Fact Sheet: Carbon

The living soil

Introduction to Soil Carbon

Soil carbon is a fundamental component of soil organic matter, playing a crucial role in soil health, fertility, and ecosystem function. It helps improve soil structure, retain moisture, and sustain a vibrant soil microbial community, all of which contribute to plant growth and environmental resilience.

Organic Carbon

  • Overview: Organic carbon measures the total amount of carbon contained in organic compounds within the soil. This encompasses all organic matter components, from freshly added plant residues to well-decomposed humus.

  • Importance: It signifies the overall level of organic matter in the soil, impacting soil structure, moisture retention, and long-term nutrient availability. Organic carbon is vital for enhancing water retention, supporting a robust microbial ecosystem, and sustaining plant growth over extended periods.

Active (Labile) Carbon

  • Overview: Active or labile carbon refers to the portion of soil organic carbon that is most readily available for microbial decomposition. It represents a more immediate and dynamic component of soil organic matter.

  • Importance:

    • Indicator of Microbial Activity: Labile carbon is a critical indicator of the soil's biological health because it represents the fraction of organic matter readily available for microbial use.

    • Nutrient Cycling Speed: The presence of labile carbon influences the rate at which nutrients are cycled within the soil, with higher levels indicating more active nutrient cycling and faster availability for plant uptake.

    • Soil Resilience and Response: This component of soil carbon responds quickly to management changes and environmental conditions, making it a sensitive gauge of how soil management practices impact biological functions.

Importance of Differentiating the Two

  • Immediate Plant Benefits: The labile portion of soil organic carbon is crucial for short-term plant growth and health because it directly influences nutrient cycling and availability.

  • Long-term Soil Health: The stable portion, while not immediately beneficial to plants in terms of nutrients, plays a critical role in maintaining soil structure, water retention, and overall soil health over longer periods.

Why Both Tests are Important

  • Comprehensive Soil Health Assessment: Together, these tests provide a complete picture of the soil's carbon status. While total organic carbon gives an idea of the potential long-term benefits and overall soil fertility, active carbon tells you how "active" and healthy the soil is right now in terms of biological activity and nutrient processing.

  • Targeted Management Practices: Understanding both total and labile carbon levels can help tailor management practices more precisely. For instance, if organic carbon levels are high but labile carbon is low, efforts might be focused on increasing microbial activity to enhance nutrient cycling.

Total (Combustible) Carbon

  • Overview: Total carbon includes both the organic and inorganic carbon present in the soil. It is typically measured by combusting the soil sample and quantifying the resulting carbon dioxide, hence often referred to as combustible carbon.

  • Importance:

    • Comprehensive Carbon Assessment: Total carbon measurement provides a complete overview of all carbon present, reflecting the soil's overall carbon pool and capacity for long-term carbon storage.

    • Indicator of Soil Stability and Quality: High levels of total carbon can indicate a soil rich in organic matter, contributing to stability, fertility, and quality. It helps assess the soil's ability to support sustainable plant growth and ecosystem services.ot immediately provide nutrients to plants.

Types of Nitrogen Tests.

Types of Soil Carbon Tests

  1. Organic Carbon (W&B)

    • Method: This test measures the amount of carbon in organic compounds within the soil, typically using methods like the Walkley-Black procedure.

    • Importance: Indicates the level of organic matter and provides insights into soil fertility and nutrient-holding capacity.

  2. Active (Labile) Carbon

    • Method: Measures the most reactive, easily decomposable fraction of soil organic carbon, often using colorimetric or spectroscopic methods.

    • Importance: Labile carbon is a good indicator of soil biological activity and overall soil health, reflecting the soil's ability to support plant growth and nutrient cycling.

  3. Total Carbon (Combustion)

    • Method: Determines the total carbon content, both organic and inorganic, by combusting the soil sample and measuring the resulting carbon dioxide.

    • Importance: Provides a comprehensive overview of all carbon sources in the soil, crucial for understanding long-term carbon storage and its effects on soil properties.

