an area of higher concentration
to an area of lower concentration.
• Diffusion delivers appreciable amounts of phosphorus, potassium, zinc, &
iron to the root surface.
• Diffusion is a relatively slow process compared to the mass flow of
nutrients with water movement toward the root.

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Nutrient absorption by plant roots
Plant absorption of ions from the soil solutions/root space can be
described by;
i. Passive absorption( Apoplastic)
ii. Active absorption (Symplastic)

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i. Passive Absorption
• Rapid movement of ions into the free spaces (space between cell wall
and plasma membrane called apoplast)
• Ions moves along the concentration gradient( higher to lower)
• Do not require energy(ATP) thus called passive.
The major hypothesis (theories) that explain the mechanism of passive
absorption of ions are:
1. Mass flow theory. 2. Contact Exchange theory.
3.Carbonic Acid Exchange Theory 4. Donnan Equilibrium

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Mass flow theory
• According to this theory ions are absorbed by the root along with mass
flow of water under the effect of transpiration.
• An increase in transpiration pull increases the uptake of ions by the roots,
(the uptake of ions by free diffusion).
• Thus, mass flow of ions through the root tissues occurs due to
transpiration pull in the absence of metabolic energy.

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Contact Exchange Theory
• According to this theory, the ions adsorbed on the clay micelles get
adsorbed to the root in exchange for hydrogen ions, previously, adsorbed on
the root.
• Ions adsorbed on solid particles oscillate within a small space. When two
particles are close enough, the oscillation space of an ion adsorbed to one
particle overlaps the oscillation space of an ion adsorbed to another particle.
Thus exchange of ions may take place.

Deukhuri, Dang
Carbonic Acid Exchange Theory
• The soil solution provides medium for exchange of ions between the root
and clay micelles.
• Carbon dioxide released in respiration of root forms carbonic acid by
reacting with water of the soil solution.
• Carbonic acid then dissociates in the soil solution to form H⁺ and HCO₃⁻
ions shown above expression.
• H⁺ ions are adsorbed to the clay micelles in exchange for cations, such as
K⁺ , which are released into the soil solution.
• From here they may diffuse to the root, where they may be adsorbed in
exchange for H⁺ ions.

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Donnan Equilibrium
• As cell membrane is composed of macromolecules of proteins and lipids
that have many carboxyl groups (-COOH) and phosphate (HPO₃⁻)
groups, from which positively charged particles like protons of hydrogen
(H⁺) can dissociate, leaving the macromolecules with negative charge
called non diffusible anions.
• To counterpart the negative charge positively charged anions moves
inside the cell membrane and thus equilibrium exists known as Donnan
Equilibrium.
• C+ (ouside). A-(outside) = C+ (inside). A-(inside)

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ii. Active Absorption
• Movement of ions through inner space (called Symplast)
• Ions moves against the concentration gradient
• Requires energy(ATP) thus called active.
• Slow movement of ions
• The major hypothesis that explain
the mechanism of active transport
of ions is:
Carrier Concept – transport by
a carrier protein.

Deukhuri, Dang

Mechanism of nutrient absorption

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