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Soil Microbiology: Ammonification in Soil

Introduction

Ammonification is a central process in the nitrogen cycle, responsible for converting organic nitrogen into ammonia (NH₃) or ammonium (NH₄⁺). This microbial-driven transformation is essential for soil fertility, ecosystem productivity, and plant nutrition. Understanding ammonification helps agronomists, microbiologists, and environmental scientists optimize soil management, fertilizer strategies, and microbial ecology research.

What Is Ammonification?

Ammonification also called mineralization of organic nitrogen—is the biochemical process where heterotrophic soil microorganisms decompose organic matter and release ammonia.

This occurs when organic nitrogen compounds such as:

  • proteins

  • nucleic acids

  • amino acids

  • urea

  • plant residues

  • manure

are broken down and converted into NH₃/NH₄⁺.

Microorganisms Involved in Ammonification

1. Ammonifying Bacteria

  • Bacillus spp.

  • Clostridium spp.

  • Proteus spp.

  • Pseudomonas spp.

  • Actinomyces spp.

2. Fungi

  • Aspergillus

  • Penicillium

  • Mucor

3. Actinomycetes

Important in humus degradation and nitrogen release.

These microorganisms utilize organic nitrogen as an energy and nutrient source, releasing ammonia as a metabolic by-product.

Biochemical Process of Ammonification

  1. Proteolysis

    Soil microbes secrete extracellular enzymes (proteases) to break down proteins into peptides and amino acids.

  2. Deamination

    Amino acids undergo oxidative or reductive deamination, releasing NH₃.

  3. Ammonia Release

    NH₃ may:

    • remain as ammonia gas (alkaline soils), or

    • convert to ammonium (NH₄⁺) in moist or acidic soils.

This ammonium can later undergo nitrification, forming nitrate (NO₃⁻), which plants can absorb.

Factors Affecting Ammonification

1. Soil Temperature

Optimum: 25–35°C

Higher temperatures accelerate microbial decomposition.

2. Soil Moisture

Moderate moisture enhances microbial activity; waterlogged soils slow down ammonifiers.

3. Soil pH

  • Neutral to slightly alkaline soils favor ammonification.

  • Very acidic soils inhibit microorganism growth.

4. Organic Matter Content

More organic substrates → more ammonification.

5. C:N Ratio

A balanced carbon-to-nitrogen ratio supports efficient nitrogen mineralization.

Importance of Ammonification in Soil Ecosystems

1. Nutrient Recycling

Ammonification releases nitrogen in a plant-available form, promoting soil fertility.

2. Supports the Nitrogen Cycle

It is the first step before nitrification and denitrification.

3. Soil Productivity

Without ammonification, organic nitrogen would accumulate, and plant growth would drastically decline.

4. Indicator of Soil Health

High ammonification activity indicates:

  • active microbial populations

  • good organic matter turnover

  • healthy soil structure

5. Role in Agriculture

Essential for:

  • composting

  • sustainable farming

  • biofertilizer development

  • nitrogen-use efficiency

Applications and Laboratory Measurement

1. Ammonification Test

Performed by incubating soil with a nitrogen-rich substrate and measuring NH₄⁺ formation.

2. Analytical Methods

  • Nessler’s reagent for ammonium

  • Ion-selective electrodes

  • Spectrophotometry

  • Steam distillation (Kjeldahl method)

3. Agricultural Monitoring

Used to estimate:

  • soil fertility

  • decomposition rates

  • organic matter quality

Conclusion

Ammonification is a vital microbial process that sustains plant growth and maintains soil fertility. Driven by bacteria, fungi, and actinomycetes, it transforms organic nitrogen into ammonia a critical nutrient for crops and natural ecosystems. Understanding ammonification helps improve soil management, enhance agricultural productivity, and maintain ecological balance.