Fact Sheet: Nitrogen

The green machine

Introduction to Nitrogen

Nitrogen (N) is a vital macronutrient for plants, essential for the synthesis of amino acids, proteins, and chlorophyll. It is a key component of plant growth and productivity, influencing leaf development, coloration, and overall health. Nitrogen in soil exists in several forms, primarily as nitrate, ammonium, and organic nitrogen, each playing a unique role in plant nutrition.

Detailed Overview of Nitrogen Forms in Soil

Nitrogen in soil exists in various forms, each playing a crucial role in soil fertility and plant nutrition. The main forms are:

  1. Nitrate Nitrogen (NO3-N)

    • Characteristics: Nitrate is the most oxidized form of nitrogen and is highly soluble in water, making it readily available to plants. However, its high mobility means it can easily leach out of the soil profile, especially in sandy or heavily watered/irrigated soils.

    • Importance: Since nitrate is the primary form of nitrogen that plants absorb, its availability directly influences plant growth and productivity. Understanding nitrate levels helps manage fertilization schedules to maximise crop yield and minimise environmental impact.

  2. Ammonium Nitrogen (NH4-N)

    • Characteristics: Ammonium is a positively charged ion (cation) that adheres to negatively charged soil particles, making it less prone to leaching than nitrate. It is converted to nitrate in the soil through the process of nitrification.

    • Importance: Ammonium can provide a more stable source of nitrogen in cooler or less aerated soils where nitrification rates are slow. Knowing ammonium levels can be important in acidic soils since it can help in maintaining soil pH levels.

  3. Organic Nitrogen

    • Characteristics: This form includes nitrogen that is part of organic matter, such as proteins, amino acids, and urea. Organic nitrogen must be mineralised (converted into ammonium and then nitrate) by soil microbes before it becomes available to plants.

    • Importance: Organic nitrogen is a significant reservoir of nitrogen in many soils, especially those with high organic matter. Understanding the total organic nitrogen helps in predicting the long-term availability of nitrogen and the soil's capacity to sustain plant growth without additional fertilizers.

  4. Total Nitrogen

    • Characteristics: This is the sum of all forms of nitrogen in the soil, including nitrate, ammonium, and organic nitrogen. It represents the overall nitrogen content of the soil.

    • Importance: Total nitrogen gives an overview of all nitrogen sources, providing insights into the soil's comprehensive fertility status. This measure is crucial for managing nitrogen in a way that ensures sufficient availability throughout the growing season and minimises loss to the environment.

Types of Nitrogen Tests.

Understanding the different nitrogen measurements provided in soil tests can help gardeners manage their soil's nutrient levels effectively:

  1. Nitrate Nitrogen (NO3-N)

    • Method: Usually measured through soil extraction followed by colorimetric or spectrometric analysis.

    • Typical Values: Low: <10 mg/kg, Optimal: 10-30 mg/kg, High: >30 mg/kg

    • Interpretation: Indicates the amount of nitrogen in the nitrate form, which is immediately available for plant uptake. Nitrate is highly mobile in soil and can be easily leached away.

  2. Ammonium Nitrogen (NH4-N)

    • Method: Measured similarly to nitrate but reflects the ammonium form of nitrogen.

    • Typical Values: Low: <5 mg/kg, Optimal: 5-20 mg/kg, High: >20 mg/kg

    • Interpretation: Represents nitrogen in the ammonium form, which is less mobile than nitrate and can be held in the soil longer but is also readily converted into nitrate by soil microbes under most conditions.

  3. Total Nitrogen

    • Method: Often determined through Kjeldahl digestion or similar comprehensive methods.

    • Typical Values: Low: <0.1%, Optimal: 0.1-0.5%, High: >0.5%

    • Interpretation: Reflects the total amount of organic and inorganic nitrogen forms in the soil. It provides a measure of the overall nitrogen reservoir, including what's immediately available and what could potentially become available.

Interpreting Results

  • Low Levels (Nitrate <10mg/kg, Ammonium <5mg/kg, Total Nitrogen <0.1%):

    Indicate nitrogen deficiency which can lead to stunted growth, chlorosis (yellowing of leaves), and reduced yield.

  • Optimal Levels (Nitrate 10-30mg/kg, Ammonium 5-20mg/kg, Total Nitrogen 0.1-0.5%):

    Adequate for supporting healthy plant growth, promoting vigorous green foliage and optimal productivity.

  • High Levels (Nitrate >30mg/kg, Ammonium >20mg/kg, Total Nitrogen >0.5%):

    Can lead to excessive vegetative growth at the expense of flowering and fruiting; can also increase the risk of leaching, potentially contaminating groundwater.

Factors Affecting Nitrogen Availability

  • Soil Texture and Organic Matter:

    Sandy soils or soils with low organic matter can have reduced nitrogen retention. Conversely, soils rich in organic matter can provide a steady release of nitrogen through mineralisation.

  • Soil pH:

    Extremes in pH can affect microbial activity and the chemical forms of nitrogen, influencing their availability and stability in the soil.

  • Climatic Conditions:

    Temperature and moisture affect microbial activities that convert organic nitrogen to mineral forms (nitrification and ammonification) and the overall nitrogen cycle.

  • Leaching Due to Rainfall or Irrigation:

    Especially in sandy or well-drained soils, significant rainfall or excessive watering can cause rapid leaching of nitrates from the soil. This can substantially decrease the availability of nitrogen to plants and may require more frequent replenishment of nitrogenous fertilisers to maintain optimal levels.


Actionable Recommendations

  • For Low Nitrogen Soils:

    Apply nitrogen-rich fertilisers such as urea, ammonium nitrate, or composted manure to boost levels. Consider cover crops or green manures that fix atmospheric nitrogen.

  • For Optimal to High Nitrogen Soils:

    Maintain levels with careful management, avoiding excess nitrogen which can lead to environmental issues. Utilise slow-release fertilisers to provide a steady supply without the risk of leaching.

FAQs

  • What are signs of nitrogen deficiency in plants?

    Look for pale green leaves, stunted growth, and poor development of new leaves.

  • How often should I test for nitrogen?

    Annually or biannually, as nitrogen levels can change rapidly due to its high mobility and reactivity in soil.