Interpreting Results

Organic Carbon

  • Low Levels: Less than 2%

    • Implications: Indicates insufficient organic matter, which can lead to poor soil structure and reduced water retention. Plants may show signs of stress and poor growth due to limited nutrient availability.

    • Recommendations: Enhance soil organic matter through the addition of compost, well-rotted manure, or organic mulches to improve soil health and fertility.

  • Optimal Levels: 2-4%

    • Implications: Suggests a healthy amount of organic matter, supporting good soil structure, moisture retention, and nutrient supply, which is ideal for most ornamental plants.

    • Recommendations: Maintain current management practices, continue regular additions of organic materials, and monitor soil health through periodic testing.

  • High Levels: Above 4%

    • Implications: While generally beneficial, very high levels of organic matter might not necessarily improve plant health further and could complicate nitrogen availability due to microbial competition for nitrogen (immobilization).

    • Recommendations: Monitor nutrient balances, particularly nitrogen, and adjust fertilization practices to ensure that high organic matter does not lead to nutrient imbalances.

Active (Labile) Carbon

  • Low Levels: Less than 500 mg/kg

    • Implications: Indicates low microbial activity and potential underperformance in nutrient cycling and soil health.

    • Recommendations: Enhance microbial life and nutrient availability through the addition of compost teas, manures, and other biologically active amendments.

  • Optimal Levels: 500-2,000 mg/kg

    • Implications: Suggests a healthy level of microbial activity and soil fertility, supporting robust nutrient cycling and plant health.

    • Recommendations: Maintain organic matter inputs and consider cover cropping or green manures to sustain a vibrant soil biology.

  • High Levels: Above 2,000 mg/kg

    • Implications: Extremely active soil biology, which can be beneficial but may also lead to rapid nutrient cycling that could outpace plant needs.

    • Recommendations: Monitor nutrient levels closely, particularly nitrogen, as high microbial activity can sometimes lead to temporary nutrient imbalances.

Total Carbon (Combustion)

  • Low Levels: Less than 1%

    • Implications: Indicates a severe deficiency in organic matter, which may compromise soil structure, water retention, and nutrient availability.

    • Recommendations: Substantial addition of organic materials is needed to build up soil organic content.

  • Optimal Levels: 1-3%

    • Implications: Indicates a healthy balance of organic matter sufficient for most ornamental gardens, supporting good soil structure and fertility.

    • Recommendations: Continue regular organic matter additions and sustainable soil management practices to maintain these levels.

  • High Levels: Above 3%

    • Implications: Indicates very high organic matter, which is generally beneficial but could be more than necessary and potentially lead to issues like excessive moisture retention and nutrient imbalances.

    • Recommendations: Manage additions of organic matter carefully and monitor for signs of excessive soil moisture or nutrient lock-up.

Factors Affecting Carbon Levels

  • Decomposition Rate:

    Influenced by temperature, moisture, and microbial activity, which can vary widely depending on climate and soil conditions.

  • Organic Matter Inputs:

    The type and amount of organic residues returned to the soil, including plant litter, crop residues, and organic amendments.

  • Tillage Practices:

    Frequent tillage can increase the decomposition rate of organic matter, reducing soil carbon levels over time.

Actionable Recommendations

  • Increase Carbon Inputs:

    Apply organic mulches, compost, or green manures to boost soil organic matter.

  • Reduce Soil Disturbance:

    Adopt no-till or reduced tillage practices to conserve organic matter and enhance carbon sequestration.

  • Regular Testing:

    Monitor soil carbon levels periodically to adjust management practices accordingly and ensure optimal soil health.

FAQs

  • How often should I test my soil for carbon?

    Every 3-5 years, or more frequently if significant changes in land use or management practices occur.

  • What are the benefits of high soil carbon?

    Improved soil structure, enhanced nutrient and water retention, increased biodiversity, and greater resilience to erosion and weather extremes.

Conclusion

Maintaining healthy soil carbon levels is vital for sustainable soil management and robust plant growth. Regular assessment and mindful management of soil carbon can lead to improved soil health, enhanced productivity, and a reduced environmental footprint.