Nutrient Absorption and Forms of Nutrients in Plants
- Nutrient Absorption
Definition:
Nutrient absorption is the process by which plant roots take up essential mineral nutrients from the soil solution and transport them into plant tissues for growth and development.
- Nutrients are absorbed mostly in ionic form (charged particles).
- The process occurs mainly through root hairs and epidermal cells.
- Nutrient uptake depends on soil properties, root activity, and concentration gradient.
- Mechanisms of Nutrient Absorption
Plant roots absorb nutrients through two major mechanisms:
(A) Passive Absorption
- Nutrients move from the soil solution into the root cells without the expenditure of metabolic energy (ATP).
- Movement occurs along the concentration gradient — from higher to lower concentration.
- Accounts for 90–95% of total ion absorption.
Mechanisms involved:
- Diffusion: Movement of ions from high to low concentration across membranes.Example: Uptake of K⁺, NO₃⁻ ions.
- Mass Flow: Nutrients move along with the flow of water during transpiration and root absorption. Example: Uptake of Ca²⁺, Mg²⁺, and nitrate ions.
- Ion Exchange: Exchange between ions adsorbed on root surface (e.g., H⁺, OH⁻) and those in soil solution. Example: H⁺ from roots exchanged for K⁺ from soil colloid.
Key Characteristics:
- No energy required.
- Fast process under high transpiration.
- Operates mainly in outer root tissues.
(B) Active Absorption
- Nutrients are absorbed against the concentration gradient (low → high) with the help of metabolic energy (ATP).
- It is an energy-dependent process occurring through carrier proteins located in plasma membranes.
- Requires enzymes like ATPase to transport ions.
Steps involved:
- Nutrient ions bind to carrier proteins on the root cell membrane.
- ATP provides energy to move ions into the cytoplasm.
- Ions accumulate inside the root and are translocated to xylem.
Example: Uptake of phosphate (H₂PO₄⁻), sulphate (SO₄²⁻), nitrate (NO₃⁻), and other ions of low mobility.
Key Characteristics:
- Requires energy from respiration.
- Occurs mainly in living root cells (cortex and endodermis).
- Enables selective ion uptake.
- Forms of Plant Nutrients Absorbed
Plants absorb nutrients mainly in ionic or soluble forms.
The form depends on the chemical nature of the nutrient element.
|
Nutrient Element |
Form Absorbed by Plants |
|
Nitrogen (N) |
NO₃⁻ (nitrate), NH₄⁺ (ammonium) |
|
Phosphorus (P) |
H₂PO₄⁻, HPO₄²⁻ |
|
Potassium (K) |
K⁺ |
|
Calcium (Ca) |
Ca²⁺ |
|
Magnesium (Mg) |
Mg²⁺ |
|
Sulphur (S) |
SO₄²⁻ |
|
Iron (Fe) |
Fe²⁺, Fe³⁺ |
|
Manganese (Mn) |
Mn²⁺ |
|
Zinc (Zn) |
Zn²⁺ |
|
Copper (Cu) |
Cu²⁺ |
|
Boron (B) |
BO₃³⁻, B₄O₇²⁻ |
|
Molybdenum (Mo) |
MoO₄²⁻ |
|
Chlorine (Cl) |
Cl⁻ |
|
Nickel (Ni) |
Ni²⁺ |
|
Carbon (C) |
CO₂ (absorbed through leaves) |
|
Hydrogen (H) |
H⁺ (from water) |
|
Oxygen (O) |
O₂, H₂O |
- Combined and Uncombined Forms of Nutrients
Nutrients occur in the soil in two basic forms:
|
Form |
Description |
Examples |
|
Combined Forms |
Nutrients are present as part of organic or inorganic compounds. They are not directly available to plants and become available only after mineralization or weathering. |
Nitrogen in proteins, Phosphorus in organic matter, Sulphur in sulphides. |
|
Uncombined Forms |
Nutrients exist as free ions in soil solution and are directly available for plant absorption. |
K⁺, Ca²⁺, NO₃⁻, SO₄²⁻, Cl⁻. |
➡️ Plants mostly absorb nutrients in uncombined ionic form, whereas combined forms serve as a reserve pool that releases nutrients gradually.
- Factors Affecting Nutrient Absorption
- Soil moisture: Adequate moisture enhances nutrient movement.
- Soil temperature: High temperature increases root activity.
- Soil aeration: Proper aeration facilitates respiration and active uptake.
- Soil pH: Affects solubility and availability of nutrients.
- Root health and surface area: More root hairs → higher absorption.
- Microbial activity: Affects nutrient mineralization and solubility.
- Concentration gradient: Drives passive movement of ions.
