Mineralization, Availability & Reaction Products of Nutrients in Soils

Mineralization of Nutrients

Definition: Mineralization is the microbial conversion of organic forms of nutrients in soil into inorganic forms that are available for plant uptake. It is an essential process for the cycling of Nitrogen, Sulphur, and Phosphorus in soils.

Nitrogen (N) Mineralization

• Organic N in soil (proteins, amino acids, nucleic acids) → Ammonium (NH₄⁺) → Nitrate (NO₃⁻).
• Steps:
  1. Ammonification: Organic N → NH₄⁺ by decomposer microbes (bacteria, fungi).
  2. Nitrification: NH₄⁺ → NO₂⁻ → NO₃⁻ by Nitrosomonas & Nitrobacter.
• Factors affecting N mineralization:

• • Soil temperature: Optimal 25–35°C.
  • Soil moisture: Field capacity ideal.
  • Soil pH: 6–8 optimal.
  • Organic matter content & C:N ratio (optimum 20–30).
  • Microbial population & activity.

Sulphur (S) Mineralization

• Organic S (from amino acids, proteins) → SO₄²⁻ (sulphate) by Thiobacillus spp.
• Required for protein synthesis and oilseed crops.
• Influenced by soil moisture, temperature, and microbial activity.

Phosphorus (P) Mineralization

• Organic P → H₂PO₄⁻ or HPO₄²⁻ by microbial phosphatases.
• Depends on soil pH, organic matter, microbial population.
• Usually slow, often supplemented by chemical fertilizers.

2. Availability of Nutrients in Soil

Definition: Nutrient availability refers to the proportion of nutrients in soil that is readily accessible to plants. It is influenced by soil properties, chemical reactions, and environmental conditions.

Factors Affecting Nutrient Availability

• Soil pH:
  1. Acidic soils (pH < 6) → Al³⁺, Fe³⁺ toxicity; P, Mo, Ca unavailable.
  2. Alkaline soils (pH > 8) → Zn, Fe, Mn, Cu deficiencies.
• Soil texture:
  1. Clay soils → high nutrient retention, slow release.
  2. Sandy soils → low retention, prone to leaching.
• Organic matter: Source of N, S, P; enhances CEC and microbial activity.
• Redox potential: Waterlogged soils → NO₃⁻ loss via denitrification; Fe & Mn more soluble.
• Moisture & temperature: Affect microbial activity and nutrient diffusion.
• Fertilizer use and cropping intensity → high NPK can induce secondary/micronutrient deficiencies.

Forms of Nutrients in Soil

Reaction Products of Nutrients in Soil

Nitrogen (N)

• Nitrification: NH₄⁺ + O₂ → NO₂⁻ → NO₃⁻ (soil acidification occurs).
• Denitrification: NO₃⁻ → N₂ / N₂O (under anaerobic/waterlogged conditions).
• Volatilization: NH₄⁺ → NH₃ (loss in alkaline soils).

Phosphorus (P)

• Acid soils: P reacts with Fe³⁺ / Al³⁺ → insoluble Fe/Al-P.
• Alkaline soils: P reacts with Ca²⁺ → Ca-P compounds (apatite, tricalcium phosphate).
• Fixation: Reduces P availability, major limiting factor for crop yield.

Potassium (K)

• Exchangeable K⁺: Readily available.
• Non-exchangeable K: Fixed in interlayers of mica/illite → slowly available.

Sulphur (S); SO₄²⁻ may leach or react with Ca²⁺ → gypsum (CaSO₄·2H₂O) in calcareous soils.

Micronutrients

• Fe, Mn: Oxidized to insoluble oxides under aerobic conditions.
• Zn, Cu: Adsorbed by organic matter or precipitated as hydroxides at high pH.
• B: Soluble as boric acid; leaches easily in sandy soils.

Nutrient Transformations in Soil (Detailed)

Nitrogen Transformations

• Ammonification (Mineralization of N)
  • Organic N → NH₄⁺ (ammonium).
  • Carried out by heterotrophic microbes.
  • Influenced by temperature, moisture, pH, and residue type.
• Nitrification
  • NH₄⁺ → NO₂⁻ → NO₃⁻ (by Nitrosomonas and Nitrobacter).
  • Acidifies soil slightly (H⁺ released).
  • Nitrate is highly mobile → prone to leaching in sandy soils.
• Denitrification
  • NO₃⁻ → N₂ / N₂O under anaerobic/waterlogged conditions.
  • Causes N loss from soil.
• Volatilization; NH₄⁺ → NH₃ (especially in alkaline soils or surface-applied urea).
• Immobilization
  • Microbes temporarily assimilate inorganic N into organic forms → unavailable to plants.
  • Occurs when C:N ratio of residue is high (>30).

Phosphorus Reactions

• Acid soils (pH <6.5); H₂PO₄⁻ reacts with Fe³⁺ and Al³⁺ → insoluble Fe/Al-P (reduces availability).
• Alkaline soils (pH >7.5); HPO₄²⁻ reacts with Ca²⁺ → Ca-P compounds (apatite, tricalcium phosphate).
• Organic P Mineralization; Microbial phosphatases hydrolyze organic P → plant-available orthophosphate.
• Fixation and Occlusion; Over time, P becomes occluded → slowly available or unavailable. Major reason for P deficiency in highly weathered soils.

Potassium Transformations

• Exchangeable K⁺: Readily available for plants.
• Non-exchangeable/fixed K: Trapped in interlayers of mica/illite → slowly released.
• Leaching: Minor in clay soils, significant in sandy soils with high rainfall.

Sulphur Transformations

• Organic S → SO₄²⁻ (sulphate) via microbial mineralization.
• SO₄²⁻ can form gypsum (CaSO₄·2H₂O) in calcareous soils.
• Leachable, especially in sandy soils → deficiency risk.

Micronutrient Reactions

Factors Controlling Nutrient Availability

• Soil pH: Most nutrients optimal at 6.5–7.5 (except Fe, Mn, Zn more available in acidic soils).
• Redox potential: Controls Fe, Mn, N availability in waterlogged soils.
• Soil texture & CEC: High clay/organic matter → more retention of cations.
• Organic matter: Source of N, S, P; binds micronutrients; improves CEC.
• Moisture & Temperature: Affect microbial activity, mineralization, and solubility.

Practical Implications

• Acid soils: Apply lime → improves pH → enhances N, P, Mo availability.
• Alkaline soils: Apply gypsum → reduces sodicity → improves Zn, Fe availability.
• High P soils: Risk of Zn, Fe, Mn deficiency → foliar micronutrient sprays recommended.
• Flooded rice soils: Mn, Fe become more available; N losses via denitrification → split N application advised.
• Sandy soils: Risk of leaching of NO₃⁻, SO₄²⁻, K⁺ → fertigation and split fertilization recommended.

Key Exam Facts

• Mineralization: Essential for N, S, P availability.
• P fixation: Acid soils → Fe/Al-P; Alkaline soils → Ca-P.
• K fixation: Clay minerals trap K → slowly available.
• N losses: Volatilization, leaching, denitrification.
• Micronutrient availability: Highly dependent on pH, OM, redox potential, and soil type.
• Liquid fertilizers: Supply nutrients in immediately available form, bypassing slow mineralization.