Forms of Nutrients in Soil
Plants absorb nutrients from soil in specific ionic forms.
However, in soil, these nutrients exist in several chemical and physical pools, ranging from readily available to unavailable forms.
Understanding these forms helps in fertilizer management and soil fertility evaluation.
1. Classification of Nutrient Forms in Soil
Nutrients in soil occur in different chemical forms that differ in their availability to plants. These forms act as nutrient pools and regulate how much nutrient reaches the soil solution for plant uptake. Based on availability, nutrient forms are grouped into four major categories.
1.1 Soil Solution Form (Most Available Form): This is the immediately available form of nutrients.
Key Features
- Nutrients are dissolved in soil water.
- Directly absorbed by roots through mass flow and diffusion.
- Controls the instant supply of nutrients to plants.
Examples
- N: NO₃⁻, NH₄⁺
- P: H₂PO₄⁻, HPO₄²⁻
- K: K⁺
- Ca²⁺, Mg²⁺
- SO₄²⁻
- Micronutrients: Zn²⁺, Fe²⁺, Mn²⁺, Cu²⁺, boron (H₃BO₃)
- This form determines nutrient concentration in the root zone.
1.2 Exchangeable Form (Readily Available Form): A buffering reservoir that maintains soil solution concentration.
Key Features
- Nutrients are held on clay and organic colloids through electrostatic attraction (CEC).
- Easily exchanged with other cations in soil solution.
- Readily replenishes solution nutrient levels during plant uptake.
Examples
- K⁺, Ca²⁺, Mg²⁺ held on clay particles
- NH₄⁺ adsorbed on exchange sites
- These forms are readily available but not as immediate as soil solution forms.
1.3 Non-Exchangeable or Fixed Form (Slowly Available): These forms contribute to long-term nutrient supply.
Key Features
- Present in primary & secondary minerals or in organic matter.
- Release nutrients only through weathering (minerals) or
mineralization (organic matter). - Act as long-term nutrient reserves.
Examples
- Organic N, organic S, organic P (phytin)
- Mineral forms of K in feldspar, mica
- Micronutrients in oxides or sulfides (Fe, Mn, Zn, Cu)
- These forms do not contribute immediately to plant nutrition.
- Nutrient-wise Forms in Soil
Each nutrient in soil exists in different chemical forms that vary in availability to plants. Some forms are readily available, while others serve as slow-release or reserve sources. Understanding these forms is essential for fertilizer management and soil fertility evaluation.
a) Macronutrients
i) Nitrogen (N) – Forms
- Nitrogen occurs in both organic and inorganic
- Organic N: Proteins, amino acids, humus (unavailable; becomes available after mineralization).
- NH₄⁺ (Ammonium): Plant-available; held on exchange sites.
- NO₃⁻ (Nitrate): Plant-available; highly mobile in soil.
- NO₂⁻ (Nitrite): Intermediate form during nitrification; rarely accumulates.
- Availability: Plants absorb N mainly as NO₃⁻ and NH₄⁺.
ii) Phosphorus (P) – Forms
- Phosphorus exists in soil as organic and inorganic compounds.
- Organic P: In phytin, phospholipids; becomes available after mineralization.
- H₂PO₄⁻ and HPO₄²⁻: The two available forms (dominant at different pH levels).
- Fe-P, Al-P, Ca-P complexes: Fixed forms; not readily available.
- Availability: Highly dependent on soil pH and fixation.
iii) Potassium (K) – Forms
- Potassium occurs in four major pools:
- Solution K⁺: Directly available.
- Exchangeable K: Readily available reservoir.
- Fixed K: Slowly available, trapped between clay layers.
- Mineral K: Structural K in feldspar, mica (unavailable).
- Availability: Mostly as K⁺.
iv) Calcium (Ca) and Magnesium (Mg) – Forms
- These cations occur in:
- Ca²⁺ / Mg²⁺ in solution: Available.
- Exchangeable Ca & Mg: Easily available.
- CaCO₃, MgCO₃: Slow-release forms.
- Mineral forms: Unavailable until weathered.
- Availability: Higher in neutral to alkaline soils.
v) Sulphur (S) – Forms
- Sulphur exists in:
- SO₄²⁻ (Sulfate): Available form.
- Organic S: Released through mineralization.
- Sulfides (under waterlogged conditions): Unavailable.
- Availability: Plants take up S as SO₄²⁻.
b) Micronutrients
i) Iron (Fe) – Forms
- Fe²⁺: Available form (reduced soils).
- Fe³⁺: Less available (oxidized soils).
- Fe-oxides, Fe-hydroxides: Unavailable reserves.
- Availability: High in acidic, low in calcareous soils.
ii) Manganese (Mn) – Forms
- Mn²⁺: Available form.
- MnO₂ and Mn oxides: Unavailable.
- Availability: High under reducing (waterlogged) conditions.
iii) Zinc (Zn) – Forms
- Zn²⁺: Available form.
- ZnO, ZnS, mineral Zn: Unavailable.
- Availability: Reduced in alkaline and sandy soils.
iv) Copper (Cu) – Forms
- Cu²⁺: Available.
- Organic-bound Cu: Slowly available.
- CuO, CuS: Unavailable.
- Availability: Tightly held in organic soils.
v) Boron (B) – Forms
- H₃BO₃ (boric acid): Available form.
- Adsorbed B: Slowly available.
- Availability: Declines under dry conditions.
vi) Molybdenum (Mo) – Forms
- MoO₄²⁻ (molybdate): Available.
- Organic Mo: Slowly available.
- Availability: Higher in alkaline soils.
vii) Chlorine (Cl) – Forms: Cl⁻: Plant-available and very mobile.
viii) Nickel (Ni) – Forms: Ni²⁺: Available form. Requirement: Needed in trace amounts.
Importance of Understanding Nutrient Forms
- Predicts nutrient availability: Knowing the form of a nutrient helps determine how easily plants can absorb it and how quickly it will be released in soil.
- Improves fertilizer selection and placement: Helps choose the right type of fertilizer and apply it in a way that matches nutrient behavior in soil.
- Aids in diagnosing deficiencies and toxicities: Different nutrient forms influence how and where deficiencies or excesses appear in crops.
- Essential for nutrient management planning: Understanding nutrient forms helps design balanced, efficient nutrient management strategies to improve crop yield and soil health
